^ . ■ y f*-'yyp:''\?;>i-< \Avl- }#.:* > -K t . . ->f -• ■ ■:, Sft-V; ■ •,. .-.V- >■. >-.. :{Z ■ ••■'■i-'iM:'- 7 . ''"V.^Y •■■*'■ >'•'■' '5 \y ', • <" -t\. ps?>.'.. fit;- s^; V'i- ;:•■•'■■'if\ •K« * ■0f, , ~ '.'-Jit* '- ^fc^ Snentinc -■ ij.f M IS....k- :m.| Min.ilMl- A. E.FOOTE. M. D I'luhi-lclphia, Pa. "*ZW SURGEON GENERAL'S OFFICE Section,.. No, ® + *™i™*™i+*A~*.i*&*i**^**"-^® k MECHANICAL DENTISTRY. LINDSAY & BLAKISTON'S DENTAL PUBLICATIONS. 1. HARRIS'S DICTIONARY OF MEDICAL TERMINOLOGY, DENTAL SURGERY, AND THE COLLATERAL SCIENCES. Third Edition, carefully revised and enlarged. Royal octavo. Cloth, $6 50. Sheep, $7 50. 2. HARRIS'S PRINCIPLES AND PRACTICE OF DENTAL SURGERY. Ninth Edition, with 320 Illustrations. Royal octavo. Cloth, $6 00. Sheep, $7 00. 3. BOND'S PRACTICAL TREATISE ON DENTAL MEDICINE. Being a Compendium of Medical Science, as Connected with the Study of Dental Surgery. Third Edition. Octavo. Sheep, $3 00. 4. ROBERTSON'S MANUAL ON EXTRACTING TEETH. Founded on the Anatomy of the Parts involved in the Operation, &c, &c. Second Edition. 12mo. $1 50. 5. TAFT'S PRACTICAL TREATISE ON OPERATIVE DENTISTRY* A new edition, thoroughly revised, containing 86 Illustrations. Octavo. Sheep, $4 50. 6. FOX AND HARRIS ON THE HUMAN TEETH. Their Natural History, Structure, &c. With 250 Illustrations. Octavo. $4 00. 7. RICHARDSON'S PRACTICAL TREATISE ON MECHANICAL DENTISTRY. With over 159 Illustrations. Octavo. Sheep, $4 50. 8. HANDY'S TEXT-BOOK OF ANATOMY, AND GUIDE TO DISSECTIONS. With 312 Illustrations. Octavo. Sheep, $4 00. 9. PIGGOT'S DENTAL CHEMISTRY AND METALLURGY. In one volume, octavo. Sheep, $3 50. 10. TOMES' SYSTEM OF DENTAL SURGERY. With 208 beautifully executed Illustrations. Octavo. Sheep, $4 50. PRACTICAL TREATISE MECHANICAL DENTISTRY. BY * JOSEPH RICHARDSON, D.D.S., M.D., FORMERLY PR0FESS0IC OF MECHANICAL DENTISTRY AND METALLURGY IN THE OHIO COLLEGE OF DENTAL SURGERY. Second Edition, my mudt tfntatjjcflN^ ONE HUNDRED AND FIFTY-NINE ILLUSTRATIONS. PHILADELPHIA: LINDSAY AND BLAKISTON. 1869. fjroob. )Qb'StiCi>l Entered, according to Act of Congress, in the year 1868, by LINDSAY & BLAKISTON, In the Clerk's Office of the District Court for the Eastern District of Pennsylvania. HENRY B. ASHMEAD, PRINTER, Kos. 1102 and 1104 Samson Street. JAMES TAYLOR M.D., D.D.S., SUERITUS PROFESSOR OP THE INSTITUTES OP DENTAL SCIENCE IN THE OHIO COLLEGE OF DENTAL SURGERY, AS AN ACKNOWLEDGMENT OF PROFESSIONAL EMINENCE AND PRIVATE WORTH, &%i# Wolumc te gratefully htjewifrrtf, BY HIS FRIEND AND FORMER PUPIL, THE AUTHOE. I I PREFACE TO SECOND EDITION. The exhaustion of a large edition of the present work under circumstances which have, in a marked degree, affected the sale of all similar publications adversely within the past eight years, and the assurance of its favorable acceptance by the profession, conveyed in the various friendly, impartial, and generally commendatory notices which greeted its first appearance, afford adequate encour- agement for the preparation of a second edition, which, it is hoped, will, in a still greater degree, commend itself to the favorable consideration of the profession. Aiming, in the first instance, to make this treatise a dis- tinctly practical guide to the manipulations of the dental laboratory, discarding everything, in point of facts or modes of practice, which was not well grounded in theory and confirmed by experience, the Author finds but little em- braced in the first, which it is deemed necessary to exclude from the second, edition. Some verbal corrections and changes of phraseology have been made, and numerous inter- polations and additions of matter introduced throughout the body of the work, while other portions have been almost entirely re-written. The chapter on Vulcanite Base has been so revised and extended as to comprehend all that is valuable and approved Vlll PREFACE. in that particular process, and will, it is believed, be found to be as comprehensive and reliable in text and illustration as any description of the method yet published. In the initial portion of the chapter devoted to Artificial Dentures with Continuous Gums, the practitioner and student will be gratified to find descriptions of this unrivalled process from the pen of its distinguished Author himself, embracing a concise and intelligible account of the various modes of procedure employed at this time by Dr. Allen. The Author's special and grateful acknowledgments are due to Prof. Norman W. Kingsley for the able, comprehen- sive, and highly instructive contribution on Artificial Vela and Palates, which appears at the close of the volume. The accompanying illustrations, explanatory of the text, derive additional value from having been executed under his own immediate supervision, and will, it is believed, convey a more satisfactory and intelligible understanding of the mechanism of the apparata employed by him, than any heretofore im- parted by published descriptions. An additional chapter has been embodied in the work, embracing all the information of a practical character that could be obtained in reference to the working of Aluminium as a base for artificial teeth. Processes in a stage of but partial or incomplete develop- ment have not been treated of for the reason that it • is judged inconsistent with the design and character of a work like the present to embrace descriptions of immatured methods or modes of manipulation, the practicability of which has not been ascertained, or the merits of which have not been passed upon or confirmed by the common experi- PREFACE. IX ence of the profession. Time must be given to sanction or condemn whatever does not at present stand approved. In the preparation of the present, as in the former, edi- tion, the Author has not hesitated to appropriate whatever was deemed valuable and approved from all the sources at his command, but he has aimed, in every instance where the results of the labors and experience of others have been em- bodied, to accord the proper credit. For the many personal favors and courtesies generously extended to him by indi- vidual members of the profession, his thanks are, in an espe- cial manner, due. Terre-Haute, Ind., November, 1868. PREFACE TO FIRST EDITION. In the preparation of the following treatise, the author has endeavored to present, in as concise and methodical a form as possible, the material facts and principles which relate to the Mechanical Department of Dental Practice in its present advanced condition. In the accomplishment of this under- taking, the primary and leading purpose has been to furnish the student and more inexperienced practitioner with a practical guide to the manipulations of the laboratory, with accompanying elucidations of the elementary principles which underlie the practice of this important specialty. In further- ance of this design, all matters, discussions and commentaries not strictly material, have been carefully excluded. The arrangement and treatment of the various subjects embraced are such, it is believed, as will best facilitate the student in the acquisition of a knowledge of the department alluded to, and the practitioner in the intelligent and success- ful conduction of the manipulations which appertain to this branch of Practical Dentistry. In the belief that these objects have been mainly accom- plished, the work is respectfully submitted to the profession. J. Eichardson. Cincinnati. CONTENTS. PAGE Dedication,.............v Preface,.............vn PART FIRST. METALS EMPLOYED IN DENTAL LABORATORY OPERATIONS, WITH PRELIMINARY OBSERVATIONS ON THE DIFFERENT MODES OF APPLYING HEAT. CHAPTER I. PAGE Different Modes of Applying Heat,.......25-45 Blowpipe,............25 Mouth Blowpipe,..........26 Bellows Blowpipe,..........29 Self-acting or Spirit Blowpipe,........30 Hydrostatic Blowpipe,.........32 Lamps,.............37 Oil Lamp.............37 Spirit Lamp............37 Furnaces,............38 Draft or Wind Furnace,.........38 Baking Furnace,..........40 Fuel,.............41 Supports,.............44 Crucibles,............45 CHAPTER II. Gold,..............46-52 Geological Situations...........46 Geographical Distributions,.........47 Properties of Gold............47 Influence of Alloying on the Properties of Gold,.....48 Properties of Particular Alloys of Gold,.......50 XIV CONTENTS. CHAPTER III. PAGE Refining Gold,............53~59 Elements Employed........... Separation of foreign metals from Gold,.......54 CHAPTER IV. Alloys of Gold for Dental Purposes,.......60-73 Reducing Metals, ...........60 Required fineness of Gold Plate,........60 Formulas for Gold Plate used as a Base for Artificial Dentures, . . 61 Formulas for Gold Plate used for Clasps, Wire, Stays, Metallic Pivots, &c, 63 Gold Solders,............64 Method of reducing Gold to a lower or higher standard of fineness, and of determining the caret of any given alloy.......65 Table of Gold Coinage of different Nations,......69 CHAPTER V. Method of converting Gold Alloys into the required forms for dental purposes,........... 74-80 Manner of procuring an Ingot,........74 Forging,.............75 Laminating or Rolling,..........75 Thickness of Gold Plate required as a Base for Artificial Dentures, . 77 Thickness of plate for Clasps, Stays, Ac,.......78 Reduction of Gold Solders into proper forms for use, .... 78 Method of obtaining Gold Wire,........78 Method of constructing Spiral Springs,.......80 CHAPTER VI. Silver...............81-83 General Properties of Silver,.........81 Alloys of Silver,...........81 Reduction of Silver to the required forms for dental purposes, . . 82 Formulas for Silver Solders,.........83 CHAPTER VII. Platinum and the Platinoid Metals,.......84-87 Alloys of Platinum,...........86 Platinoid Metals,...........86 CONTENTS. XV CHAPTER VIII. PAGE Aluminium,.............88-91 General Properties of Aluminium,........88 Alloys,.............90 CHAPTER IX. Copper, Zinc, Lead, Tin, Antimony and Bismuth,.....92-97 Copper,.............92 Alloys of Copper...........92 Zinc,......... 94 Lead, ........ 94 Alloys of Lead,...........95 Tin,..............95 Antimony,.......• 96 Bismuth,............ 97 CHAPTER X. General properties of alloys, and their treatment and behavior in the process of compounding,........98-101 PART SECOND. ARTIFICIAL DENTURES. CHAPTER I. Treatment of the mouth preparatory to the insertion of artificial dentures,..........102-110 Useless and diseased remains of teeth,...... 103 Removal of salivary calculus or tartar,...... 106 Diseased conditions of the mucous membrane and gums, . . . 107 Caries or decay of the remaining teeth,...... 107 Surgical treatment of the mouth after the extraction of teeth, . . 107 Time necessary to elapse after the extraction of teeth before inserting artificial dentures, . ,........ 108 CHAPTER II. Materials and methods employed in obtaining impressions of the mouth, . .........110-126 Wax,............. HO Manner of obtaining an impression of the mouth in wax for partial upper dentures,.......... 112 XVI CONTENTS. PAGE Manner of obtaining an impression of the lower jaw in wax for par- tial dentures,........... 115 Manner of obtaining an impression of the mouth in wax for entire upper dentures,.......... 115 Manner of obtaining an impression of the lower jaw in wax for entire dentures,............ 118 Gutta-percha,........... 119 Plaster of Paris, . ......... 119 Manner of obtaining an impression of the mouth in plaster for partial upper dentures,.......... 121 Manner of obtaining an impression of the mouth in plaster for entire upper dentures,.......... 123 Manner of obtaining an impression of the mouth in plaster for entire lower dentures............ 125 CHAPTER III. Plaster Models,...........127-133 Manner of obtaining a plaster model from an impression in wax for partial dentures,.......... 127 Manner of obtaining a plaster model from an impression in wax for entire dentures, *......• . 129 Manner of obtaining a plaster model from an impression in plaster for partial dentures,.......... 131 Manner of obtaining a plaster model from an impression in plaster for entire dentures,.......... 133 CHAPTER IV. Metallic Dies and Counter-dies,.......134-149 Manner of obtaining a Metallic Die,....... 134 Molding,........... 134 DiPPing............ 140 Counter-Die,.......... 141 Essential Properties of a Die,........ 142 Table of Fusible Alloys,......... 14g CHAPTER V. Partial Dentures,......... 150-172 Means employed in retaining partial sets of teeth in the mouth, . . 150 Pivot Teeth.......... 250 Circumstances modifying the success of the operation, . 151 CONTENTS. xvii PAGE The roots of teeth to which artificial crowns are usually attached, . 153 Preparation of the root,......... 153 Fitting the crown,......... 158 Attaching the crowns by means of wood pivots,..... 159 Pivots of metal and wood.......... 162 Metal pivot,.......... 162 Pivot plate,............ 165 Substitution of rubber for wood pivot,...... 167 Wired rubber pivot, with rubber base,...... 167 Canal of root lined with rubber tube for the reception of gold pivot vulcanized into a crown, with rubber base,..... 169 Substitution of a plate for a pivot tooth,...... 170 CHAPTER VI. Partial dentures retained in the mouth by means of clasps at- tached TO THE NATURAL TEETH,........173-193 Remarks on the use of clasps,........ 173 The teeth to which it is most proper to attach clasps, .... 175 Separation of the teeth, by filing, for the reception of clasps, . . 177 Modifications in the form of clasps........ 178 Modifications in the form of plates for partial dentures supported in the mouth by clasps,......... 182 Swaging or stamping the plate,........ 187 Uniting the plate and clasps,........ 190 CHAPTER VII. Partial dentures supported in the mouth by means of cylinders of wood attached to tubed plates........194-196 CHAPTER VIII. Partial dentures supported in the mouth by pivoting the plate to the roots of the natural teeth,......197-199 CHAPTER IX. Partial dentures supported in the mouth by atmospheric pres- sure, ............. 200-204 Modifications in the form of the base,....... 200 Manner of forming an air-chamber,....... 202 2 XV111 CONTENTS. CHAPTER X. PAGE Method of obtaining an antagonizing model for partial den- tures ; selecting, arranging, and antagonizing the teeth ; IN- VESTING, adjusting stays, soldering, etc.,.....205-219 CHAPTER XI. Entire dentures,...........220-246 Method of constructing a plate base for an entire denture for the upper jaw.............. 220 Modifications in the form of plates for entire upper dentures, . . 223 Method of constructing a plate base for an entire denture for the under . 228 jaw,............. 228 Antagonizing model for an entire upper and lower denture, . . 229 Antagonizing model for an entire upper denture with the natural teeth of the lower jaw remaining,........ 233 Selecting, arranging, and antagonizing the teeth; rimming the plate; attaching spiral springs; investing, lining, soldering, and finishing, 234 CHAPTER XII. Substances used in the manufacture of porcelain as applied to dental purposes,...........247-249 Silex,............. 247 Felspar,............ 247 Kaolin,............ 248 Coloring materials, .......... 249 CHAPTER XIII. Porcelain Block Teeth,.........250-270 Composition and preparation of the Body,...... 251 Composition and preparation of Crown Enamels,.....252 Composition and preparation of Gum Enamels.....• 253 Antagonizing model for an entire upper and lower denture constructed of block teeth,.......... 255 Antagonizing model for an entire upper denture with the natural teeth of the opposing jaw remaining,....... 255 Forming a matrix for molding the body preparatory to carving the ***&,........... 256 Molding the porcelain paste preparatory to carving the teeth, . . 259 Carving the teeth,........ 262 CONTENTS. XIX PAGE Crucing or Biscuiting,.......... 262 Application of the Crown and Gum Enamels...... 263 Final Baking,.......... 264 Fitting and attaching the blocks to the metallic base, .... 264 Dr. Calvert's method,.......... 266 CHAPTER XIV. Uniting single porcelain teeth to each other and to a me- tallic BASE WITH A FUSIBLE SILICIOUS COMPOUND, FORMING A CON- TINUOUS artificial gum,.........271-293 Introductory remarks,......... 271 Dr. Allen's Modes of Practice,........275 Dr. Hunter's Formulas and Modes of Practice...... 283 Application of Continuous Gum to Partial Sets,.....290 CHAPTER XV. Vulcanite Base,...........294-351 General Properties of India-rubber,....... 294 Compounding Rubber for Dental Purposes,...... 295 Objections to the Use of Rubber Considered,..... 305 Method of Constructing an Entire Denture in a Base of Rubber, . 309 Arranging the Teeth,.......... 310 Formation of the Mold or Matrix,....... 312 Packing the Mold,.......... 317 Vulcanizing,........... 325 Finishing............. 338 Partial Dentures Constructed in a Base of Rubber, .... 339 Metallic Clasps attached to Rubber Plates,...... 341 Substitution of Plate for Rubber Teeth,...... 342 Repairing,............ 342 Refitting Gold or Rubber Plates with Rubber Lining, .... 343 Attaching Porcelain Teeth to a Metallic Base by means of Rubber, . 346 Manner of Obtaining an exact Duplicate of a Set of Teeth on Rubber, 349 CHAPTER XVI. Cheoplastic method of mounting ARTIFICIAL TEETH, . . . 352-359 Method of Constructing Entire and Partial Dentures in a Base of Cheoplastic Metal,.......... 352 Finishing,............ 358 Repairing,............358 XX CONTENTS. PAGE CHAPTER XVII. Aluminium Base............360-413 Aluminium Plate Base,......... 360 Aluminium Solder........... 362 Cast Aluminium Base........... 362 Dr Bean's Method,.......... 363 Preparing the Expanded Mold and Casting the Matrix, . . . 363 Constructing the Model Set,......... 377 Investment in the Flask and Completion of the Mold, . . . 383 Casting the Plate,........... 389 Finishing the Plate,.......... 392 Securing the Teeth to the Plate,........ 397 Casting Shells for Lower Sets,........ 402 Repairing Aluminium Work,........ 405 Casting Lower Sets of Tin and Silver,...... 406 Mending Defects in Aluminium Plates, ...... 409 Refining Aluminium Scraps and Dross,...... 411 Alloys of Aluminium,.......... 412 Preparation of the Alloy of Silver and Tin,..... 413 CHAPTER XVIII. Defects of the palatal organs and their treatment by arti- ficial means,...........414-435 Dr. Kingsley's Artificial Velum and Palate, ..... 414 Palatine Defects,........... 414 Obturators,............ 415 Artificial Palates,.......... 418 Artificial Palates for Congenital Fissure,...... 424 Method of making an Artificial Palate....... 427 MECHANICAL DENTISTRY. PART FIRST. METALS EMPLOYED IN DENTAL LABORATORY OPERATIONS, WITH PRELIMINARY OBSERVATIONS ON THE DIF- FERENT MODES OF APPLYING HEAT. CHAPTER I. DIFFERENT MODES OF APPLYING HEAT. The application of heat to the various mechanical processes of the dental laboratory would seem to require a brief descrip- tion of some of the agencies employed for the purpose. As full a description of the appliances used will be introduced as is compatible with the scope of the present work. BLOWPIPE. Various modifications in the form of the blowpipe have been introduced from time to time, and are named according to the means used to produce the blast, as, Mouth, Bellows, Self-acting or Spirit, and Hydrostatic Blowpipe. In addition to the varieties mentioned, there are others used in producing extreme degrees of heat, as the "oxygen blowpipe" with which the flame is blown with a jet of oxygen; and another, with which the two gases, oxygen and hydrogen, 3 26 MECHANICAL DENTISTRY. are burned, called the "oxy-hydrogen blowpipe.'' The latter is capable of producing a heat that immediately fuses the most refractory substances, as quartz, flint, rock-crystal, plumbago, &c. With it, gold is volatilized, and iron rapidly consumed when placed in the flame; while platinum, next to iridium, the most infusible of all known metals, has been melted in quantities exceeding one hundred ounces by means of this powerful instrument. As, however, these blowpipes are of no special practical utility in the dental laboratory, any further reference to them will be omitted. Mouth Blowpipe.—This instrument has been long in use, is simple in its form and construction, and, for general use in the application of moderate degrees of heat, is both conve- nient and economical. Those accustomed to its use are en- abled to produce a continuous blast of considerable force, and soon acquire the facility of regulating the heat produced with equal if not greater precision than can be readily attained in the use of either of the other varieties mentioned. The most simple form of the mouth blowpipe is shown in Fig. 1. It consists usually of a plain tube of brass, larger at the end applied to the mouth, and tapering gradually to a point at its other extremity, the latter being curved and Fig. 1. tipped at the point with a conical-shaped, raised margin to protect it from the action of the flame: the calibre of the in- strument terminates here in a very small orifice. The point of the instrument, as well as that part of it received into the mouth, is sometimes plated with a less oxydable metal than brass, as silver or platinum. The stem is generally from twelve to twenty inches in length, and the mouth extremity from one-half to three-fourths of an inch in diameter. MOUTH BLOWPIPE. 27 In operations requiring protracted blowing, a somewhat different form of the instrument will be required, owing to the accumulation of moisture within the tube, which, being forcibly expelled from the orifice, spirts upon whatever is being heated and interrupts the blast; also, on account of the fatigue which in process of time renders the muscles of the mouth and face engaged in the act to a great extent power- less. The difficulties mentioned may be obviated, in a great mea- sure, by applying the form of blowpipe represented in Fig. 2. To the mouth extremity is attached a circular concave flange or collar which receives and supports the lips. To the shaft, near its curved extremity, is adjusted either a spherical or Fig. 2. cylindrical chamber which collects and retains the moisture as it forms within the pipe. By allowing that part of the tube connected with the curved end to pass part way into the chamber, a basin is formed at the depending portion of the latter, which, by collecting the fluids, will effectually prevent them from overflowing and passing into the tube beyond. There are other modifications of the mouth blowpipe some- what allied in form to the one last described, but as they are constructed more especially for chemical examinations or analyses, and as they possess no advantages for dental pur- poses over those already mentioned, a description of them is not deemed necessary. Mechanism involved in the act of producing a continuous blast with the mouth blowpipe.—As a steady, continuous cur- rent of air from the blowpipe is preferable to the interrupted 28 MECHANICAL DENTISTEY. jet, in all those operations where it is desired to produce a steadily augmenting heat, the following remarks, explanatory of the method of producing it, are subjoined in the belief that they will render easier a process not always readily acquired. " The tongue must be applied to the roof of the mouth, so as to interrupt the communication between the passage of the nostrils and the mouth. The operator now fills his mouth with air, which is to be passed through the pipe by compress- ing the muscles of the cheeks, while he breathes through the nostrils, and uses the palate as a valve. When the mouth becomes nearly empty, it is replenished by the lungs in an instant, while the tongue is momentarily withdrawn from the roof of the mouth. The stream of air can be continued for a long time without the least fatigue or injury to the lungs. The easier way for the student to accustom himself to the use of the blowpipe, is first to learn to fill the mouth with air, and while the lips are kept firmly closed to breathe freely through the nostrils. Having effected this much, he may introduce the mouth-piece of the blowpipe between his lips. By infla- ting the cheeks, and breathing through the nostrils, he will soon learn to use the instrument without the least fatigue. The air is forced through the tube against the flame by the action of the muscles of the cheek, while he continues to breathe without interruption through the nostrils. Having become acquainted with this process, it only requires some practice to produce a steady jet of flame. A defect in the nature of the combustible used, as bad oil, such as fish oil, or oil thickened by long standing or by dirt, dirty cotton-wick, or an untrimmed one, or a dirty wick-holder, or a want of steadiness of the hand that holds the blowpipe, will prevent a steady jet of flame. But, frequently, the fault lies in the orifice of the jet, or too small a hole, or its partial stoppage by dirt, which will prevent a steady jet of air and lead to difficulty. With a good blowpipe, the air projects the entire flame, forming a horizontal, blue cone of flame, which con- BELLOWS BLOWPIPE. 29 verges to a point at about an inch from the wick, with a larger, longer, and more luminous flame enveloping it, and terminating to a point beyond that of the blue flame."* Bellows Blowpipe.—This form of blowpipe, although su- perseded in a great degree by other modifications more re- cently introduced, is well adapted to all the minor operations of the shop, provided it is so constructed as to produce a con- tinuous and equable current of air. A very efficient form of this apparatus, contrived by Drs. McDaniels and Roudebush, is exhibited in Fig. 3. It consists of three distinct chambers; two lower ones, separated from each other in the centre, and one above, communicating by valvular openings with the former. The basement boards of the lower compartments are ac- curately joined to each other in the centre with an inclination like that of an ordinary roof; are each about six inches square and have apertures in the centre with valves opening upward. The upper borders of these chambers are formed by the treadle or foot-board, con- sisting of a plain board, equal in length and breadth to both chambers, and adjusted by means of hinges to the basement pieces where they unite in the middle;—strips of India rub- ber cloth being glued along the joints on either side to pre- vent the escape of air from one chamber into the other. The upper chamber is formed from a strip of board of equal length and half the breadth of the treadle, the ends and sides being closed, as also the lower apartments, with pieces of leather so arranged as to admit of expansion and closure of the air chambers. The lower chambers communicate with the upper or receiving chamber by means of an aperture in the top of each of the former with valves opening into the receiver. * " The Practical use of the Blowpipe." Anon. 30 MECHANICAL DENTISTRY. The movable top of the upper chamber should be bound down with compressed spiral wires arranged internally, or by strips of India rubber tacked to the borders of the chamber externally so that when the lid of the latter is forced open by the injection of air from the lower chambers, a steady com- pression of the contained air will be made, forcing it with a steady and continuous impulse into the rubber tube con- nected with the receiver; the force of the current being regu- lated by the power exerted in depressing either end of the treadle with the foot. The modus operandi of this blowpipe when in use is briefly as follows: One end of the treadle being forced down, the contained air of the corresponding chamber underneath is urged through the valvular opening above into the receiver, while the air at the same time rushes in through the opening in the bottom of the opposite chamber, filling the latter : the other end of the treadle being pressed down, the air in the chamber below is, in like manner, also thrown into the receiver through the opening on the same side above; thus, by an alternate action of the treadle, the receiving chamber is kept constantly replenished. Self-acting or Spirit Blowpipe.—With this instrument the flame is blown with a vapor of boiling alcohol or whiskey. Several varieties have been introduced within the past few years, some of which are well adapted to dental laboratory operations, while others, owing to too great a complexity of mechanism in their construction, by which their action is rendered uncertain and liable to frequent and perplexing interruptions, have not been generally adopted; others, again, are regarded as too unsafe and inconvenient for labor- atory uses. One of the best implements of the kind, simple in its con- duction and design, and at the same time safe, convenient and manageable, is what is known as "Hollely's Self- acting or Spirit Blowpipe." A description of this apparatus will serve to convey a sufficiently distinct idea of the prin- SELF-ACTING OR SPIRIT BLOWPIPE. 31 ciple involved in the various modifications of the blowpipe under consideration. The accompanying cut, Fig. 4, exhibits the different parts in their proper relation when in use. It consists of a cylin- drical boiler, usually of brass, with a square-capped safety valve on top, so arranged as not to admit of complete closure, thereby rendering an explosion of the boiler impossible. The pressure of the contained vapor, and, by consequence, the Fig. 4. power of the jet may be increased by turning the thumb-screw connected with the valve forward, and lessened by a reverse movement of the same. The valve fixture may be removed by unscrewing it, and alcohol introduced into the boiler through the opening; the boiler being from one-half to two- thirds full when in use. To the top of the latter near the side two pipes are attached communicating with the interior of the boiler, and which are provided with a gauge-cock, by which the operator is enabled, by a partial rotary movement of a thumb-screw, to control, somewhat, the force of the vapor current through the pipes, and also to transfer the jet from one pipe to the other, as he may desire a pointed or spreading flame ; or a commingling jet from both pipes may be simultaneously produced whenever a very large and di- verging flame is required. 32 MECHANICAL DENTISTRY. A small alcohol lamp being lighted, is placed underneath the boiler, when, after the lapse of a few minutes, a current of vapor will be forcibly expelled from the orifices of the pipes*, and may be directed against the flame of any of the lamps in common use for heating purposes. A blowpipe like the foregoing will be found serviceable in all operations requiring a protracted blast, and, in common with the bellows and hydrostatic blowpipes, will be found of especial service to those whose eyes and lungs are injuriously affected, as they sometimes are, by the use of the mouth blow- pipe. It is also portable, as the various parts of which it is composed may be readily detached and packed within a small space. Hydrostatic Blowpipe.—This blowpipe is of comparatively modern origin, and for many purposes in the dental labora- tory is the best that has yet been introduced. With it, a constant, equable, and forcible cur- rent of air may be produced and maintained for a length of time equal to the capacity of the water-chamber; and for all operations requiring a high and steady heat it is invaluable. Fig. 5 represents an apparatus of this kind, which is so simple and economical in its construction, that every dentist may, with little trouble and at a trifling cost, provide one for his laboratory. By reference to the annexed illus- tration, it will be seen to consist of a tank or cylinder made of sheet- iron, zinc or copper, of variable dimensions, usually, however, from four to six feet in length and from twelve to twenty inches in diameter. To the side of the HYDROSTATIC BLOWPIPE. 33 cylinder near the bottom a pipe with faucet is attached, called the "supply pipe," and is designed to convey water to the tank; another is attached to the bottom, termed the "waste pipe," and is used to discharge the contained water. To the centre of the top of the cylinder is adjusted a stop- cock to freely admit the ingress of air when the water is being discharged, without which the sides of the cylinder would tend to collapse on the formation of a vacuum within. An India rubber tube is also united to the top of the cylinder on one side, and is attached at its other end either to an ordinary mouth blowpipe or gas jet. The water being admitted to the tank through the supply pipe, the contained air is compressed and forcibly expelled through the air-tube and orifice of the blowpipe upon the flame; the force of the jet being regulated by the stop-cock connected with the supply-pipe. When the cylinder becomes filled, or partially so, the faucet on the top should be opened and the water drawn off through the discharging pipe. A small glass tube, communicating with the interior of the cylinder on one side near the bottom, extending up the side, and again entering the cylinder at the top, will be found useful as a guide to determine, at any time, the depth of water in the tank, the water in the tube maintaining the same level with that in the former. Those unable to command the facilities afforded by water power, may avail themselves of the following contrivance which combines mainly all the advantages of the blowpipe just described. A cylinder of the form mentioned, but with an open top, is partially filled with water, and another, similiarly formed but inverted, or with its closed end above, is made to fit and slide into the first. Weights are then placed on the top of the inverted tank, when the air within the latter will be forcibly compressed and impelled, as in the other case, through an air-tube attached to its upper closed extremity. 34 MECHANICAL DENTISTRY. The following description of a hydrostatic blowpipe, (Fig. 6,)* of simple but in- FlG- 6- genious construction is inches high, 15 inches long, and 8 inches wide. B represents a side view; and the dotted line between B and C a partition passing from within about an inch of the front upper angle down to within an inch of the lower back angle of the tank: or, in other words, this partition runs diagonally from the front upper angle to the lower back angle. This partition must be water tight, except at the bottom of the tank, where an open space is left of about an inch, so that when water is poured into the open compartment B—which is nearly as wide at the top as the tank is long—it can readily flow into the compartment C. When the tank is filled nearly half full it will be seen that the great bulk of the water will be in the front compartment C. The bellows is an ordinary large size house-bellows. The end of it rests upon the top of the * The accompanying illustration was kindly furnished, at the instance of Prof. K., by Dr. S. S. White, for which and other similar favors the author would express his grateful acknowledgments. HYDROSTATIC BLOWPIPE. 35 tank, and the handle of the bottom part is made fast to an upright strip of board. A cord connected with the upper handle, having a weight of four or six pounds attached to the other end, passes over a small pulley for the purpose of raising the bellows. A cord connected with the same handle is attached to a small foot-lever or treadle, and by this means the bellows is easily worked with the left foot. To the nozzle of the bellows there is fastened one end of a plumber's small brass joint; the other end of the joint is soldered to a piece of small-sized lead pipe which communicates with the compartment C at the top. In this brass joint is placed a leather valve—which any dentist of ordinary ingenuity can construct for himself—to prevent the air from returning back into the bellows. Connected with the lead pipe, running from the nozzle to the compartment C, is another piece (D), passing up through the work-bench or table, as the case may be, to which an old fashioned brass mouth blow-pipe is soldered. When the bellows is moved, the air is forced out of the pipe D and also into the compartment C, which be- comes a reservoir for the air. For as the air presses down upon the surface of the water in C it is forced in the di- rection indicated by the arrow into the compartment B, which is open at the top. As the water is forced by the air in C into B its weight acts upon the air in C to drive it out of the pipe D, and in this way a steady and continuous blast, capable of being graduated to the wish of the operator by the greater or smaller quantity of water acting upon it, may be maintained with the most perfect ease. Any tinman can make the tank and its connections. Tin or zinc will do, but in the long run tinned copper would be the most economical. The apparatus should be placed at one end of the bench, or in some convenient place where it will not occupy much room. " It will be observed that in using this blow-pipe both hands are free to handle the piece of work, and the lungs and mouth are also free to perform their natural functions. 36 MECHANICAL DENTISTRY. I am sure any one using this blow-pipe for one month could never be prevailed upon to abandon it.....The intensity of the flame is such as to enable me to melt an ounce of gold in a few minutes; and it will be found sufficient for all the practical requirements of a dentist's blow-pipe. With the ordinary movable gauze gas burner for soldering, the size of the flame can be graduated by a slight change in the position of the burner, as may be desired." The oil or alcohol lamp in common use will serve for either of these blow-pipes, but a gas-jet, whenever it can be conveniently com- ..^^sA^te. manded, is preferable to either. Fig. ^O^ A 7 represents an improved apparatus .sLw employed in the application of the air- ;fr^ jet to the gas flame. a? A movable gas-jet, attached to two ~^e&m5*^^^\ short arms of an ordinary gas-pipe, is ."»"". .....www'i" made to receive within it the blow-pipe point connected with the rubber tube, the air-tube, terminating a little with- in the open mouth of the gas jet; it is thus a tube within a tube, with a space between them for the admission and pas- sage of gas. The gas being admitted by turning the screw connected with the gas-pipe, is ignited, and the current of air admitted from the rubber tube by turning a similar screw attached to the air-pipe, when the jet of air will strike the centre of the flame and project it upon the substance to be heated. The connected portions of the air and gas jets are so attached to the main pipe as to admit of an upward and downward motion, while the volume of gas and air is readily graduated by the stop-cocks attached to the former. This arrangement is one of the most complete and efficient for blowpipe purposes that has yet been devised. The jet may be elevated or depressed at will, while the force of the air-current and the volume of gas-flame can as readily be increased or diminished. The operator is thus enabled, with LAMPS. 37 the greatest ease, to produce a heat adapted to the most deli- cate operations, or to instantly change it to a heat so intense that pure gold in considerable quantities is almost immedi- ately fused in the flame. It is, therefore, well adapted to all operations in the laboratory, but will be found of especial utility in the construction of work requiring pure gold as a solder. LAMPS. The lamps most commonly used by dentists in blowpipe manipulations are oil and spirit lamps. Oil Lamp.—When oil is burned, the form of lamp repre- sented in Fig. 8 may be used. It should hold from one to two pints, and should have a spout one inch or more where it joins the body of the lamp, tapering gradually to three- fourths of an inch at the top. The spout should be well filled with wick, but not so tightly as to prevent it from being freely saturated. The best combustible is pure sweet oil, but common lamp or lard oil is generally employed and answers every practical purpose. The wick should be kept well cleaned and trimmed, and fresh oil should be substituted whenever that in use becomes thickened by dirt or otherwise deteriorated. Spirit Lamp.—Alcohol is preferred by many on account of its greater cleanliness, although it does not afford so great a heat as oil. When spirit is employed a somewhat different form of lamp should be used. With one like that described for oil, there is danger of explosion in the event of the flame, mixed with air, communicating with the alcohol contained in the lamp, and which is more liable to happen when the spout 38 MECHANICAL DENTISTRY. is but loosely filled with wick. To provide against such casualty, it is not unusual to pack the wick too closely, which, by obstructing the flow of alcohol into the spout, lessens the heat of the flame. The proximity of the flame, also, to the body of the lamp, produces undue waste of alcohol by evapo- ration. The objections stated may be obviated by employing a lamp of the form shown in Fig. 9. With a lamp like that repre- sented, the spirit is entirely uninfluenced by the heat of the flame, while explosion is rendered impossible. The centre of the up- right portion of the spout is traversed by a small tube extending throughout, and open at both ends to admit of the application of a jet of air to the lower orifice, impelling the flame from the centre and thereby intensifying the heat. Around this central tube the wick is arranged; the space occupied by the latter communicating with the body of the lamp through the horizontal arm of the spout. FURNACES. It would be inconsistent with the design of the present work to introduce a description of any forms of furnace other than those commonly used by the dentist. Those used in the arts, or for chemical and pharmaceutical purposes, em- brace almost endless varieties, and have no special adaptation to the uses required of them in the dental laboratory. Draught or Wind Furnace.—A very convenient, portable and economical furnace may be made of sheet-iron, of any DRAUGHT OR WIND FURNACE. 39 desired shape or dimensions, though usually of small size, and cylindrical in form. A light grate, or heavy piece of sheet- iron perforated with holes to admit of the passage of air, should be adjusted near the bottom, while above and below the grate are two openings, the lower one communicating with the ash-pit, and the upper one for the introduction of fuel and substances to be heated. By surmounting this simple apparatus with a pipe, or connecting it with the flue of a chimney, it will be found efficient in many of the minor operations of the shop, as melting metals, heating pieces pre- paratory to soldering, annealing, &c. A more durable and serviceable draught furnace, however, may be built of masonry, a convenient form of which is repre- sented in Fig. 10. The construction of this stationary fix- ture is so plainly exhibited in the cut that any extended description of it is deemed unneces- sary. The upper holes represent the entrance to the fire- chambers which are distinct from each other; the lower ones communicate with the ash-pit which is common to both chambers. Two fire apartments are here shown; one for melting and re- fining the more pre- cious metals, heating up operations for soldering, &c.; the 40 MECHANICAL DENTISTRY. other being used exclusively for fusing the baser metals, as zinc, antimony, lead, &c. These furnaces are sometimes constructed with a single fire-chamber, but the one exhibited is, in every way, preferable. Baking Furnace.—The chief purposes to which these fur- naces are applied are the manufacture of porcelain teeth, single and in sectional blocks, the preparation of silicious compounds, and the construction of what is known as " con- tinuous gum-work." The most recent and approved form of this furnace is exhibited in Fig. 11. The body of the furnace rests upon a cast-iron frame-work or basement, which serves the purpose of an ash-pit. The grate immediately over this inclines from each side of the furnace toward the bottom and the centre of the ash-pit, to afford more ample room for fuel directly under- neath the lower muffle. The upper portion or body of the furnace is made of fire-clay, and contains three muffles arranged horizontally; the upper two, termed " annealing muffles," are designed, more especially, for dry- ing substances, partial heating preparatory to final baking, and to receive substances from the lower muffle to be gradually cooled. The lower or main muffle is for general baking purposes requiring the employment of extreme degrees of heat. Each muffle is provided with fire-clay slabs or slides, on which substances to be heated are placed and Fig. 11. FUEL. 41 introduced into the muffles; and also plugs of the same mate- rial to close the openings to the former. Openings are made on each side of the furnace, intermediate between the muffles, for the introduction of fuel, and to afford ready access to the latter with tongs or other implements. These entrances are also provided with plugs which are applied during the process of heating. This furnace should be connected with a flue having a strong and unobstructed draught. FUEL. Under this head are comprehended such combustible sub- stances as are used for fires or furnaces, as wood, coal, char- coal, and coke. For dental furnace operations, only the latter two are, as a general thing, admissible. Bituminous, or pit-coal, is unfit for the uses required of fuel by the dentist, and is, therefore, seldom used. Anthracite coal, if carefully selected, may be employed, pro- vided it is clean, free from slate, and does not yield a fusible ash. As charcoal and coke are the fuels chiefly used in the processes of the laboratory, these substances will be more particularly described. Charcoal.—Charcoal is obtained by igniting wood and then excluding it from the air while burning ; the volatile products are thus driven off while the carbon remains. When com- bustion has proceeded slowly for a certain length of time, the openings to the bed or mound are closed, and the wood al- lowed to char. When it is desired to maintain a high heat in a small com- pass, the charcoal best adapted to the purpose is that obtained from what is termed hard wood, as the beech, the oak, the alder, the birch, the elm, etc. A cubic foot of charcoal de- rived from these woods weighs, upon an average, from twelve to thirteen pounds ; while a similar bulk obtained from soft wood, as the fir, the different kinds of pine, the larch, the linden, the willow, and the poplar, averages only from eight 4 42 MECHANICAL DENTISTRY. to nine pounds.* There is, therefore, economy in the use of the former when purchased by the* bulk; and of this class the beech-wood charcoal is the best, on account of its greater specific gravity. The more heavy charcoals require a stron- ger draught than those of a lighter character, as a more generous supply of oxygen is necessary to their perfect com- bustion. Charcoal should be kept as dry as practicable, since it rapidly absorbs moisture from the atmosphere, by which its calorific energy is materially impaired. Coke.—This substance, like charcoal, is a carbonaceous residuum obtained from pit-coal that has been exposed to ig- nition for some time excluded from the contact of air, the volatile products of the coal, like those of Wood, having been driven off by the heat. Coke differs in appearance as well as in quality. The principal part of that obtained from gas- houses is of a dull, iron-black color, very spongy and friable, and is more rapidly consumed, and produces less heat than the harder and more compact kinds. The best coke for fur- nace use is that used by brassfounders, and has a steel-gray color with a somewhat metallic lustre; is compact in its structure, and splits into pieces having a longitudinal frac- ture. Coke does not readily ignite, and, at first, generally re- quires the admixture of charcoal to effect its combustion; it also requires a strong draught to burn it, but when thoroughly ignited, it produces an intense and persistent heat. It is the principal fuel used in baking mineral teeth, porcelain blocks, and the silicious compounds employed in the fabrication of continuous gum work. Professor Piggot, in his remarks on the comparative value of fuels, observes : " Practically, for the purposes of the chem- ist, the best fuel is charcoal or coke, or a mixture of the two. The ash of charcoal being infusible, it passes through the bars of the grate as a white powder. Should potash, how- * Ure. FUEL. 43 ever, be in large excess, it corrodes the bricks, by forming with them a silicate of potash, which runs down the walls and chokes the bars. In small quantities, this action is bene- ficial, as it furnishes a protective varnish, and unites the bricks and lutes, by forming a sort of cement, which inti- mately combines with them. " Coke contains a very variable amount of ash, which is composed chiefly of oxide of iron and clay. The latter is not fusible by itself, but may soften. When pure, it forms a harmless slag, which injures neither the furnace nor the cru- cibles. Usually, however, the oxide of iron predominates. In this case the ash is very injurious, for it is reduced to a protoxide, which is not only fusible, but powerfully corrosive to all argillaceous matters, so that both the crucibles and furnaces suffer."* In order that the greatest amount of heat may be evolved from these fuels, it is necessary that the conditions necessary to secure their perfect ignition should be strictly observed; these have reference to an unobstructed circulation of air that oxygen may be freely supplied to them. To this end the furnace should be kept clean, the bars of the grate un- broken, and a good draught obtained. The condition in which the fuel is applied will also modify the results : thus, for example, if the lumps are too large they will absorb heat, and caloric will be lost; if too small, they will be too rapidly consumed. It is essential, also, to have the fuel as free as possible from dust and dirt, as these fine particles, in any considerable quantities, obstruct the draught and prevent a thorough ignition of the mass. Coke, especially, should be preserved clean, and should be broken into fragments not larger than an inch or an inch and a half in diameter, and, as nearly as possible, in the form of blocks or cubes, as these leave more open spaces for the free circulation of the air. * Dental Chemistry and Metallurgy, p. 274. 44 MECHANICAL DENTISTRY. SUPPORTS. There are many processes in the dental laboratory requir- ing the application of heat, for which a suitable holder or support should be provided. A very convenient form of holder used in soldering may be made of a circular or semi- eliptical piece of heavy sheet-iron, the margin being serrated and turned at right-angles, forming a cup. To the under side and centre of this, an iron rod, ten or twelve inches long, may be permanently riveted; or it may be made to revolve on the handle, so that the heat may be thrown directly upon any particular part of the piece to be soldered without dis- turbing the latter. A small hand-furnace (Fig. 12) is sometimes used, and will be found a very convenient and useful apparatus, not only for Fig. 12. soldering, but for preparatory heating. It consists of a fun- nel-shaped receptacle made of sheet-iron, with a light grate or perforated plate of the same material adjusted near the bottom, and an opening on one side, underneath the grate, for the admission of air. The upper part of the holder is CRUCIBLES. 45 surmounted by a cone-shaped top, which may be readily re- moved by a handle attached to it; while to the bottom of the furnace is attached an iron rod five or six inches long, and terminating in a wooden handle. The piece to be soldered is to be placed inside on a bed of charcoal, the top adjusted to its place, and the fuel ignited; when the operation is suffi- ciently heated, the top may be lifted off, and the piece, re- maining in the furnace, soldered with the blowpipe in the usual manner, the furnace thus serving the purpose of a holder. A support in very common use consists simply of a large, close-grained piece of charcoal, invested in plaster one-half or three-fourths of an inch thick, one end or side being left open and scooped out to receive whatever is being heated. Or a plaster cup two or three inches deep may be made, and its interior partly filled with a mixture of plaster, sand, asbestos, and pulverized charcoal. CRUCIBLES. Crucibles are small conical-shaped vessels used by the den- tist principally for the purposes of melting and refining metals used for plates, compounding metallic alloys, preparing and compounding the various ingredients employed in the manu- facture of porcelain teeth and continuous gum work, etc. They combine in a high degree the properties of infusibility, exemption from the attack of substances fused in them, the power of resisting sudden alternations of temperature, and impermeability to fluids and gases. The Hessian crucibles, which are in most common use among dentists, are composed of silica, alumina, and oxyd of iron. For a more particular description of the various components entering into the struc- ture of crucibles, as well as the manner of manufacturing them, the reader is referred to Piggot's " Dental Chemistry and Metallurgy," and other works treating more fully of the subject. CHAPTER II. GOLD. Gold has been known from a period of great antiquity, having, according to the writings of Moses, been wrought into articles of jewelry more than three thousand years ago. As a base or support for artificial dentures, it has entirely superseded the use of the various animal substances formerly employed, and, by the mass of practitioners at the present time, it continues to be the most highly esteemed metal for the purpose mentioned, notwithstanding the more recent in- troduction of approved processes in which, as a base, this metal is wholly discarded. Gold is found only in the metallic state, and occurs either crystallized in the cube, and its allied forms, or in threads of various sizes, twisted and interlaced into a chain of minute octahedral crystals; also in spangles or roundish grains. These latter, when they occur of a certain magnitude, are called pepitas, some specimens of which have been obtained of great size. In 1810 a mass of alluvial gold weighing twenty-eight pounds was found in the gravel pits of the creeks of Rockhole, in North Carolina. A lump of gold ore weighing three cwt., was forwarded from Chili, South Ame- rica, as a contribution to the World's Exhibition in London. New Granada, California, Russia, and Australia, have each produced pepitas or masses of gold weighing respectively twenty-seven and a half, twenty-eight, seventy, and one hun- dred and six pounds. Geological Situations.—The crystalline primitive rocks, the compa^, transition rocks, the trachytic and trap rocks, and alluvial grounds, are the formations in which gold occurs. GOLD. 47 Unlike many other metals, it is never in such large quantities as to constitute veins by itself, but is either disseminated through the rocky masses, or spread out in thin plates or grains on their surface, or confined in their cavities in the shape of filaments or crystallized twigs. The minerals com- posing the veins are either quartz, calcspar, or sulphate of baryta. The ores associated with the gold in these veins are principally iron, copper, arsenical pyrites, galena, and blende. The most abundant sources of gold, however, are in alluvial grounds, where it is found distributed in the form of spangles in the sands of certain plains and rivers, especially at their re-entering angles, at the season of low water, and after storms and temporary floods. Sufficient reasons have been advanced in support of the belief that gold, found in alluvial situations, belongs to the grounds traversed by these rivers, instead of being washed, as was formerly supposed, from the mountains in which their waters have their origin. Geographical Distributions.—The European mines, more particularly distinguished for their richness, are in Hungary and Transylvania, especially the former. Gold also occurs, but more sparingly, in Ireland, Sweden, Siberia, Germany, Russia, and Spain. In Asia and Africa, the mines which yield most abundantly, are situated in the southern portions of these continents. From the latter, the ancients derived the greater portion of their gold. Several of the South American provinces yield this metal in considerable quanti- ties. Washings are also common in several States of the Union, but California stands unrivaled, except by Australia, in the immense productiveness of its mines, and its resources in respect to this rare and valuable metal are reckoned inex- haustible. Properties of Gold.—Pure gold is distinguished from all other metals by its brilliant orange-red or yellow color, being the only simple metal that possesses this complexion. It is susceptible of a high polish, but is inferior in brilliancy to steel, silver or mercury. Its specific gravity varies some- 48 MECHANICAL DENTISTRY. what, according as it is fused or hammered; the former having a density of 19*26; the latter ranging from 19*4 to 19-65. It is only excelled in density, therefore, by platinum, the specific gravity of which is 21'25. Gold surpasses all other metals in malleability. The average thickness of ordinary gold leaf is jg^o^o °f an inch> but the ultimate degree of attenuation of which pure gold is susceptible exceeds considerably this estimate. It is also distinguished for its ductility. A single grain of gold may be drawn into wire 500 feet in length, while an ounce may be made to extend 1,300 miles. It is somewhat softer than silver, and possesses great tenacity, though inferior in this quality to iron, copper, platinum or silver. A thread of gold T^fo of an inch in diameter will sustain a-weight of 150 pounds. Gold fuses at 2016° with considerable expansion, and, on cooling, contracts more than any other metal. On account of the want of affinity of gold for oxygen, it remains unaltered in the longest exposure; it is incapable of being oxydized in any heat that may be applied to it, and is only volatilized with great difficulty in the resistless heat of the oxy-hydrogen blowpipe. It is unaffected by the most concentrated of the simple acids, but is readily soluble in. aqua regia or nitro-muriatic acid, and nitrofluoric acid. It will thus be seen that gold possesses, in an eminent degree, those general properties which render it peculiarly fit for the purposes to which it is applied in the practice of dental prosthesis. Influence of Alloying on the Properties of Gold.—The term alloy signifies a compound of any two or more metals, as brass, which is an alloy of copper and zinc. Alloys, in respects to their uses, are practically new metals, and differ in many important respects, both in their chemical and physical characteristics, from the constituent metals of which they are composed. A more particular account of the influence of alloying upon the general properties of metals, and their management and behavior in the process of com- GOLD. 49 pounding, will be given under the head of alloys of the baser metals. As gold combines readily with most metals, some of the more prominent conditions which distinguish its alloys will be given. The malleability of gold is, strictly speaking, always im- paired by its union with other metals. This effect is eminently characteristic of certain contaminations, as those with arsenic, tin, antimony, bismuth, lead, &c.; while with certain other metals, as silver, copper and platinum, unless in excess, this property of gold is so little affected, as in no material degree to interfere with its being worked into any desired form for dental purposes. The ductility of gold is also usually diminished by its incorporation with foreign metals; sometimes in a remarkable degree. Gold is always rendered harder, and its tenacity is generally increased, by alloying, while its density varies with the particular metal or metals with which it is combined. Thus, the alloy of gold with either zinc, tin, bismuth, antimony or cobalt, has a den- sity greater than that of the mean of its constituents, while the alloys of gold having a less specific gravity than the mean of their components are those with silver, iron, lead, copper, iridium or nickel. Gold is ordinarily more fusible when al- loyed, the alloy always melting at a less heat than that required to fuse the most refractory constituent, and often- times less than the more fusible component. The alloy of gold and platinum furnishes an example of the former; the platinum, which in its uncombined state is infusible in the highest heat of a blast furnace, forming a fusible compound with gold, the melting point of which is far below that of platinum. Gold solder, composed of gold, copper and silver, affords a familiar illustration of the latter; the alloy melting at a less heat than that required to fuse its least refractory component, silver. Gold, which in its pure state has less affinity for oxygen than any other metal, is rendered more or less oxydable when combined with other metals. That gold alloys tend to be formed in definite proportions 50 MECHANICAL DENTISTRY. of their constituents would appear from the phenomenon observed in the native gold of the auriferous sands, which is an alloy with silver in the ratio of 1 atom of silver, united to 4, 5, 6, 12 atoms of gold, but never with a fractional part of an atom. The same circumstance is noticed in connection with the amalgam of silver and mercury. But as alloys are generally soluble in each other, the definiteness of this atomic combination is obscured and disappears in most cases. Properties of Particular Alloys of Gold.—The metals with which gold is liable to become contaminated in the dental laboratory are zinc, tin, lead, antimony, bismuth, iron or steel, mercury and arsenic; as also excess of silver, copper and platinum. As several of these metals when alloyed with gold, even in very minute quantities, are highly destructive in their influence upon those properties which adapt this metal to the various wants of the mechanical operator, and as their separation is often attended with considerable difficulty, annoyance, and loss of time, it is practically im- portant that care should be taken to prevent, as far as practicable, the admixture of any one or more of them with the gold scrap, filings or sweepings, which are to be re-converted into proper forms for use. The accidental intrusion of these metals, however, is, to some extent, unavoidable, and as an acquaintance with the more prominent characteristics or sensible properties of the resulting alloys sometimes furnishes valuable indications in the selection of the proper re-agents employed in their purification, a description of individual alloys is introduced. Tin, antimony, bismuth, lead and arsenic are peculiarly prominent in their effects upon the malleability of gold; either of these metals in exceedingly minute quantities ren- dering gold intractable. One part of antimony with nine of gold, forms a pale, brittle alloy, and in the proportion of one part of the former GOLD. 51 to 1920 of gold, the resulting compound is too brittle to admit of successful lamination. An alloy of arsenic with gold containing %\^ of the former is a gray brittle metal, while in the proportion of vfa, the malleability of the gold is seriously impaired without suffering any change of color. So energetic is the influence of this metal on gold that the latter is rendered brittle when sub- jected even to the vapor of arsenic. Tin, lead and bismuth are somewhat analogous to arsenic in their influence upon the malleability of gold, either of them, in almost inappreciable quantities, rendering the latter metal unmanageable under the rollers. One part of lead or bismuth to 1920 of gold converts the latter into an unmalleable metal, while tin exceeds either in its remarkable tendency to render gold hard and brittle. Alloys of gold with tin are of a light color; those with lead are of a darker complexion. Zinc with gold forms a brittle alloy, and when combined in equal proportions, is exceedingly hard, white, and brittle. Uniting or incorporating itself less intimately with the gold than either lead or tin, however, it not unfrequently happens that portions of the ingot will be brittle while others remain, in some degree, malleable; so that the bar, when rolled out in the form of plate, will be perforated or cracked at those points where the zinc preponderates, while remaining portions of the plate retain a moderate degree of softness and plia- bility. The working properties of gold are not sensibly affected by the incorporation of very small quantities of iron, as an alloy of these metals, in the proportion of one part of the latter to eleven of gold, remains malleable. Platinum, in itself a highly refractory metal, is, as before stated, rendered fusible in combination with other metals. When combined with gold in small proportions, the latter is rendered harder and more elastic without having its mallea- bility practically impaired. Platinum very readily affects 52 MECHANICAL DENTISTRY. the color of gold, the smallest quantities rendering the alloy pale and dull-colored. Silver unites with gold in every proportion, and is the chief metal employed in the reduction of gold to the required forms for dental uses. It renders gold more fusible, and im- parts to it increased hardness without materially affecting its malleability. The alloy is light-colored in proportion to the amount of silver introduced. Copper, like silver, is usually combined with gold in the formation of plate, solders, etc., and hardens and renders gold tougher without practically impairing its malleability. It imparts to the alloy a deeper red color, and in the form of plate is capable of receiving a polish excelling in richness and brilliancy any other metal. The foregoing alloys of gold, it will be perceived, are such as result from the incorporation with gold of minute propor- tions of any one of the base metals mentioned, and possess certain physical characteristics that indicate, with tolerable certainty, the particular alloying component. Thus, for ex- ample, if the alloy is light-colored and very brittle, the pres- ence of tin may be suspected; if brittle and dull-colored, lead is indicated; if grayish or dull-colored, but still malleable, tough and elastic, platinum is probably present; if unequally malleable, or brittle in spots, the presence of zinc may be inferred. Alloys of gold, however, embracing several or all of these metals in varying proportions, are sometimes accidentally formed, in which case the more distinctive features which characterize the binary compounds are lost or obscured. CHAPTER III. REFINING GOLD. Elements Employed.—The separation of foreign metals from gold by what is termed the " dry method," or roasting, is effected by the action on them of either oxygen, chlorine, or sulphur, converting them into oxyds, chlorides, or sulphu- rets. Certain compound substances are used for this purpose which, when heated and decomposed, yield these elements in sufficient quantities for the purposes specified. The refining agents in common use are nitrate of potassa, (nitre, or salt- petre,) which yields oxygen; chloride of mercury (corrosive sublimate,) which yields chlorine ; and sulphuret of antimony, (crude antimony,) which yields sulphur. Other compounds contain these elements, but those mentioned are generally preferred, because they contain them abundantly, are readily decomposed by heat, and do not materially interfere with the process of separation by the introduction of troublesome com- ponents into the alloy. Before considering specifically the different modes of refining alloys of gold, it will be convenient to classify the different forms of gold as they occur in working this metal in the laboratory. 1. Plate-scrap or clippings, and plate-filings. These, if proper care is taken to prevent the introduction of fragments of platinum, impure filings, or particles of base metals, only require, provided they were originally of suitable fineness, to be re-melted and again converted into plate or other forms for use. 2. Mixed filings, and fragments containing solder, plati- num, etc. These, when melted alone, produce an alloy more 54 MECHANICAL DENTISTRY. or less impoverished in proportion to the quantity and quality of the foreign metals introduced in finishing pieces constructed of gold, and should be either separately refined by roasting, or reduced to pure gold by the " humid method," to be described hereafter. 3. Sweepings. This form embraces many impurities, earthy and metallic, and should first be thoroughly washed to remove the earthy constituents, after which the remaining metals may either be mixed with class second, or separately refined. Another, and perhaps better, method, is to fuse together the sweepings and substances hereinafter mentioned, in the following proportions: of sweepings, eight parts; chloride of sodium, four parts; impure carbonate of potassa, four parts; impure supertartrate of potassa, one part; and nitrate potassa, half part. Mix them thoroughly together, and melt in a crucible. The crucible with its contents should remain in the fire for some time, in order to secure a complete separation of the metals from extraneous matter. It is evident from the above classification that much time and labor may be saved by preserving these forms of gold separately as they accumulate in the shop. Separate lap- skins or receptacles, therefore, should be appropriated to the working of gold, one to receive scrap and unmixed plate- filings which may be reconverted into plate without refining; another to collect the solder-filings, and such impure frag- ments as. require purification. Separation of Foreign Metals from Gold.—The most troublesome ingredients which find their way into gold alloys are what are commonly called base metals, as tin, lead, zinc, iron, antimony, bismuth, etc. In attempting to separate these metals from gold, it is not a matter of indifference what re-agent is employed, inasmuch as distinct affinities exist, which may be advantageously consulted. If, for example, zinc or iron, or both of these metals are present in small quantities, any compound which yields oxygen will, by virtue of the affinity of the latter for these metals, effect their REFINING GOLD. 55 separation by converting them into oxyds ; hence, when these metals are to be got rid of, nitrate of potassa is employed. But oxygen has but a feeble affinity for tin, and when this metal is present, its separation is better effected by some compound which parts with chlorine in the act of decompo- sition ; chloride of mercury is therefore used for the purpose. When the alloy of gold contains a number of these metals at the same time, and is very coarse, sulphuret of antimony, which is a powerful and efficient re-agent, should be resorted to, unless the operator should prefer, and which is the better way, to reduce the alloy to pure gold by the " humid method." After all traces of iron or steel have been removed from the gold fragments and filings by passing a magnet repeatedly through them, the latter should be placed in a clean crucible, lined on the inside with borax, and covered either with a piece of fire-clay slab, or broken crucible. Sheet-iron has been recommended for the latter purpose, but should never be used, as, when highly heated, scales form on the surface, and are liable to drop in upon the fused metals. If the operation of roasting is likely to be protracted, an inverted crucible, with a hole in the bottom, may be securely luted to the top of the one containing the metals; the refining agents and fluxes being introduced through the opening in the upper crucible. These are then placed in the furnace on a bed of charcoal, or what is better, a mixture of charcoal and coke; the latter being built up around the crucible, and over it when covered with a second crucible; care being taken that no fragments of fuel are permitted to fall in upon the fused metals. Small portions of borax may first be added, and when the metals are fluid, the refining agents may be intro- duced in small quantities from time to time, and the roasting continued from half an hour to an hour, according to the coarseness of the alloy. The roasting may be conducted first with borax and nitre to effect partial separation, when the crucible may be removed from the fire, and the metals allowed 56 MECHANICAL DENTISTRY. to cool gradually. The crucible may then be broken, and the button of gold at the bottom removed and separated from the slag that covers it with a hammer. The button is then put into a fresh crucible and re-melted. If there is any known base metal present likely to render the gold brittle, the particular re-agent which will most readily attack it may now be used. If, however, as is generally the case, the alloy is of uncertain composition, or contains various metals having distinct affinities, the process becomes, to some extent, ex- perimental, and it may become necessary to use first one refining agent and then another, until sufficient separation is effected. Generally, it will be sufficient to use the nitrate of potassa alone, as most metals are oxydable. After roasting with nitre for half or three-fourths of an hour, adding small portions at a time, the melted metals may be poured into ingot moulds previously warmed and oiled. If, after ham- mering, annealing, and rolling the ingot, it should still be found brittle, it must be remelted, and chloride of mercury used as the refining agent. This will remove any traces of tin which may be present. If the alloy, however, is greatly impoverished, it may be more advantageously treated with sulphuret of antimony; in which case the metals should be melted in a large crucible with about twice or three times their weight of the native sulphuret, which should be added in small quantities at a time. The heat decomposes the sulphuret of antimony; the sulphur uniting with the base metals forming sulphurets, and the antimony uniting with the gold forming a leaden-colored alloy. The antimony may be parted from the gold alloy in the following manner: Place the mixture in a clean crucible, and when melted, force a current of air with a pair of bellows upon its surface; this oxydizes the antimony, which passes off in the form of vapor. The current should be mild at first, as too great a draught is apt to carry off portions of gold by a too hurried volatilization of the antimony. A current strong enough to produce visible fumes will be sufficient. REFINING GOLD. 57 When these cease, the crucible may be covered, and as the melting point of the gold rises with the escape of antimony, the fire should be urged to a stronger heat, and before pour- ing, a forcible current of air should again be thrown upon the surface of the melted metals to effectually dissipate any remaining portions of the antimony. If, after treatment with the re-agents enumerated, the alloy should be found malleable, but stiff or elastic, and dull-col- ored, it is probably due to the presence of platinum ; and any further attempts to reduce it by roasting will prove unavail- ing. It must then be subjected to the process which will be hereafter described for the separation of gold and platinum. When it is desired to reduce the alloy to pure gold, which is generally advisable whenever the gold to be refined consists of very coarse filings, fragments of plate containing large quantities of solder, linings with platinum pins attached, particles of base metals, etc., the " humid process," as it is called, should be employed. The solvents in common use for this purpose are the nitric, sulphuric, and nitro-muriatic or hydrochloric acids; but as the desired results can be more conveniently and directly obtained by the use of the latter, or hydrochloric acid, this most available method alone will be given. The following practical remarks on the process are copied from an article on the " Management of Gold,"* by Professor George Watt. " When the alloy is composed of metals differing but little in their affinities for oxygen, chlorine, etc., we resort to one of the 'wet methods.' And, in connection, we will only describe the one which we consider the most convenient and effectual for the practical dentist. It is effectual in all cases, as it always gives us pure gold. " Let us, then, suppose that our gold alloy has become con- taminated with platinum to such extent that the color and elasticity of the plate is objectionable. The alloy should be * Dental Register of the West, vol. xii. p. 251. 5 58 MECHANICAL DENTISTRY. dissolved in nitro-muriatic or hydrochloric acid, called aqua regia. The best proportions for aqua regia are three parts of hydrochloric acid to one of nitric. If the acids are at all good, four ounces of the aqua regia will be an abundance for an ounce of the alloy. The advantage of using the acids in the proportion of three to one, instead of two to one, as directed in most of the text books, is, that when the solution is completed, there is but little, if any, excess of nitric acid. If the acids be "chemically pure," four parts of the hydro- chloric to one of the nitric, produces still better results. " By this process the metals are all converted into chlorides; and, as the chloride of silver is insoluble, and has a greater specific gravity than the liquid, it is found as a grayish-white powder at the bottom of the vessel. The chlorides of the other metals, being soluble, remain in solution. By washing and pouring off, allowing the chloride of silver time to settle to the bottom, the solution may be entirely separated from it. The object is now to precipitate the gold while the others remain in solution. This precipitation may be effected by any one of several different agents, but we will mention only the proto-sulphate of iron. " This salt is the common green copperas of the shops, and, as it is always cheap and readily obtained, we need look no farther. It should be dissolved in clean rain-water, and the solution should be filtered, and allowed to settle till perfectly clear. Then it is to be added gradually to the gold solution as long as a precipitate is found, and even longer, as an ex- cess will the better insure the precipitation of all the gold. The gold thus precipitated is a brown powder, having none of the appearances of gold in its ordinary state. The solution should now be filtered, or the gold should be allowed to settle to the bottom, where it may be washed after pouring off the solution. It is better to filter than decant in this case, as, frequently, particles of the gold float on the surface, and would be lost in the washings by the latter process. " Minute traces of iron may adhere to the gold thus pre- REFINING GOLD. 59 cipitated. These can be removed by digesting the gold in dilute sulphuric acid; and, when the process is properly con- ducted, thus far, the result is pure gold, which may be melted, under carbonate of potash, in a crucible lined with borax, and reduced to the required carat." CHAPTER IV. ALLOYS OF GOLD FOR DENTAL PURPOSES. Gold, in its pure state, is rarely employed by the dentist in laboratory processes on account of its softness and flexi- bility ; it is, therefore, usually alloyed with such metals as impart to it—without practically impairing either its mallea- bility, pliancy or purity—the degree of hardness, strength and elasticity necessary to resist the wear and strain to which an artificial piece constructed from it is unavoidably exposed in the mouth. Reducing Metals.—The metals with which gold is usually combined are copper and silver. It is sometimes reduced with silver alone, many regarding the introduction of copper ^to the alloy as objectionable, as plate derived from it is sup- posed to be more readily tarnished and to communicate to the mouth a disagreeable metallic taste. This is unquestionably true, if, as is sometimes the case, the copper used is in excess; when, in addition to the effects mentioned, gold, so debased, may become a source of positive injury to the organs of the mouth, as well as to the general health. The small propor- tions of copper usually employed in forming gold plate, how- ever, are not likely to produce, in any objectionable degree, the consequences complained of, unless the fluids of the mouth are greatly perverted. If gold coin is used in the formation of plate, it may be sufficient to add silver alone, inasmuch as copper is already present; though, usually, additional quan- tities of the latter metal are added. Required Fineness of Gold Plate.—Alloys of gold to be per- manently worn in the mouth, should be of such purity as will most certainly, under all the contingencies of health and ALLOYS OF GOLD FOR DENTAL PURPOSES. 61 disease, resist any chemical changes that would tend to com- promise either the comfort or health of the patient. Evils of no inconsiderable magnitude are sometimes inflicted, either through ignorance, carelessness or cupidity, by disregard of this important requirement. If the general health of the patient remained always uniformly unimpaired, with the secretions of the mouth in their normal state, gold degraded to eighteen or even sixteen carats fine, would undergo no material changes in the mouth. But it must be remembered that, in addition to the corrosive agents introduced into the mouth from with- out, a variety of diseases, local and constitutional, effect im- portant changes in the otherwise bland and innoxious fluids contained therein, which, from being alkaline or neutral, become more or less acidulated. Indigestion, with acid eructations; gastro-enteritis; ague ; inflammatory and ty- phoid fevers; brain affections; eruptive diseases; rheumatism; gout, &c, are some of the local and constitutional disorders almost uniformly imparting to the mucous and salivary secre- tions an acid re-action. These readily attack the impover- ished gold too frequently employed as a base for artificial teeth; and as a natural sequence to such practice we find supervening, inflammation of the mucous membrane and gums, with chronic periodontitis and loosening of the teeth; apthous ulcers; gastric irritation; general nervous disorders; decay of the teeth; foetid breath; disagreeable metallic taste in the mouth, &c. Gold plate intended to be introduced into the mouth, should not, therefore, as a general thing, be of a less standard of fineness than twenty carats. It may exceed this degree of purity in some cases, but will rarely or never, un- less alloyed with platinum, admit of being used of a higher carat than the present American coin, which is 21'6 carats fine. Formulas for Gold Plate used as a Base for Artificial Den- tures.—Any of the following formulas may be Employed in the formation of gold plate to be used as a base or support for artificial dentures. The relative proportions of the alloying 62 MECHANICAL DENTISTRY. components may be varied to suit the peculiar views or neces- sities of the manipulator. The estimated carat of the appen- ded formulas are based on the fineness of the American gold pieces coined in 1837 and thereafter. GOLD PLATE EIGHTEEN CARATS FINE. Formula No. 1. Formula No. 2. 18 dwts. pure gold, 20 dwts. gold coin, 4 dwts. fine copper, 2 dwts. fine copper, 2 dwts. fine silver. 2 dwts. fine silver. GOLD PLATE NINETEEN CAEATS FINE. Formula No. 3. Formula No. 4. 19 dwts. pure gold, 20 dwts. gold coin, 3 dwts. copper, 25 grs. copper, 2 dwts. silver. 40-}- grs. silver. GOLD PLATE TWENTY CARATS FINE. Formula No. 5. Formula No. 6. 20 dwts. pure gold, 20 dwts. gold coin, 2 dwts. copper, 18 grs. copper, 2 dwts. silver. 20-f- grs. silver. GOLD PLATE TWENTY-ONE CAEATS FINE. Formula No. 7. Formula No. 8. 21 dwts. pure gold, 20 dwts. gold coin, 2 dwts. copper, 13+ grs. silver. 1 dwt. silver. Formula No. 9. 20 dwts. gold coin, 6 grs. copper. 7f grs. platinum. GOLD PLATE TWENTY-TWO CABATS FINE. Formula No. 10. 22 dwts. pure gold, 1 dwt. fine copper, 18 grs. silver, 6 grs. platinum. ALLOYS OF GOLD FOR DENTAL PURPOSES. 63 The union of platinum with gold, as in Formula No. 10, furnishes an alloy rich in gold, while it imparts to the plate derived from it a reasonable degree of stiffness and elasticity ; preserves in a good degree the characteristic color of fine gold; and does not materially impair its susceptibility of re- ceiving a high polish. The amount of gold coin given in Formula No. 9 may be reduced with platinum alone, adding to it from eight to twelve grains ; in which case, although the carat of the alloy is lowered, its absolute purity remains unaffected, and plate formed from it will better resist any changes in the mouth than gold coin itself. Formulas, for Gold Plate used for Clasps, Wire, Stays or Linings, Metallic Pivots, dfc.—Gold used in the formation of clasps, stays, &c, is improved for these purposes by the addi- tion of sufficient platinum to render it firmer and more elastic than the alloys ordinarily employed in the formation of plate as a base. The advantages of this elastic property, in its application to the purposes under consideration, are, that clasps formed from such alloys will adapt themselves more accurately to the teeth, as, when partially spread apart on being forced over the crowns, they will spring together again and accurately embrace the more contracted portions. In the form of stays or backings, additional strength being imparted, a less amount of substance will be required; the elasticity of these supports, also, will not only lessen the chances of accident to the teeth themselves in mastication and otherwise, but preserve their proper position when tem- porarily disturbed by any of the forces applied to them. The same advantages last mentioned are obtained from this prop- erty in the use of metallic pivots. t Formula No. 1. Formula No. 2. 20 dwts. pure gold, 20 dwts. coin gold, 2 dwts. fine copper, 8 grs. fine copper, 1 dwt. fine silver, 10 grs. silver, 1 dwt. platinum. 20 grs. platinum. The alloy derived from either of these formulas will be twenty carats fine. 64 MECHANICAL DENTISTRY. Gold Solders.—Solders are a class of alloys by means of which the several pieces of the same or of different metals are united to each other. They should be more fusible than the metals to be united, and should consist of such compo- nents as possess a strong affinity for the substances to be joined. They should also be as fine as the metals to which they are applied will admit of without endangering the latter. Solders of different degrees of fineness, therefore, should always be provided, from which the one most suitable for any given case may be selected. The use of solders of doubtful or unknown composition should be avoided, and hence they should be compounded either from pure gold or gold coin. The following formula taken from Prof. Harris' work on " Dental Surgery," page 664, recipe No. 3, may be used in connection with eighteen or twenty carat gold plate, and is sixteen carats fine. 6 dwts. pure gold, 2 dwts. roset copper, 1 dwt. fine silver. Recipes Nos. 1 and 2, page 663 of same work, are too coarse to be introduced into the mouth; the former being a fraction below fourteen carats, while the latter is still more objectionable, exceeding but little twelve and one-half carats. Formula No. 1 of the following recipes is a fraction over fifteen carats fine; and No. 2 furnishes a solder eighteen carats fine. Formula No 1. Formula No. 2. 6 dwts. gold coin, Gold coin, 30 parts. 30 grs. silver, Silver, 4 " 20 grs. copper, Copper, 1 " 10 grs. brass. Brass, 1 " Zinc, as will be observed by the incorporation of brass in the above formulas, is sometimes employed, principally with a ALLOYS OF GOLD FOR DENTAL PURPOSES. 65 view of rendering the alloy more fusible. Its employment under any circumstances is objected to by many on the ground that it more readily tarnishes in the mouth, is more brittle, and furnishes more favorable conditions for galvanic action. These objections only hold good when zinc is used in excess. When employed in quantities sufficient only to make the gold flow readily and evenly at a diminished heat, it is claimed that the base metal used in these alloys is chiefly consumed in the process of soldering, leaving a residuum of gold equal, or nearly so, in fineness to the plate. If such is the case, they are desirable alloys for soldering purposes, inasmuch as the importance of having the various parts of a piece of dental mechanism differ as little as possible in their affinity for the acids of the mouth is generally recognized. Method of reducing Gold to a lower or higher standard of Fineness, and of determining the Carat of any given Alloy.— In the process of compounding gold for dental purposes, the manipulator should always aim at exactness in the quantity and relative proportions 01 the reducing components, and should be able to determine precisely the purity of the metals he employs. Gold alloys are too often arbitrarily com- pounded, and used without any adequate knowledge of their quality or properties; and formulas, taken on trust, are em- ployed without any certain knowledge of the quality of the alloys they produce. That we may know certainly the quality of the gold alloys used in the laboratory without resorting to the inconvenient process of analysis or assaying, they should always be made either from pure gold or gold coin, the standard of these being definitely fixed. But as the process of procuring pure gold is somewhat tedious and troublesome, gold coin is very generally employed for the purpose. The amount of alloy necessary to reduce either pure or coin gold to any particular standard, whether higher or lower, and the method of ascer- taining the carat or fineness of any given alloy, may be readily determined by a few simple rules. The following 66 MECHANICAL DENTISTRY. practical remarks on the method are copied from an article on "Alloying Gold,"* by Professor G. Watt. " 1. To ascertain the carat of any given alloy.—The pro- portion may be expressed as follows : " As the weight of the alloyed mass is to the weight of gold it contains, so is 24 to the standard sought. Take, for example, Harris' No. 3 gold solder : Pure gold, ... 6 parts. " silver, . . . 2 " " copper, . . . 1 " Total, .... 9 . " The proportion would be expressed thus,— 9 : 6 : : 24 : 16. " From this any one 6an deduce the following " Rule.—Multiply 24 by the weight of gold in the alloyed mass, and divide the product by the weight of the mass; the quotient is the carat sought. " In the above example, 24 multiplied by 6, the quantity of gold, gives 144, which, divided by 9, the weight of the whole mass, gives 16. Hence, an alloy prepared as above, is 16 carats fine. " A.s another example, under the same rule, take Harris' No. 1 solder. 22 Carat gold, ... 48 parts. silver, . . . 16 " copper, . . . 12 " Total,.....76 " Now, as the gold used is but 22 carats fine, one-twelfth ■of it is alloy. The one twelfth of 48 is 4, which subtracted from 48, leaves 44. The statement then is : 76 : 44 : : 24 : 13-9. " This solder, therefore, falls a fraction below 14 carats. * Dental Register of the West, vol. x. p. 396. ALLOYS OF GOLD FOR DENTAL PURPOSES. 67 " 2. To reduce gold to a required carat.—The proportion may be expressed as follows : " As the required carat is to 24, so is the weight of the gold used to the weight of the alloyed mass when reduced. The weight of gold subtracted from this, gives the quantity of alloy to be added. " For example, reduce 6 ounces of pure gold to 16 carats. " The statement is expressed thus : 16 : 24:: 6 : 9. " Six subtracted from 9 leaves 3, which is the quantity of alloy to be added. From this is deducted the following " Rule.—Multiply 24 by the weight of pure gold used, and divide the product by the required caret. The quotient is the weight of the mass when reduced, from which subtract the weight of the gold used, and the remainder is the weight of alloy to be added. " As another example under the same rule, reduce 1 penny- weight of 22 caret gold to 18 carets. " As the gold is only 22 carets fine, one-twelfth of it is already alloy. The one pennyweight, therefore, contains but twenty-two grains of pure gold. The statement is, therefore, thus expressed : 18 : 24 : : 22 : 29£. " Twenty-two subtracted from 29£ leaves 7£. Therefore, each pennyweight of 22 caret gold requires 7J grains of alloy to reduce it to 18 carets. " 3. To reduce gold from a lower to a higher caret.—This may be done by adding pure gold, or a gold alloy finer than that required. The principle of the rule may be set forth in the following general expression : " As the alloy in the required caret is to the alloy in the given caret, so is the weight of the alloyed gold used, to the weight of the reduced alloy required. This principle may be practically applied by the following : Rule.—Multiply the weight of the alloyed gold used by 68 MECHANICAL DENTISTRY. the number representing the proportion of alloy in the given caret, and divide the product by that representing the pro- portion of alloy in the required caret; the quotient is the weight of the mass when reduced to the required caret by adding fine gold. " To illustrate' this, take the following example : " Reduce 1 pennyweight of 16 caret gold to 18 carets. " The numbers representing the proportions of alloy in this example are found by respectively subtracting 18 and 16 from 24. The statement is, therefore : 6:8::1:1*, from which it follows that to reduce one pennyweight of 16 caret gold to 18 carets, there must be one-third of a penny- weight of pure gold added to it. " But, suppose that, instead of pure gold, we wish to effect the change by adding 22 caret gold. The numbers, then, respectively representing the proportions of the alloy would be found by subtracting, in the above example, 16 and 18 from 22, and the statement would be 4 : 6 :: 1: 1 J. " It follows, then, that to each pennyweight of 16 caret gold, a half pennyweight of 22 caret gold must be added to bring it to 18 carets. " By the above rules we think the student will be able, in all cases, to calculate the fineness or quality of his gold, and to effect any reduction, whether ascending or descending, which he may desire." To facilitate the student in accurately compounding gold alloys from coins of that metal, the following table, giving the weight in grains, fineness, and the value of the gold coins of different nations, is given in this connection. alloys of gold for dental purposes. 69 Table of Coinage of Different Nations. NATIONS. Weight. Fineness. Value. Grains. Thous. d. c. m. Argentine Republic. Doubloon, Province of Rio de la Plata, 1828-32 418 815 14 66 1813-32 415 868 15 51 The same variation of fineness and weight in coins of the same date are to be found in the silver coinage of this republic. Austria. Ducat of Maria Theresa, 1762 53-5 965 2 26 9 Sovereign of Maria Theresa, 1778 170 917 6 713 Ducat of Leopold II., 1790 53-5 986 2 27 2 " of Francis I., 1809-34 537 983 2 27 4 Quadruple of Francis I., 1830 215-5 983 912 2 Sovereign of Francis I., 1831 1745 898 6 74 8 of Ferdinand I., 1838 1745 901 6 77 1 Half-sovereign of Ferdinand I., 1839 87 902 3 38 Ducat of Ferdinand I., 1838 53-7 985 2 27 8 Quadruple of Ferdinand I., 1810 215-5 985 914 Hungary ducat of Ferdinand I., 1839 53-7 986 2 281 Baden. Ten guilder (five guilder same quality) of Louis, Grand Duke, 1819 105-5 900 4 08 6 Bavaria. Ducat of Maximilian Joseph and Charles Theo- dore, 1764-97 53 980 2 23 7 Ducat of Maximilian Joseph II., 1800 53 984 2 24 6 " of Louis, 1832 53 5 987 2 27 4 Belgium. Forty francs, 199 895 7 67 Twenty francs in proportion, same fineness. Sovereigns same as Austrian coinage. Bolivia. Doubloon, 1827-36 416 5 870 15 58 Brazil. Moidore of Maria I. and John III., 1779 1255 914 4 94 Half-Joe of Peter II, 1833-38 221-5 915 8 72 7 The other moidores and half-joes are of the same fineness with the moidore of 1779, varying slightly in weight. Britain. The gold coins of this kingdom are of the uniform fineness of 915 5, but below the legal standard about one-thou- sandth. The par value of the pound sterling is about $4.84. Sterling gold is worth 946 cents per penny- weight. Brunswick. X. Thaler of Charles, 1745 202 898 7 812 of Charles William Ferdinand, 1805 204 896 7 87 2 " of Wm. Fred, and George, Regent, 1813-19 2045 896 7 891 of Charles. 1824-30 205 896 7 91 of William, 1831-38 205 894 7 89 3 V. Thaler of Charles, • 1748-64 102 903 3 96 6 MECHANICAL DENTISTRY. Table of Coinage of Different Nations.—Continued. Doubloons, Doubloons, Central America. Chili. Colombia. 1824-33 1819-24 1835 and seq Doubloon of eight escudos, Colombia, Bogotan Mint, 1823-36 Popayan Mint, 1823-36 " of New Granada, Bogota, 1837 Half-doubloon of Ecuador, Quito, 1836 Quarter-doubloon of Colombia, Bogota, 1823-35 of Ecuador, Quito, 1835 Eighth-doubloon of Colombia, Bogota, 1823-36 Popayan, These last coins vary in fineness from 849 to 854, and in weight from 44J to 61 J. The sixteenth-doubloons are of the same quality. Denmark. Specie ducat of Frederick V., of Christian VII., Current ducat of Christian VII., Christian d'or of Christian VII., Double Frederick d'or of Frederick VI., Egypt. Sequin fundoukli of Achmet III., of Mahmoud L, of Mustapha III., of Abdul Hamed, 1749 1795-1802 1783 1775 1813-39 1115 (1703) 1143 (1730) 1171 (1757) 1187 (1773) ofSalimlll., 1203(1789) Half-sequin fundoukli of Mahmoud II., 1233 (1818) Bedidlik, 100 piastres, of Abdul Majeed, 1255 (1839) Nusfiix. 50 piastres, " " Kairia Hastreen, 10 piastres, " " The first date given above is the year of the Hegira; the second, the Christian era. France. Louis d'or of Louis XV., " of Louis XVI., Double Louis d'or of Louis XV., of Louis XVI., Napokon, 20 francs, of Napoleon 1726-73 1786-92 1744 1786-92 1803-14 The subsequent gold coinage of France is of the uniform fineness of 899, except the twenty franc pieces of Louis Phillippe, coined in 1840-41, which are 900. Greece. Twenty drachms of Otho, Ducat of George III., Hanover. 1833 1776 Weight. Fineness. Value. Grains. Thous. d. c. m. 417 833 14 96 417 867 15 57 417 872 15 66 4168 870 ■ 15 617 4165 858 15 39 4168 870 15 617 209 844 7 59 6 104 865 3 87 4 104 844 3 78 51 865 190 51 852 1871 535 988 2 27 6 53-7 979 2 26 4 48 876 1811 103 905 4 014 2045 895 7 88 2 53 958 213 7 39 940 157 9 39 848 142 4 39 781 1312 39 786 132 39 645 108 3 39 690 115 9 18 670 519 1322 874 4 97 6 661 875 2 491 27 874 1017 124 897 4 79 116 5 900 4516 250 902 9 711 235 901 9119 99 2 899 3 841 89 900 3 45 53-5 993 2 28 8 ALLOYS OF GOLD FOR DENTAL PURPOSES. Table of Coinage of Different Nations.— ■Continued. nation. Weight. Fineness. Value. Grains. Thous. d. c. m. Hanover.—Continued. Pistole or five thaler of George III., 1803 102 896 3 93 6 " " " 1813-14 102 890 3 91 Ten thaler of George III., 1813-14 2045 890 7 83 8 William IV. and Ernst. August. 1835 and seq. 205 895 7 90 2 Hesse. Ten thaler of Frederick II., 1773-85 202 890 7 74 2 Five thaler of Frederick II., 1771-84 101 893 3 88 4 of William IX., 1788-89 1015 892 3 89 9 of William I., 1815-17 1015 894 3 90 8 HlNDOSTAN. Mohur of Bengal, 1770 190 982 8 03 5 " " 1787 191 989 8134 " " 1793 191 993 816 8 " " 1818 204-7 917 8 08 4 " of Madras, 1818 180 917 7109 " of Bombay, 1818 179 920 709 2 Half-mohur of Bengal, 1787 95 984 4 02 6 Star pagoda of Madras, 525 800 180 9 Pondicherry pagoda of Pondicherry, 52 5 708 1601 Porto Novo pagoda of Portuguese Companj t 52 5 740 167 3 Mecklenburg Schwerin. Ten thaler of Frederick Francis, 1831 204 5 896 7 891 Mexico. Dublooon of Mexico, Augustin, Emperor, 1822 416 5 864 15 49 8 " " Mexican Republic, 1824-30 416 5 865 15 516 Other doubloons minted at Mexico weigh 417 grains, and are from 867 to 869 thousandths fine. The doubloon of Guanaxuato varies from 860 to 867 in fineness. Doubloon of Durango. 417 868 15 58 8 i> u 417 865 15 53 4 u h 1833-36 4175 872 15 67 9 '• of Guadalaxara, 416 865 15 49 7 Milan. Zecchino, or Sequin, of Maria Theresa and Joseph II., 1770-84 53 5 990 2 281 Doppia, or Pistole, of Joseph II, 1783 97 5 908 3 813 Forty lire of Napoleon, 1805-14 199 899 7 70 6 Sovereign of Francis I., 1831 174 5 898 6 74 8 " of Ferdinand I., 1838 1745 901 6 771 Half-sovereign, 1839 87 902 3 38 Naples and Sicily. Six ducat, of Ferdinand IV., 1783 135 893 519 2 Onzia of Sicily of Charles, 1751 68 959 2 516 Onzia of Ferdinand I., 1818 58 995 2 48 5 Twenty lire of Joachim Napoleon, 1813 99 900 3 848 Netherlands. Ducat, 1770-1810 53 5 980 2 25 8 " of William I., 1833-39 53 7 981 2 26 9 Ten guilders of William I., 1816-39 103 5 899 4 00 7 TO MECHANICAL DENTISTRY. Table of Coinage of Different Nations.— Continued. Weight. Fineness. Value. Persia. Toman of Fatha AH Shah. Kajar, 1230-40 (1814-24 " of Mohammed Shah, Shakinshah, 1255 l>o!*, Half-toman of Mohammed Shah, 1252 1637, Poland. Ducat of Stanislaus Augustus, Portugal. Moidore of Peter II., of John V., Half-joe, of Maria I. and Peter III., " of Maria I., " of John VI., Joannese of John V., Crown of Maria II., Prussia. Frederick d'or of Frederick II., " of Frederick William II., of Frederick Wilhelm III., Double Frederick d'or of Fred. Wilhelm III., Ducat of Frederick William II., 1752-82 1795-1*0 1799-1612 1800-11 1«1 1787 712 1791 53 5 16891 165 1705 165 1714-26 165 1727-77 217 1778-85 220 1787-1804 221 1822-21 221 173U 439 1«8 148 102 102 102 2u5 2u5 53 5 991 965 968 984 908 \fj.a 913 914 913 914 909 912 912 901 «97 901 903 979 3042 2 23 3 1121 2 26 6 6 45 2 6 59 4 6 48 8 8 02 8 65 669 9 8 65 2 17 24 2 5 813 3 95 8 3 94 3 95 8 7 92 3 7 97 2 2 25 6 Rome. Sequin of Pius VI., Doppia of Pius VI. •' of Pius VII. Gold scudo of Republic, Ten scudi of Gregory XVI. 84-5 1799 910 996 906 901 833 2 25 2 3276 3 27 9 646 1830 267 5 ' 9lX) i io 36 6 Russia. Imperial of Elizabeth, 1750 The gold coins of Russia, though irregular in weight, are of the same standard fineness during the reigns oi Elizabeth and Catharine II. Ducat of Paul I., 1798 Three roubles of Nicholas, 1838 Half-imperial of Nicholas, 1839 Sardinia. Pistole of Victor Amadeus, &c, Carlino (island) of Victor Amadeus, &c., Marengo of Republic, Eighty lire, Genovine of Ligurian republic (Genoa) Saxony. Double August d'or of Fred. August. III., Double Anton d'or of Anthony, Ducat of Anthony, 1773 1800 1798 66 60-5 100-5 148 247 98 398 388 1784-1817 204 5 1826 205 1830-36 205 1830 53 7 91c 969 917 917 905 890 898 898 908 896 898 900 979 9 97 2 75 4 2 389 3 96 9 5 76 8 9 46 7 3 79 15 39 2 1517 2 7 891 7 92> 7 946 2 2o4 ALLOYS OF GOLD FOR DENTAL PURPOSE: i o Table of Coinage of Different Nations.—Continued. Weight. : Fineness.' Value. Spain. Cob doubloon of Philip V., American, Doubloon of Ferdinand VI., American, of Charles III., American, " of Charles III., Spanish, " of Charles IV. and Ferdinand VII. American, 1733-44 416 1751 416 1772-84 416 1760-88 416 1789-18211 416-5 Pistole of Philip V., Spanish, of Charles III . American, 1745 103 1774-82 103 " of Ferdinand VII., American, 1613-24 104 Escudoof Charles III. Spanish, 17>0-66 52 " of Charles IV.. 1769-18"S 52 " of Ferdinand VII . American, 1S09-20 52 Half-doubloon of Charles III., Spanish. 176'">-82 206 of Charts IV., American. 1769-1606 208 " of Ferdinand VII., Spanish, 181'j-24 208 Sweden. Ducat of Gustavus III. and Gustavus IV., 1777-1800 53 " of Charles John XIV., 1838 54 Switzerland. Pistole of Berne, 1796 116 " of Basle, 1795 118 '• of Soleure, 1798 116 " of Helvetian Republic, 1800 116 Ducat of Berne, 1794 525 of Basle, 53 Tunis. Half-sequin of Abdul Hamed, 1773 19 Turkey. Sequin fondonk of Selim III., zermahboub of Selim III., Ohikilik of Mahmoud II., Twenty piasters, of Mahmoud II., Yirmilik, 20 piasters of Abdul Medjid, Tuscany. Ruspone of Francis III. to Leopold III. of Louis I. and Cha: '?s I., " of Leopold II., Sequin of Leopold, of Leopold II., Eagle, Ducat of Charles. United States. Wurtenburg. 1789 1789 1822-24 1827 1840 173S-1800 160 1801-07 161 1824-34J 161 1765-79 1824-34 1792-1834 1834-37 1837 and seq 1790-1818 52-5 36 25 27-5 245 53 535 270 53 Grains. Thous. d. c. m * Varies from 893 to SdS. t Varies from 883 to 893, the oldest pieces being the best. 6 CHAPTER V. METHOD OF CONVERTING GOLD ALLOYS INTO THE REQUIRED FORMS FOR DENTAL PURPOSES. Manner of procuring an ingot.—The gold, with its alloy- ing constituents, is put into a clean crucible, lined on the inside with borax, and placed in the furnace. When the con- tained metals are perfectly fused, the crucible should be removed from the fire with a pair of tongs, and the contents poured quickly but carefully into the ingot molds; the latter being placed conveniently near the mouth of the furnace, as the molten metals soon become chilled on exposure to the open air. Before pouring, the molds, if made of iron, should be moderately heated and oiled, or coated with lamp-smoke by holding their inner surfaces over the flame of an oil lamp or gas jet. Ingot molds are constructed of various substances, but those in most common Fig. 13. use, and at the same time most convenient, are formed of iron; and for gold, are generally about two inches square, and from one-eighth to one- sixth of an inch thick. (Fig. 13.) They should be slightly concave on their inner surfaces to compensate for the greater shrinkage of the gold in the centre than at the mar- gins of the ingot. CONVERTING GOLD ALLOYS INTO REQUIRED FORMS. 75 Soapstone is sometimes employed for the same purpose, and is preferred by many. It should also be warmed and oiled before pouring the metals. Molds made from charcoal are also sometimes used, but as they require to be frequently renewed, are not generally employed. Molds are formed from this latter substance by selecting two pieces with even surfaces, or dividing a single piece with a saw, when either the required size and shape of the mold may be cut out in one half, or a strip of sheet iron, a little broader than the required thickness of the ingot, being bent into proper form, is placed between, and the edges partially imbedded in, the two pieces of charcoal and the latter secured by binding them together with wire. Molds made from this material do not require to be either heated or oiled. It not unfrequently happens that, at the first pouring, the metals arrange themselves in the ingot in accordance with the density of the several components; those of greater specific gravity passing to the bottom, and the lighter metals remaining above. Whenever this occurs, the ingot must be broken into pieces and re-melted; this should be repeated, if necessary, until the alloy assumes a perfectly homogeneous appearance. It should then be annealed in hot ashes, which softens the gold and removes the adhering grease. Forging.—Before laminating the ingot, it should be reduced somewhat in thickness by placing it on an evenfaced anvil or other equally smooth and resistant surface, and subjecting it to repeated blows with a tolerably heavy hammer. It should be frequently annealed, and the process of forging continued, alternately hammering and annealing, until the ingot is re- duced one-half or more in thickness. Laminating or rolling.—The reduced ingot, well annealed, is next laminated or spread out into a sheet of greater or less thinness by passing it repeatedly between two strong, highly polished, cylindrical steel rollers. The mills used for the purpose are variously constructed; the plainest forms (Fig. 14) being very simple in their mechanism, while others, or 76 MECHANICAL DENTISTRY. Fig. 14. Fig. 15. geared mills, are more complicated, and are constructed with a view to a greater augmentation of power, and precision, and Flo 16 certainty of action. The latter, (Figs. 15-16,) if of approved pattern, mater- ials and manufacture, are, upon the whole, more econ- omical and reliable than the cheaper varieties. The rollers, for the purposes of the dentist, should be from three to four inches in length. In laminating, the roll- ers should first be adjusted equi-distant at both ends, and this uniformity, as they are approximated from time to time, should be preserved throughout. At every passage of the gold bar between the rol- lers, the distance between the latter should be dimin- ished; care being taken that the approximation be not CONVERTING GOLD ALLOYS INTO REQUIRED FORMS. 77 sufficient to clog or impede the free action of the mills. The gold, which in time becomes hard and brittle and liable to crack in the mills, should be frequently and well annealed by bringing it to a full red heat; this restores the pliancy of the gold and facilitates the operation in the press. When the ingot has been extended in one direction as far as may be desired, it should always be re-annealed before turning it in the mills; a neglect of this precaution will ser- iously interfere with the working of the gold by twisting or doubling the plate upon itself; and in some instances, provi- ded the gold has not been well annealed throughout the operation, or is in any considerable degree unmalleable, the plate will be torn across and rendered unfit for use. A thin or retreating edge may be given to the plate at any desired point or points by passing such portions part way between the rollers and withdrawing; repeating this, with the rollers brought a little nearer to each other every time the plate is introduced between them, and decreasing the distance the plate passes each time until it is reduced to as thin an edge as may be desired. The degree of attenuation obtained by rolling is determined by what is called a gauge plate, (Fig. 17.) This instrument is usually circular or oblong in form, and is marked at intervals on its edge by cross-cut grooves or fis- sures, which successively diminish in size and are indexed by num- bers ranging from 6 to 40. The size of the grooves diminish with the ascending numbers. During the operation of rolling, the plate should be tested, from time to time, by the gauge to deter- mine when it has undergone sufiicient attenuation. Thickness of Gold Plate required as a Base for Artificial Dentures.—In prescribing the thickness of plate proper for 78 MECHANICAL DENTISTRY. the purpose indicated, no estimate can be given that will apply to all cases, as certain conditions of the mouth, to be mentioned hereafter, will suggest some modifications in this respect. Usually, however, plate for entire upper sets should correspond in thickness with number 26 of the guage plate; for the under jaw, number 24 may be used; while for partial upper pieces, an intermediate number may be chosen, unless atmospheric-pressure plates are used, when the number recommended for full upper sets may be employed. Thickness of Plate for Clasps, Stays, <§c.—Plate for these purposes should correspond with from 20 to 22 of the gauge; a less amount of substance, as before stated, being required when the alloy has incorporated with it a small proportion of platinum. Reduction of Gold Solders into Proper Form for Use.—The method of converting gold solders into the form of plate, does not differ from that already described in the manufacture of plate as a base, except that when zinc or brass is used, the latter should be added after the other constituents are com- pletely fused, and then instantly poured to prevent undue wasting of the base metals by a too protracted heat. The solder should be reduced to plate somewhat thinner than that used for upper sets, 28 of the gauge-plate. It is customary sometimes to roll the solder into very thin rib- bons, but this is objectionable for the reason that a greater amount of the alloying metals being exposed in a given sur- face to the action of the heat in soldering, are burnt out or oxydated, which interrupts the flow and weakens the attach- ment between the solder and plate. Method of Obtaining Gold Wire.—To convert gold or its alloys into the form of wire, the operator should be provided with a draw-plate, a vice, and a pair of hand-pincers. A draw-plate (Fig. 18) is an oblong piece of steel pierced with a regular gradation of holes, or a series of progressively dimin- ishing apertures, through which the gold bar, reduced to a rod, is forced and made to assume the form and dimensions CONVERTING GOLD ALLOYS INTO REQUIRED FORMS. 79 of the hole through which it is last drawn. The holes are formed with a steel punch, and are enlarged on the' side where the wire enters and diminish with a gradual taper to Fig. 18. the other side. A draw-bench is sometimes employed in ex- tending the wire, the power being applied by a toothed wheel, pinion, and rack work, and is moved by the hands of one or two persons. For the purposes of the dentist, however, it will be sufficient to fix the draw-plate securely between the jaws of a bench-vice, and, by seizing hold of one end of the gold rod with a strong pair of clamps or hand-pincers serrated or cut like a file on the inside of the jaws, the wire may be drawn steadily through the plate, passing from the larger to the smaller holes until a wire of the required size is obtained. In drawing the wire, the motion should be steady and uni- form, for if drawn interruptedly or by jerks, the wire will be marked by corresponding inequalities. The gold rod should also be annealed from time to time, and the holes kept well greased or waxed. The process described above will answer equally well in reducing any of the ductile metals to wire, as silver, copper, platinum, &c, so that any further description of the method, in connection with these metals, will be unnecessary. 80 MECHANICAL DENTISTRY. Method of Constructing Spiral Springs.—Inasmuch as spiral springs have been, to a great extent, superseded by more approved agencies employed in the retention of artificial teeth in the mouth, and as all the principal dental furnishing houses are supplied with these appliances already prepared for use, the author does not deem it necessary to enter into a description of the various apparata used in making them. The following simple contrivance will meet the limited re- quirements of those who are obliged or prefer to manufacture their own springs. The wire, obtained as already described, is held between two blocks of wood fastened between the jaws of a bench-vice. By means of a small hand-vice, one end of the wire is clamped to a uniformly cylindrical and well-tem- pered steel rod or wire four or six inches long, and about the size of a small knitting needle, and which being made to re- volve while resting on the blocks of wood, the wire is wound firmly and compactly around it producing a uniform coil. CHAPTER VI. SILVER. General Properties of Silver.—Pure silver, when planished, is the brightest of the metals. Fused, or in the form of ingot, its specific gravity is 10'47; but when hammered or condensed in the coining press, its density is increased, and its specific gravity becomes 10*6. It fuses at an extreme red heat, generally estimated at 1*873° Fah. It is remarkably laminable and ductile; yielding leaves not more than T o o^uo" of an inch thick, and wire 400 feet of which may be drawn weighing but a single grain. It exceeds gold in tenacity or cohesion, but is inferior to platinum in this respect. A silver wire *078 of an inch in diameter will sustain a weight of 187'13 ponnds. Fine silver is unaffected by moisture or pure atmospheric air, but is readily tarnished with a film of brown sulphuret by exposure to sulphuretted hydrogen. The sulphuret of silver thus formed may be easily removed by rubbing the metal with a solution of cameleon mineral, pre- pared by calcining equal parts of black or peroxide of manganese and nitre. Unlike gold and platinum, it is readily soluble in nitric acid; this and sulphuric acid being the only simple ones that dissolve it. Silver becomes very brilliant when heated; boils and vaporizes above its fusing point; and when cooled slowly, its surface presents a crystal- line appearance. Alloys of Silver.—Silver combines readily with most metals, forming compounds of variable degrees of malleability, ductility, density, &c. . Tin, zinc, antimony, lead, bismuth and arsenic, render it brittle. A very minute quantity of tin is fatal to the 82 MECHANICAL DENTISTRY. ductility of silver. Silver does not easily combine with iron, although the two metals may be united by fusion. Gold, copper, platinum, iridium, steel, manganese, and mercury, also form alloys with silver. An alloy of nine parts of silver and one of copper is the Government standard of the United States coinage since 1837. To this, three cent pieces form an exception; these being composed of three parts silver and two of copper. The coins of silver having a greater average fineness than those of our own country, are Brazil, Britain, Chili, France, Greece, Hindostan, Persia, Portugal, Rome, and Tuscany. A common impression prevails that the Mexican silver coin contains more than an average percentage of silver, and is therefore sought after on account of its supposed purity. This is true of some pieces coined at different periods, but the average fineness of the Mexican, as well as Spanish coins, is exceeded by those of the United States mints. Reduction of Silver to the Required Forms for Dental Pur- poses.—Owing to the very soft and flexible nature of silver in its pure state, it is usual, when converting it into plate or other forms for use, to employ an alloy of the metal. Hence silver coins, which are made harder by the copper they con- tain, are generally selected for the purpose. The employment of silver, thus debased, as a base for dental substitutes is re- garded by many as unsafe and injudicious. Although the influences of an alloy so readily acted upon as this by the various agents which affect it chemically, cannot always be certainly predicted in every case, yet no reasonable doubt can be entertained but that, under the favoring conditions which usually exist in the mouth, the evils accruing, directly and indirectly, to the organs of the mouth, and through them to the general system, are positive and undoubted. If used at all, therefore, it should be alloyed with the least practicable amount of copper, or what is better, pure silver should be reduced with platinum alone, in sufficient quantities to impart to the plate an adequate degree of strength and elasticity. SILVER. 83 The tendency of silver to tarnish in the mouth when alloyed with copper, may be diminished by boiling the finished piece in a solution of cream of tartar and chloride of soda, or com- mon salt, or by scrubbing it with aqua ammonia, which removes the superficial particles of copper, and exposes a surface of fine silver. When platinum is introduced as the sole alloying component, the purity of the silver is not only preserved, but the alloy is less easily acted on chemically, while the plate derived from it is rendered sufficiently inflexible and elastic. From three to five grains of platinum may be added to one pennyweight of pure silver. On account of the strong affinity of sulphur for silver, the fuel most proper to be used in melting it is charcoal. The various processes employed in the conversion of silver into the required forms for use are similar to those described for gold, and need not be recapitulated. Formulas for Silver Solders.—Silver solders are usually composed of silver, copper, and zinc, in variable proportions. Alloys formed from the following formulas are such as are generally employed in soldering silver plate derived from the coins of that metal. Three-cent pieces, composed of two parts silver and one of copper, may also be used for the same purpose. Formula No. 1. Formula No. 2. Silver 66 parts. Silver 6 parts. Copper 30 " Copper 2 " Zinc 10 " Brass 1 " When the material to be united is composed of pure silver and platinum, silver coin alloyed with one-tenth zinc may be used as a solder. In compounding silver solders, the silver and copper should be first melted, and the zinc or brass afterwards added, when they should be quickly poured to prevent undue waste, by oxydation, of the more fusible component. The ingot when cold, should be rolled into plate a little thicker than that recommended for gold solder. CHAPTER VII. PLATINUM AND THE PLATINOID METALS. Platinum is a grayish-white metal, resembling in some measure polished steel. It is harder than silver, and has a density greater than any other known metal, its specific gravity being 21*25. It remains unaltered in the highest heat of a smith's forge, and can only be fused by means of the oxy- hydrogen blow-pipe and galvanism. A white heat does not tarnish it, nor is it in any way affected by exposure either in the air or water. It is insoluble in any of the simple acids; nitro-muriatic acid being the only one that dissolves it. It is sufficiently malleable to be hammered into leaves so thin as to be blown about by the breath. It may be drawn into wire the two-thousandths of an inch in diameter, and a still greater attenuation may be obtained by coating the wire with silver, drawing it out, and dissolving off the latter metal. Platinum is very soft and flexible, and when rolled into thin sheets, say 28 or 30 of the gauge-plate, and well annealed at a strong white heat for eight or ten minutes, it may be readily forced into all the inequalities of a zinc die without producing any appreciable change in the face of the latter. The following interesting and practical observations on the method of melting and welding platinum scraps, are taken from a recent dental publication,* by E. A. L. Roberts. By this process, the operator will be enabled to re-convert his waste scraps of platinum into convenient forms for use, and which he could not otherwise avail himself of on account of the infusible nature of this metal in its uncombined state. * Dental Instructor, vol. i. p. 10. PLATINUM and the platinoid metals. 85 "Platinum used by dentists should be soft, tough, and without flaws. These qualities can be attained only by thorough melting and welding. The welding must be done at a white heat. When the surface is cool enough to be visible, the metal is too cool to be welded, and every blow is injurious, because it has a tendency to shatter and shake it to pieces. The necessary delicacy of this process, and the uncertainty of success, has led some writers to declare that platinum is incapable of being welded. The platinum must be perfectly clean, and must be heated in a muffle. When welded, the metal should be handled with tongs plated with platinum, and hammered with a clean hammer on a clean anvil, both of which should be as hot as possible, without drawing the temper of the steel. The hammer used in weld- ing should weigh about a pound, to prevent drawing the metal; but when welded, the metal may be forged with a heavier hammer. " The scraps or sponge should be condensed in a square mold, very compactly, two pieces of which, weighing from ten to twenty ounces, may be put into a muffle together. When the heat becomes so great, that on opening the door the metal becomes invisible, bring one of the pieces in the tongs quickly to the anvil, give it three or four quick, sharp blows, in rapid succession. Return the piece to the muffle, and proceed with the other piece in like manner, and thus alter- nately till both are thoroughly welded. " We use one of our improved tooth-furnaces of the largest size, fourteen by ten inches, inside measure. " Platinum should never be thrown into water while hot, as that tends to make it crystallize. It should be thoroughly hammered, as it makes it tough and fibrous. The following process gives the best results in melting this metal. Con- dense the scraps, sponge or filings in an iron mold. Lay the condensed mass on a concave fire-brick, and heat it to white- ness. Take the brick from the muffle, and place it in a sheet- iron pan, coated with plaster and asbestos. The pan should 86 MECHANICAL DENTISTRY. be deep enough and broad enough to catch all the globules and other loose particles of the metal. Place it under the jet of the oxy-hydrogen blowpipe, in the following manner: " The pan is provided with a handle, opposite to which is a ring, which is to be attached to an iron hook and rod, sus- pended from the ceiling by a slip of India rubber, which enables the operator to hold the pan conveniently at any dis- tance from the jet of the burning gases. The hydrogen is first lighted, and gives a powerful flame, but as the oxygen combines with it, the flame subsides into an intense focus of heat, in which the metal is soon brought to a state of fusion. Begin at one end and melt along toward the other, till the whole is fused in one mass. The platinum in this condition, when cool, is quite crystallized and sonorous. It breaks very easily, like spelter-zinc. Heat it very hot and forge it. A continuation of this process renders it soft, tough, and fibrous. When reduced to the width desired, and to the thickness of one-fourth of an inch, it should be made very hot, and passed instantly through the rollers." Platinum, in mechanical practice, is chiefly employed as a base for continuous gum work; it is also used as a coloring ingredient of porcelain, and for pins in the manufacture of mineral teeth; and, to a limited extent, in some of the minor operations of the shop. Pure gold is the only proper solder for this metal. Alloys of Platinum.—Platinum unites with most of the base metals, forming alloys of variable degrees of hardness, elasticity, brittleness, color, fusibility, &c, but their practical value to the dentist is not sufficient to justify a separate description of their properties. Alloyed with gold it forms a straw-colored' alloy, the shade depending on the quantity of gold added. Silver hardens it, the resulting alloy being unaffected by sulphur. Platinoid Metals.—The platinoid metals, palladium, iridium, osmium, rhodium, and ruthenium, are native contaminations, PLATINUM AND THE PLATINOID METALS. 87 the alloys of these metals having a close general resemblance to platinum. Among the platinoid metals, palladium is the only one that has been used for dental purposes, and that only to a limited extent. It is of a steel-gray color, and when planished, is a brilliant steel-white metal not liable to tarnish in the air. Though closely resembling platinum, it may be readily dis- tinguished from the latter metal by the following tests : 1. It has little more than one-half the density of platinum. 2. If a piece of it is heated to redness, it assumes a bronze-blue shade of greater or less intensity, as it is cooled more or less slowly; but if it is suddenly chilled by immersing it in cold water, it instantly resumes its original lustre. 3. When a drop of the tincture of iodine is let fall upon its surface and evaporated over the flame of a lamp, a black spot remains, which does not occur With platinum. Palladium melts at about 9500° Fah., and does not oxydize at a white heat. Its specific gravity is from 11*8 to 12*14. CHAPTER VIII. ALUMINIUM. Aluminium is the metallic basis of alumina, the latter being the characteristic ingredient of common clay. It is only within the past few years that the attention of chemists has been directed to the production of this remarkable metal with a view to its general introduction into commerce and the arts. Prior to the researches of M. Deville, who, under the patron- age of the present Emperor of the French, commenced his researches in 1854 for the production of this metal on a large scale, the small quantities produced and the corresponding exorbitant prices it commanded, rendered it entirely unavail- able for other purposes than merely scientific experiment. The improvements in the method of obtaining it, however, which have been recently introduced, cannot fail, by render- ing its production more economical, to supply it in much larger quantities and at a corresponding reduction in the cost of the metal. The following account of the properties of this metal is taken from a paper read before the Society of Arts, London, by its Secretary, P. Le Neve Foster. It embodies the most authentic and complete description of the properties of alu- minium that has yet been published. " One of the most striking properties of aluminium is its extreme lightness, its specific gravity being 2*6, nearly that of glass, whilst that of platinum is 21*5, gold 19*5, silver 10*5, copper 8*96, zinc 7*2, tin 7'3. " The metal is malleable, ductile, almost without limit; it can be reduced to very thin sheets, or drawn into very fine threads. Its tenacity, though superior to that of silver, is ALUMINIUM. 89 less than that of copper; but no very accurate experiments have been made in this respect. " When pure, it is about as hard as silver. Its elasticity is not great. It files readily, and is said not to injure the file. It conducts electricity with great facility, so that it may be considered as one of the best conductors known, almost equal in this respect to silver, and more than eight times a better conductor than iron. It melts at a temperature a little above that of zinc, between zinc and silver. In its chemical qualities it would seem to take an intermediate rank between what are termed the noble metals and the common metals, being, as Deville states, one of the most unalterable of metals. "It might be imagined that it would as readily re-assume its oxygen as it parted with it with difficulty when in its state of oxyd. This, however, is not the case; it appears to be as indifferent to oxygen as either platinum or gold. In air and in oxygen it undergoes no sensible alteration, and it even resists it at the highest temperature which Deville could produce in a cupelling furnace, a temperature higher than that employed in assaying gold. Water has no action, ac- cording to Deville, on aluminium, neither at its ordinary temperature, nor when boiling, nor even upon the metal at a low red heat, near its melting point. According to Professor Crace Calvert, this statement must be received with some degree of caution, as in experiments he has made he considers that oxydation does take place slowly when the metal is im- mersed in water for any considerable length of time. It is not affected by sulphur or sulphuretted hydrogen, like silver, nor is acted upon to any considerable degree by any of the oxy-acids in the cold; nitric acid, whether strong or weak, at its ordinary temperature, in no way affects it, though when boiling it acts upon it slowly. Small grains of aluminium, plunged in sulphuric acid for three months, remained appar- ently unaltered. The vegetable acids, such as acetic, oxalic, and tartaric acids, have scarcely any effect on it at all. The 7 90 MECHANICAL DENTISTRY. true solvent of the metal is hydrochloric acid, which attacks it rapidly. It appears to resemble tin when brought into contact with hydrochloric acid and the chlorides. Its absolute harmlessness permits of its being employed in a vast number of cases where the use of tin would not be desirable on account of the extreme facility with which that metal is dissolved in the organic acids. " Figuier, in his scientific Year Book for 1858, just pub- lished, states that the caustic alkalies, potash, and soda, and even ammonia, dissolve aluminium sensibly. He also states that common salt and acetic acid, (vinegar,) especially when mixed, attack and dissolve aluminium. He adds, that the mixture of salt and vinegar for seasoning a salad, made in a spoon of aluminium, feebly but inevitably attacks it. " All these points, however, deserve to be inquired into, as there seems some discrepancy between different writers on them." Alloys.—" Aluminium, like iron, does not unite with mercury, and scarcely at all with lead. It, however, forms a variety of alloys with other metals. It can be alloyed with iron, and when aluminium becomes cheaper, it will be curious to see what effect mixtures of this metal with iron will have upon its quality, whether for good or for evil. It seems to unite readily with zinc, and these have been found to give the best promise as solders for aluminium ; but, unfortunately, when melted, neither of them are sufficiently liquid, and does not run readily. The joints will not bear a blow. A variety of alloys with nickel have been made, and that consisting of 100 parts of aluminium and 3 of nickel, is found to work readily, and to have gained hardness and rigidity, as compared with the pure metal. The alloys, however, with copper are the most striking; they are light and very hard, and capable of a fine polish. In the same degree that copper adds to the hardness of aluminium, so does the latter, when used in small quantities, give hardness to copper, without, however, injuring its malleability. It renders it susceptible ALUMINIUM. 91 of a fine polish, and, according as the proportions are varied, the color of the alloy becomes deep or pale gold. Alloys of copper with five and ten per cent, of aluminium, resemble gold perhaps more than any other metallic alloy hitherto employed. They do not tarnish sensibly by exposure to the air. Aluminium can be deposited by the battery, and by the same agent it can be gilt or silvered." Some attempts have been made to render aluminium available as a base for atificial teeth, but with only partial success. When in the form of plate, no suitable solder has yet been discovered by which the several parts of a dental appliance may be securely united to each other; and experi- ments in casting this metal have practically failed, owing to its extreme lightness and consequent want of fluidity. More experience in its proper management, and a further acquaintance with its capabilities in yet unknown forms of combination with other metals, may ultimately demonstrate its applicability to dental purposes.* * Some account of the more recent successes obtained in the use of aluminium for dental purposes will be found under the head of " Aluminium Base," to which the reader is referred. CHAPTER IX. COPPER, ZINC, LEAD, TIN, ANTIMONY, AND BISMUTH. COPPER. Copper is one of the metals most anciently known; is of a brownish-red color, with a tinge of yellow; has a faint but nauseous and disagreeable taste, and imparts, when exposed to friction, a smell somewhat similar to its taste. Its specific gravity ranges from 8*8 to 8*9. It is both malleable and and ductile, but excels in the former property, finer leaves being obtained from it than wire. It is inferior to iron in tenacity, but surpasses gold, silver and platinum in this respect. Copper melts at 1996° Fahrenheit. Alloys of Copper.—Copper unites readily with most metals, forming alloys of great practical value in the arts, but which have but a limited application in dental laboratory processes. Many of these alloys are curious and instructive as illustrating the singular and unaccountable influence of al- loying upon the distinctive properties of the component metals. Copper and tin, for example—the former of which is highly ductile, and the latter equally malleable—when combined in the proportion to form speculum metal, (9 C—1 T) forms an alloy distinguished for its extreme brittleness, with a surface hardness almost equal to steel. By increasing the quantity of tin until the compound assumes the pro- portions constituting gun metal, (C 2—T 1) the alloy, though neither malleable or ductile, becomes eminently tough and rigid. Other prominent examples might be given, showing how completely this combining influence defies all calculations in regard to ultimate results. The following summary em- COPPER. 93 braces the names and composition of the more familiar alloys of copper, omitting, as unnecessary in this connection, a description of their individual properties. Alloys of Copper with Zinc.—Brass is an alloy of uncertain and variable composition, consisting usually, however, of 2 to 5 parts of copper and one of zinc. Brass melts at 1869° Farh. Princes metal, and its allied compounds, Pinchbeck, Similor, and Manheim gold, consists of nearly equal parts of copper and zinc. Mosaic gold consists of 100 parts of copper and from 52 to 55 of zinc. Dutch gold, from which foil of that name was formerly obtained, is formed of 11 parts of copper with 2 of zinc. Bath metal is composed of 32 parts of brass and 9 of zinc. Brass solder consists of two parts of brass and one of zinc, to which a little tin is occasionally added. Alloys of Copper with Tin.—Bell metal usually consits of 100 parts of copper with from 60 to 63 parts of tin. Cannon metal is compounded of 90 parts of copper with 10 of tin. Cymbals and gongs contain 100 parts of copper and 25 of tin. Speculum metal consists of 2 parts of copper and 1 of tin. Copper and arsenic form a white-colored alloy, and in the proportion of 9 parts copper and one of arsenic, is white, slightly ductile, and is denser and more fusible than copper. Genuine German silver is composed of copper 40*4; nickel 31*6; zinc 25*4; iron 2*6; but the proportions of the metals of this alloy differ according to the various uses to which this compound is applied. Chinese packfong consists of 5 parts of copper alloyed with 7 parts of nickel and 7 parts of zinc. A very useful alloy, employed in making plummer blocks, bushes, and steps for the steel and iron gudgeons and pivots of machinery to run in, is said to consist of 90 parts of cop- per, 5 of zinc, and 5 of antimony. 94 MECHANICAL DENTISTRY. ZINC. Zinc is a bluish-white metal, possessing considerable lustre when broken across. The commercial variety is always im- pure, containing traces of iron, lead, cadmium, arsenic, carbon, etc. It does not easily tarnish in dry air, but soon becomes dull on exposure to moisture. In the condition in which it ordinarily occurs it is a brittle metal, but may be rendered malleable by annealing it at certain temperatures. This change in its condition is effected by subjecting it to a heat of from 220° to 300°, at which temperature it may be rolled into sheets, and retain its malleability when cold. The best annealing temperature for zinc is about 245°. A knowledge of this fact will enable the operator to avail himself of the advantages of this property by annealing his zinc die, by which its liability to crack or part under the hammer is diminished. The specific gravity of zinc varies from 6*9 to 7'2. It melts at about 773°, and when heated much above this point with contact of air, it burns with a brilliant greenish-white flame, while woolly-looking flocculi rise from the vessel in which it is being heated and float in the air. Zinc has been long and almost exclusively employed in the formation of dies used in swaging metallic plates employed in mounting artificial teeth, and experience has very justly accorded to it undisputed pre-eminence above all other un- alloyed metals for the purpose. A more particular account of its peculiar fitness for dental purposes will be given under the head of Metallic Dies and Counter-Dies. fLEAD. Lead has a grayish-blue color, with a bright metallic lustre when melted or newly cut, but it soon becomes tar- nished and dull-colored when exposed to the air. The specific gravity of commercial lead, which is usually contaminated TIN. 95 with other metals, is 11*352. It fuses at 612°. Exposed to a high heat, it absorbs oxygen rapidly, forming on its surface a gray film of protoxide and metallic lead. It is both malle- able and ductile, but soft and perfectly inelastic. Lead, either in its pure state or when alloyed with certain other metals, serves important purposes in the laboratory. In its simple or uncombined state it is useful only in forming counter-dies. Alloyed with antimony in the proportion of from \ to \ of the latter, with the addition sometimes of very small portions of copper, tin, and bismuth, it forms different grades of type metal, which is harder than lead, and very brittle, and is sometimes used for dies ; and sometimes, though very rarely, for counter-dies. When used as a counter to a zinc die, it is improved for the purpose by adding to it an equal quantity of lead; it may also be used in the form of a die in connection with a lead counter after rough stamping with zinc. The alloy known as Rose's fusible metal is composed of 2 parts of bismuth, 1 of lead, and 1 of tin, and melts at about 200°. A still more fusible alloy is composed of lead 3 parts, tin 2 parts, and bismuth 5 parts, which fuses at 197°. There are other alloys of lead, to be mentioned hereafter, melting at from 200° to 440°, which may be advantageously employed in forming dies to be used after zinc, where the latter, from its greater shrinkage, fails to bring the plate into accurate adaptation to the mouth. Soft solder is an alloy composed of lead and tin in the pro- portion of two parts of the former with one of the latter. TIN. Tin is a brilliant, silver-white metal, the lustre of which is not sensibly affected by exposure to the air, but is easily oxydized by heat. It has a slightly disagreeable taste, and emits, when rubbed, a peculiar odor. It is soft, inelastic, and, when bent, emits a peculiar crackling sound called the 96 MECHANICAL DENTISTRY. creaking of tin. It is inferior in tenacity and ductility, but is very malleable, and may be beaten into leaves the j^y- of an inch in thickness; ordinary tin foil being about y^oo °f an inch thick. It fuses at 442° ; boils at a white heat, and burns with a blue flame to binoxide. * The more common alloys of tin with other metals have al- ready been noticed. It was at one time used as a base for artificial teeth; and, more recently, it has been introduced as a component of "cheoplastic" metal, a compound used for the same purpose. In its pure state, it is sometimes used for counter-dies, and occasionally for dies. When employed for the latter purpose in connection with a lead counter, the latter should not be obtained directly from the die, as the high temperature of melted lead would produce, when poured upon tin, partial fusion of the latter and consequent adhesion of the two pieces. When tin is used in the formation of a die, therefore, either a counter previously obtained from a zinc die should be used, or the "dipping" method employed, by which the counter-die is first obtained from the plaster model, and a die from the counter. ANTIMONY. Antimony is of a silver-white color, with a tinge of blue, a lamellar texture, and crystalline fracture. It is brittle and easily pulverized. The specific gravity of the purest variety is 6*715. It fuses at about 810°, and when heated at the blowpipe, it melts with great readiness, and diffuses white vapors, possessing somewhat of a garlic smell. Antimony enters as an ingredient into the composition of type and stereotype metal, music plates, and Britannia metal. It is also a component of certain fusible alloys analogous to those already mentioned under the head of lead, and which, in the form of a die, are sometimes used on account of their slight degree of shrinkage. BISMUTH. 97 BISMUTH. Bismuth is a white-colored metal resembling, in some degree, antimony. It is soft, but so brittle as to be easily pulverized. Its specific gravity is 9*83, which may be increased somewhat by hammering. It melts at 480° Fah., and may be cooled six or seven degrees below this point without fixing ; but the moment it begins to solidify, the temperature rises to 480°, and continues stationary till the whole mass is congealed. When the temperature of the metal is raised from 32° to 212°, it expands 7^ in length. Bismuth has the property, in a high degree, of increasing the fusibility of the metals with which it is incorporated, and is a common ingredient of the more fusible alloys, some of which melt in boiling water. One part of bismuth with 24 of tin is malleable, but the alloy of these metals becomes brittle by the addition of more bismuth. Bismuth unites readily with antimony, and in the proportion of one part or more of the former to two of the latter, it expands in the act of cooling. There are many other metals and alloys besides those al- ready enumerated, but which have not been particularly described on account of their inutility in the laboratory for dental purposes. Among these may be mentioned, iron, brass, bronze, fyc, which are only employed for auxiliary pur- poses, and are both inconvenient and impracticable for dies on account of their infusible nature and consequent contrac- tion ; nickel, on account, also, of its extreme infusibility and its tendency to render the alloy, of which it is a component, less fusible; sodium, on account of the changes produced on it by exposure to the air; potassium, on account of its ex- treme sensitiveness to the influence of low temperatures, being semi-fluid at 60° Fah., nearly liquid at 92°, and entirely so at 120° ; arsenic, because it volatilizes before fusing : cad- mium, with no advantages above tin, on account of its scarcity and costliness, &c. CHAPTER X. GENERAL PROPERTIES OF ALLOYS, AND THEIR TREATMENT AND BEHAVIOR IN THE PROCESS OF COMPOUNDING. All alloys possess metallic lustre; are opaque, conduct heat and electricity; and, in a greater or less degree, are ductile, malleable, elastic, and sonorous. Some alloys, as brass, and gong-metal, are usually malleable in the cold, and brittle when hot. Metals sometimes unite in atomic ratios, forming compounds of definite or equivalent proportions of the component metals; as certain alloys of copper and zinc; gold and copper; gold and silver; mercurial alloys, &c.; while, on the other hand, many are formed in all proportions, like mixtures of salt and water. Metals differ in respect to their affinity for each other, and do not, therefore, alloy with equal facility; thus it is difficult to unite silver and iron, but the former combines readily with gold, copper, or lead. The ductility of an alloy is, in general, less than that of its constituent metals, and this difference is, in some instances, remarkably prominent, as in the case of certain alloys of copper and tin already mentioned. An alloy is generally harder than the mean hardness of its components, a property which, when taken in connection with their increased fusibility, gives to alloys peculiar value in the formation of dies for stamping purposes. To the rule stated, amalgams or mercurial alloys are cited as exceptions. The density of an alloy varies with the particular metals composing it; being generally either greater or less than the mean density of its several components. GENERAL PROPERTIES OF ALLOYS. 99 It is impossible to predict with certainty the melting point of an alloy from that of its separate constituents, but, generally, the fusibility of the alloy is increased,—sometimes in a most remarkable degree. The alloy of 5 parts of bismuth, 3 of lead, and 2 of tin, is a striking example of this fact; this compound, melting at 197°, while the mean melting point of its constituents is 514°. Silver solder is also a familiar illustration of the influence of alloying on the fusibility of metals ; copper melting at 1996°, and silver at 1873°, when combined, fuse at a heat much below that required to melt silver, the more fusible component of the alloy. Again, iron, which melts at a little less than 3000°, acquires almost the fusibility of gold when alloyed with the latter. Examples might be multiplied, but it will be sufficient to add, that, in general, metallic alloys melt at a lower heat than is required to fuse the most refractory or infusible component, and some- times than the most fusible ingredient. The color of an alloy cannot, in general, be inferred from that of it component metals; thus, it would be conjectured that copper would be rendered very much paler by adding to it zinc in considerable quantities; but the fallacy of such an inference is at once shown by an examination of some of the rich-looking gold-colored varieties of brass, as Prince's metalr pinchbeck, and similor, composed each of nearly equal parts of copper and zinc; and manheim gold, compounded of & parts copper and 1 of zinc. The affinity of an alloy for oxygen is greater than that of the separate metals;' a phenomenon that is ascribed by Ure to the increase of affinity for oxygen which results from the tendency of one of the oxyds to combine with the other; by others, it is attributed to galvanic action. According to Faraday, 100 parts of steel, alloyed with one of platinum, is dissolved with effervescence, in dilute sulphuric acid too weak to act with perceptible energy on common steel. It is offered in explanation of this fact, that the steel is rendered positive by the presence of platinum. A similar illustration ia 100 MECHANICAL DENTISTRY. afforded by the action of dilute acid on commercial zinc, which is usually an alloy of zinc with other metals. The action of air is, in general, less on alloys than on the separate metals composing them. To this, however, there are exceptions, as the alloy of 3 parts of lead and 1 of tin, which, when heated to redness, burns briskly into a red oxyd. Some points of practical interest suggest themselves in connection with the behavior and proper management of alloys in the process of compounding. As metallic alloys can only be formed by fusion, and as the affinity of the metals composing them for oxygen is greatly increased by heat, especially those denominated base, it is important that this tendency, which is incompatible with the proportional accurateness of the compound, should be, as far as practicable, guarded against. Hence, various substances having a greater affinity for oxygen than the metals to be united, as oil or grease, rosin, powdered charcoal, etc., are generally added, coating the surface of the liquid metals, and which by affording a protective covering, preserves, with little change, the proportions of the alloy. Some difficulty is occasionally experienced in obtaining a perfectly uniform alloy, on account of the different specific gravities of the metals composing it—each metal assuming the level due to its density. This partial separation is common to gold and silver, provided they have not been adequately stirred before pouring. This result is not so likely to occur when the metals employed are in small quantities, and are suddenly cooled; but when used in considerable masses, and allowed to cool slowly, it is much favored by per- mitting the metals to fix themselves in the order of their separate densities. Hence, whenever a notable difference in the specific gravity of the metals exists, the fused mass should be briskly stirred immediately before the instant of pouring it, and should be made to solidify quickly. If uniformity be not obtained in this manner, it will be necessary to re-melt GENERAL PROPERTIES OF ALLOYS. 101 and repeat the process, if necessary, until the alloy is ren- dered sufficiently homogeneous. In alloying three or more metals differing greatly in fusibility, or that have but little affinity for each other, it is better to first unite those which most readily combine, and afterward, these with the remaining metal or metals. If, for example, it is desired to unite a small quantity of lead with brass or bronze, some difficulty would be experienced in forming the alloy by direct incorporation of the metals; but union could be readily effected by first melting the lead with zinc or tin, and then adding the melted copper. PART SECOND. ARTIFICIAL DENTURES. Before considering particularly the distinct and special methods employed in the construction of artificial dentures, such preliminary processes as are common in some degree to all, will, for the sake of convenient arrangement, and the avoidance of unnecessary repetition hereafter, be first con- sidered. These processes relate, 1. To the treatment of the mouth preparatory to the insertion of artificial teeth. 2. The manner of obtaining impressions of the mouth. 3. The manner of procuring and forming plaster models of the mouth. 4. Metallic dies and counter-dies. CHAPTER I. TREATMENT OF THE MOUTH PREPARATORY TO THE INSERTION OF ARTIFICIAL DENTURES. It rarely occurs that all the structures of the mouth are in such condition as will render it proper to insert an artificial appliance without some preparatory treatment. This impor- tant requirement cannot, in any material respect, be disre- garded by the practitioner without endangering the utility and permanence of the substitute, and inflicting upon the patient a train of consequences alike distressing and pernici- ous. Every experienced dentist is familiar with the fact, that an artificial substitute resting upon diseased roots of teeth, and impinging continually upon gums already irritable and inflamed, soon becomes a source not only of annoyance TREATMENT OF THE MOUTH. 103 and discomfort to the patient, but is rendered, in a great degree, inefficient in the performance of some of its more important offices. There is, besides, a perpetual and cumu- lative aggravation of the morbid conditions, and sooner or later irretrievable destruction of the remaining natural organs will be induced. These consequences cannot be wholly averted by the most skillful manipulation, but they may be greatly magnified by a defective execution of the work, or by a faulty adaptation of the appliance to the parts in the mouth. Patients not unfrequently attempt, by every artifice or pretext that caprice or timidity may suggest, to persuade the operator to violate his own clear convictions of duty, but, unless under circumstances of peculiar exigency, he should be careful to guard himself against the imputation of inconiT. petency or bad faith by being peremptory and unyielding in his demands upon the patient to submit to the necessities and just requirements of the case, and no ordinary circumstance should influence him in opposition to his better informed judgment. The conditions, usually met with, to which it will be neces- sary to direct attention in the treatment of the mouth, are, the presence of useless and diseased remains of teeth; accu- mulations of tartar ; diseased states of the gums and mucous membrane ; and caries. Useless and Diseased Remains of Teeth.—It may be stated, as a general rule of practice, that all the remaining natural teeth that are not susceptible of being restored to a state of comparative health and usefulness, should be removed before inserting an artificial substitute. Especially should this course be pursued whenever the remaining roots are found partially or wholly necrosed, and the peridental membranes and surrounding tissues inflamed and suppurating. Such should be extracted if for no other reason than that they are offensive in the mouth, and tend, in a greater or less degree, to compromise the general health. In respect to the utility, comfort, and permanence of a 104 MECHANICAL DENTISTRY. dental appliance, the expediency of removing the roots of teeth prior to the introduction of the former, is apparent. If a dental substitute is adapted with necessary accuracy to all the parts which it covers, it will be plainly seen that the forces applied to the base at every occlusion of the jaws in the act of mastication, instead of being equalized or diffused, will be expended mainly on the fangs, inasmuch as they afford so many fixed points of resistance, whilst the adjacent soft tissues, yielding to the pressure, permit the artificial piece to bear with undue and unequal force upon the roots. The con- sequences of this action are inevitably pernicious. In a comparatively short time, inflammation and suppuration are induced about the fangs, which ultimately become loosened and painfully sensitive to the slightest pressure; the secre- tions of the mouth, becoming more and more acrimonious, act persistently and with increasing energy on oxydable materials present in the mouth, as well as upon the remaining natural teeth, inducing rapid and general decay; contiguous parts, through their immediate connection or sympathetic relations with the structures of the mouth, respond to the local dis- turbances, and the case, in time, becomes complicated with those various distressing maladies about the head and face so commonly associated with diseased conditions of the buccal cavity. At last, the patient, no longer able to endure the offensiveness and distress arising from the presence of the substitute in the mouth, or to properly masticate his food, is compelled to have the offending organs removed. The absorption of the gums and processes which follow this ope- ration, and the corresponding changes which occur therefrom in the form of the alveolar ridge, make it imperative in all cases either to reconstruct the same piece or to supply the patient with an entirely new substitute; whereas, if due regard is had to the proper preparation of the mouth in the first instance, the patient may be spared such inflictions, and the operator the discredit which almost invariably attaches to the neglect of the measures recommended. TREATMENT OF THE MOUTH. 105 An additional reason why roots of teeth should be extracted is, that their presence prevents, in some degree, an accurate and uniform adaptation of the appliance to all the parts on which it is designed to rest, and this is particularly true of those cases where atmospheric pressure is made available in the retention of the substitute. Any condition of the mouth that prominently modifies the natural and uniform pliancy of the soft parts will, just to that extent, weaken the attach- ment of the plate. This fact is made obvious when we re- flect that it is only the soft and yielding condition of the mucous membrane and gums that permits the adhesion of the artificial appliance for a single moment by atmospheric pres- sure ; for it will be readily comprehended that, if the tissues on which it rests were as hard and unyielding as bone, a dental substitute, though it were moulded directly to the parts, would not be sustained for an instant by the external pressure of the air. The retention of every root that may, by treatment or otherwise, be secured in good condition, has been insisted on by a few in the profession, on the ground that they afford a fixed and permanent basis for the dental appliance, and preserve, without change, the customary fullness and contour of the mouth. Individual instances doubtless occur that render this course admissible, but as a rule of practice, it is exposed, though in a less degree perhaps, to the same objections which have been adverted to in connection with diseased roots. However carefully or skillfully such roots may be treated and prepared, or the substitute applied, entire success and permanent benefit to the patient cannot be reasonably anticipated. It rarely happens that the fangs of teeth, whose crowns have been destroyed by caries or acci- dent, are found without having suffered, at some time and in some degree, from disease of the investing membranes. and surrounding structures, and although these conditions may have apparently subsided, or may have been temporarily subdued by treatment, yet observation of such cases leads to> 106 MECHANICAL DENTISTRY. the conclusion that, however free from indications of active disease there may appear at the time, the latent predisposition favoring a recurrence of the morbid action usually exists in such cases, and it will require no greater provocation than the continued and unequal action of an artificial fixture on them to awaken this predisposition into active development. From the views here expressed, we are convinced that, as a principle of practice, the fangs of .teeth, however apparently free from disease, should be extracted in the first instance. There are, nevertheless, circumstances which clearly justify a departure from the rule we have endeavored to enforce; as in the process of engrafting an artificial crown upon a well- conditioned root; or supplying the loss of one or more of the front teeth by attaching the artificial organs to a plate and fixing the latter in the mouth by pivoting to one or more of the natural roots. Either of these methods may, under cer- tain circumstances and within certain limitations, be prefer- able to extracting the roots of such teeth and supplying the vacuities by other means. Removal of Salivary Calculus or Tartar.—The deposits of tartar which so frequently collect at the necks of the teeth and under the free margins of the gum, not only promote in- flammation and absorption of the investing membrane and contiguous soft parts, but involve, by degrees, the alveolar processes in the destructive action; so that teeth originally firm become loosened in their sockets, and thus, in their turn, become additional sources of diseased action in the surround- ing structures. Hence it becomes absolutely necessary, as it relates to the general health of the mouth, to thoroughly remove, with suitable instruments, all traces of this concre- tion from the teeth. If any considerable number of the teeth are found coated with tartar, and it is deposited in large quantities, it will be impracticable, as a general thing, to remove thoroughly all remains of it at a single sitting. The operation should be repeated, therefore, from time to time, until every portion of TREATMENT OF THE MOUTH. 107 it is completely separated from the teeth; the latter should then be well polished with suitably shaped burnishers, and the gums, if highly inflamed and turgid, may be either freely scarified at those points where they dip between the teeth, or cleansed and treated with appropriate detergent and remedial agents. Diseased Conditions of the Mucous Membrane and Gums.— It will seldom be necessary to institute treatment for the re- duction of inflammation and ulceration of the soft tissues of the mouth after the removal of diseased fangs and tartar, in- asmuch as these conditions being generally provoked by, and associated with, the latter, will spontaneously subside with the removal of the exciting causes. If, however, there are other morbid conditions of the soft tissues, or osseous structures of the mouth not immediately arising from the presence of diseased roots and tartar, they should be treated in accordance with the particular pathological conditions present. Caries or Decay of the Remaining Teeth.—In order that all the teeth which it is deemed advisable to retain in the mouth may be permanently preserved, it will be necessary to fill, or otherwise treat, such as may be affected by caries. This" operation will be attended with more satisfactory results arid be accompanied with less pain to the patient, and diminished risk of failure, when performed after the removal of the roots of teeth and tartar, and the restoration of diseased conditions of the mouth to health, as, in this case, there will be less irritability of the general system, and reduced sensitiveness of the teeth operated on. Surgical Treatment of the Mouth after the Extraction of Teeth.—In the preparation of the mouth for entire sets of artificial teeth, it frequently becomes necessary to extract the remains of all or nearly all of the teeth of one or both jaws. In such cases, the ridge is left ragged and broken, with flaps of gum lying in loose folds along the border, and the exposed margins of the alveolar processes projecting from 108 MECHANICAL DENTISTRY. underneath. These parts, if left in this condition, will be productive of more or less inconvenience to the patient; for as the gums close over and contract upon the cutting edges of the processes, irritation and inflammation will be induced at those points where they are most prominent. Immediately after the extraction of the teeth, therefore, any flaps of gum hanging loosely around the. sockets should be clipped off, and sharp and protruding portions of processes cut away with ex- cising forceps. If, in the course of a few weeks, prominences still remain, over which the mucous membrane is stretched and irritated or inflamed, as is more frequently the case around the sockets of the cuspidati, the membrane should be divided over such points with a lancet, and the sharp points of bone underneath broken down with suitable cutting instru- ments. Time Necessary to Elapse, after the Extraction of Teeth, before Inserting Artificial Dentures.—The time that should elapse after extracting the natural teeth, before replacing them with artificial substitutes, will depend upon various circum- stances. If the appliance is only intended to meet the wants of the individual until all the changes effected by absorption of the gums and processes are fully completed, it may be inserted in from one to three weeks, depending somewhat upon the number of teeth extracted, the extent of the injuries unavoid- ably inflicted upon the parts, and the virulence of the diseased action present in the structures of the mouth at the time of the operation. If there are no unusual complications, and the space or spaces to be supplied are such as are made by the loss of only one or two teeth at intervals, the parts quickly assume their normal condition, and the piece to be tempor- arily worn may be applied within a few days. If, however, a greater number or all of the teth have been removed, more or less inflammation and tenderness will be present for from ten days to two or three weeks, and which will render the wear- ing of an artificial piece uncomfortable to the patient, and in some degree mischievous, by aggravating the morbid condi- TREATMENT OF THE MOUTH. 109 tions already existing. Another objection to the too early introduction of artificial substitutes into the mouth arises from the fact that the changes which occur in the ridge are much more rapid within the first few weeks after the extrac- tion of the teeth than at any subsequent period, so that the plate, if inserted immediately or within a few days after such operation, will soon lose its bearing upon the ridge and be- come inefficient for masticating purposes, or may even fail to be retained in the mouth without much annoyance to the patient. Two or more weeks, therefore, should elapse before applying the substitute. In the meantime, the patient should be seen frequently, and such medical or surgical treatment adopted from time to time as the case may demand. It has been objected to the insertion of what are termed temporary sets of teeth, that they tend to produce unequal absorption of the parts on which they rest. We cannot but regard this view as mainly speculative. In respect to the upper jaw, at least, there is no such pressure exerted as would result in permanent irregularities in the ridge, for, as the latter recedes in the process of absorption, it is more and more relieved from direct contact with the plate covering it-— that portion of the plate resting against the roof of the mouth preventing it from following the retreating gums. Hence it is that such substitutes soon begin to "rock" in the mouth, and are easily dislodged, when pressed upon laterally, in consequence of the want of an adequate marginal bearing. Nor does the objection hold good in reference to the inferior maxilla where the pressure of the substitute upon the ridge is continuous throughout the period of absorption ; for if the ultimate form of the ridge were influenced at all in these cases, we should have, at the conclusion of the period, the same irregularities as at first, with simple diminution of sub- stance ; for we know of no peculiarities in the physiological condition of the structures that would render one part amen- able to this assumed consequence of pressure and another exempt from it. Common observation, on the contrary, 110 MECHANICAL DENTISTRY. clearly shows that the process of absorption and deposition go on, uninterruptedly, and are, in no appreciable degree, in- fluenced by the presence of the temporary substitute in the mouth. The advantages of temporary sets of teeth to the patient., on the other hand, are unquestionable. They fulfill, in a tol- erable degree, all the requirements of artificial teeth under any circumstances, if we except that of mastication, this func- tion being, more or less, imperfectly performed with such pieces. One of their most important offices consists in main- taining unchanged the customary relation or closure of the jaws. Undue projection, and in many cases, partial luxation, of the inferior jaw results from the loss of all of the teeth, and these conditions may become permanent in their char- acter if long continued. The time occupied in the completion of those changes which occur in the alveolar border after the extraction of all or any considerable number of the teeth, cannot be definitely stated, but will range from five to eighteen months or more, accord- ing to the amount of superfluous structures to be removed, the density of the osseous tissues, and the functional activity of the absorbents. In all cases, ample time should be per- mitted to elapse in order that no appreciable change in the form of the parts may take place after the appliance has been permanently adjusted. CHAPTER II. MATERIALS AND METHODS EMPLOYED IN OBTAINING IM- PRESSIONS OF THE MOUTH. In the process of constructing a dental substitute, it is of the first importance that as accurate an impression as possible should be obtained of all those parts of the mouth with which the appliance is in any way connected. If this important prelimin- ary step is, in any essential respect, imperfectly performed, the ultimate utility of the artificial fixture will either be greatly impaired or wholly destroyed, notwithstanding all the sub- sequent manipulations may be most carefully and skillfully performed. The operator, therefore, should avail himself of every appliance and facility that will enable him to attain, in this respect, the most perfect results. The materials ordinarily employed for this purpose are, wax, gutta percha, and plaster of Paris. Wax.—There are two varieties of this substance in common use,—the yellow and white wax. The yellow variety is es- teemed preferable to the white on account of its superior toughness; the latter being, to some extent, disintegrated, or rendered less tenacious in the process of bleaching, but is frequently used and is preferred by many on account of its color. The more desirable properties of the yellow wax are often impaired by the admixture with it of tallow, with which it is, for mercenary purposes, frequently contaminated. The presence of tallow may be detected by its characteristic odor, and by the whitish or pale yellow color it imparts to the wax, which in its pure state, is of a deep, bright straw color. Wax used for impressions should always be kept in con- 112 MECHANICAL DENTISTRY. venient form for immediate use, and may be prepared either by warming it until sufficiently soft and, then rolling or pres- sing it into thin sheets; or having melted it in a properly formed vessel, immerse in it a thin strip of board, previously moistened, and withdraw quickly; this is repeated as the successive layers cool, until a coating of sufficient thickness is obtained. The latter is a convenient method of obtaining sheets of wax of a uniform thickness, a form frequently re- quired for various purposes in the dental laboratory. Manner of Obtaining an Impression of the Mouth in Wax for Partial Upper Dentures.—Until within the past few years, wax has been used almost exclusively for the purpose of obtaining an impression of the mouth in those cases where any number of the natural teeth remain in either or both jaws, and, for this purpose, is ordinarily more convenient and manageable than plaster, and, if carefully manipulated, will secure in most cases a sufficiently accurate impression of the parts. Before preparing the wax, a suitable cup or holder should be selected for the particular case in hand. These appli- ances are usually constructed either of plate or block tin, Britannia metal, or silver, and a sufficient number of the various forms required should be provided to meet perfectly every requirement in respect to the size and form of the jaws of individual cases. For upper partial or broken sets, the form of cup repre- sented in Fig. 19 may be used. It should be large enough to embrace the alveolar ridge, leaving a space of nearly a fourth of an inch between its outer rim and the external border of the gum. If it is designed to employ an atmospheric pressure plate covering nearly or quite all of the hard palate, a cup of the same general form, but with its central portion extended posteri- OBTAINING IMPRESSIONS OF THE MOUTH. 113 orly, may be used; or a full cup like that represented in Fig. 21 may be employed. Having selected a cup of the proper form and size, the wax should be warmed in a spirit flame until it acquires about the consistence of freshly made putty. Wax is sometimes softened by immersing it in hot water, but the dry heat is preferable, as the former seems to impair, to some extent, its toughness and continuity. In taking the impression, the operator should place himself behind and to the right of the patient, and should be sufficiently raised above the latter to enable him to manipu- late with the greatest ease and certainty, and at the same time, to command as full and unobstructed a view of the interior of the mouth as possible. The cup, with the wax arranged, should then be introduced into the mouth without unnecessary delay. To do this properly and without subject- ing the patient to annoyance, will occasionally require some care and expertness on account of the disproportionate size of the cup and orifice of the mouth. An ample and expanded jaw, for example, is frequently associated with a small mouth, and if in addition to this the sphincter muscle of the mouth happens to be rigid and unyielding, the introduction of a cup of sufficient size may be attended with some little difficulty and embarrassment. This impediment, however, may be readily overcome in most cases by presenting the cup obliquely to the mouth, one side resting against, and pressing outward, the corner of the mouth, while, as the opposite corner is extended with the first and second fingers of the left hand, the cup is passed in with a rotary movement. When the cup is within the mouth, it should be carefully adjusted over the ridge before pressing it up, so that no portions of the rim may cut into the soft tissues of the mouth, an accident liable to happen without care, and which will make it necessary in most cases to withdraw the cup before the impression is complete. The proper position of the cup in the mouth secured, it should be held firmly with the thumb resting on the handle above, and two or more of 114 MECHANICAL DENTISTRY. the fingers on the under surface, when it is slowly but steadily and forcibly pressed against the parts above until the ridge is completely imbedded, and the wax carried closely against the roof of the mouth. The cup should then be held stationary with one hand while with the fingers of the other the wax around the margins of the cup should be pressed closely into all the depressions occurring on the outside of the ridge between the remaining teeth, or wherever irregularities may present themselves on the external border of the jaw. The finger should also be passed to the roof of the mouth at the central and posterior edge of the cup, making pressure against the protruding wax upward and forward into the anterior and deeper portions of the palatal arch. When the operation has been conducted thus far, and before removing the cup, gentle upward pressure upon the latter may again be made,—not enough to move the entire body of wax, but only sufficient to correct any partial displacement that may have happened from accidental tilting or lateral movement of the cup during the, concluding manipulations. After the wax has remained in the mouth long enough to become in some degree hardened, it should be carefully de- tached by gentle traction upon the cup, and removed from the mouth in the same manner in which it was introduced; care being taken not to displace the wax or otherwise mar the impression. More or less dragging of the wax, however, will unavoidably occur in proportion as the teeth are irregu- larly arranged in the arch, or have contracted necks. Imperfections occurring from these sources may be1 remedied with tolerable accuracy by subsequent carving of the plaster model, but the better plan, where these conditions prevail to any considerable extent, is to substitute gutta percha for wax, the elasticity of this substance enabling it to regain the form it acquires in the mouth after having been temporarily disturbed or changed in the act of detaching it from the teeth. Inasmuch as it is necessary, in constructing partial sets of OBTAINING IMPRESSIONS OF THE MOUTH. 115 teeth, to be provided with two or more plaster models, and as the latter cannot well be obtained in perfect condition from a single impression, it is better that at least two of the latter should be secured in the first instance. Manner of Obtaining an Impression of the Lower Jaw in Wax for Partial Dentures.—If the case is one where teeth at intervals are to be supplied, the form of cup used in taking an impression for an entire lower denture (Fig. 23) may be employed; or if the vacuity exists in the front part of the ridge only, then one like that represented in Fig. 19 will answer the purpose. If, however, as is more generally the case, the front teeth remain, and those posterior to the cus- pids or bicuspids are to be replaced, the form of cup exhibited in Fig. 20 should be used; a portion being cut out from the front part of it, forming a va- cuity which receives and per- mits an unobstructed passage of the front teeth. As the latter are often very long, it is difficult, with the ordinary form of cup, to press the wax down fairly upon the ridge be- hind without bringing their cutting edges prematurely in contact with the floor of the cup in front. Instead of the opening represented in the cup, however, it will be sufficient in most cases to have it formed with a depression in front of adequate depth to receive the points of the anterior teeth. In taking an impression of the lower jaw, after having prepared and arranged the wax by softening and filling the groove of the cup flush with the margins, the operator may first take a position to the right and back of the patient, and introduce the cup into the mouth in the manner heretofore described, when he should pass to the front of the patient, and having adjusted the cup properly over the ridge, the first two or three fingers of each hand should be placed upon 116 MECHANICAL DENTISTRY. the top of each side of the cup, and a thumb upon each side and underneath the jaw, and firm and steady pressure made until the ridge is wholly imbedded. The wax may then be pressed in around the margins of the cup, and the impres- sion carefully removed from the mouth in the manner before indicated. Manner of Obtaining an Impression of the Mouth in Wax for Entire Upper Dentures.—The, form of cup employed in taking an impression of the upper jaw in the absence of all the natural teeth, is seen in Fig. 21. A number of these corresponding as nearly as pos- Fig. 21. sible in form and size to the vari- ous modifications in the configu- ration and dimensions of the maxillary arch, should be kept conveniently at hand. If the teeth have been recently extract- ed, the wax should be prepared somewhat softer than usual to prevent displacement of the gums, which, in their unabsorbed condition, possess more or less mobility. The cup should be filled flush with the edges, and built up in the centre if the depth of the palatal vault requires it, and the wax properly trimmed; it is then intro- duced into the mouth and adjusted to the ridge, as already described, and pressed to the jaw with sufficient force to fully encase all the parts to which the substitute is ultimately to be applied. The wax, as the cup is pressed up, has a ten- dency to roll out at its edges and thus depart from the upper and outer portions of the ridge; hence care must be taken to press the wax in around the marginal portions of the cup, filling up any depressions or fosses that may occur on the external border of the jaw. It is particularly necessary to observe this precaution whenever the ridge overhangs, as is prominently the case for the first few months after the ex- traction of the teeth. If the impression is an accurate one, some difficulty is oc- OBTAINING IMPRESSIONS OF THE MOUTH. 117 casionally experienced in detaching it from the mouth on ac- count of the thorough exclusion of air from between it and the mouth, the wax being held firmly in place by atmospheric pressure; in which event it is only necessary to admit the air between the two, and this may generally be readily ef- fected by placing the finger against the jaw on one side and above the wax, pressing firmly toward the centre of the arch and upward, -dragging the mucous membrane somewhat from the edge of the cup, and at the same time depressing the latter on the same side. A small portion of air being ad- mitted, it will soon diffuse itself between the adhering sur- faces and allow the wax to be readily detached. To harden the wax, and thereby prevent it from dragging at those points where the ridge overhangs, or to prevent any change of form on the application of sufficient force to detach it from the mouth when it adheres with great tenacity, a cup has been constructed with a chamber underneath into which a stream of cold water is admitted. Two short pipes, as will be seen by reference to Fig. 22, communicate with the cham- Fig. 22. ber, and these again connect with a double tube fitting them closely, and united at the other end, with two gum elastic tubes—one communicating with a vessel of water conveni- ently placed and provided with a stop-cock, the other leading to a spittoon or other waste place. The two portions of pipe may be disconnected when not in use. After taking an im- pression with this cup, and before removing the wax from the mouth, the two portions of pipe are connected and a con- 118 MECHANICAL DENTISTRY. tinuous current of cold water passed through the chamber b)T turning the faucet connected with the tank; when suffi- ciently hard, the wax is removed from the mouth before dis- jointing the pipes to prevent the water from flowing upon the patient. To provide more perfectly against failure of the wax being carried closely against the roof of the mouth in cases where the palatal vault is very deep, a piece may be cut from the central part of the cup, the wax being pressed at this point upward and forward into the deeper portions of the palatal fosse. The author would express, in this connection, his convic- tion that it is impracticable, in most cases, to obtain a fault- less impression of the mouth in wax for full upper dentures. There are points, not readily accessible to the fingers, where the wax departs from the external and posterior borders of the jaw, and is not, therefore, susceptible of easy correction; besides, when reached, and the remedy applied, there is no certain assurance that in pressing the wax in at one point we are not displacing it at another. For this reason, we invariably use plaster in these cases, and we have sufficient reason to believe that the results are more uniformly suc- cessful. Manner of Obtaining an Impression of the Lower Jaw in Wax for Entire Dentures.—The method pursued in securing an impression of the lower jaw in wax for an entire denture, differs in no essential respect from that described when taking an impression for lower partial pieces, the form of cup being represented in Fig. 23. When the parts are imbedded in the wax, the latter should be pressed in around the inner bor- der of the holder, but more espe- cially near the posterior part of the ridge on each side where the latter overhang and ap- OBTAINING IMPRESSIONS OF THE MOUTH. 119 proximate each other, forming corresponding excavations underneath. After adjusting the wax to the ridge along the border of the cup, the latter should again be pressed directly down upon the jaw before removing it, to correct any partial deformity that may have occurred during the previous mani- pulations. Gutta Percha.—This material is rarely used except in ob- taining impressions for partial pieces, and is more particu- larly indispensable whenever a perfect representation of the parts in plaster is essential to the success of any method in which the base is moulded or cast upon the model, as in the case of the "Vulcanite" or "Cheoplastic" processes. It takes the form and position of the teeth readily, and pre- serves them unchanged, by virtue of its elasticity, when removed from the mouth. The general management of this substance in the process of obtaining an impression with it, is in most respects, similar to that of wax, when used for the same purpose. It should, however, be prepared by softening it in hot water, but as the heat required to render it sufficiently plastic is greater than could be well endured without inflicting injury upon the soft tissues of the mouth, and subjecting the patient to pain in its application, it is customary, after having heated it sufficiently and packed the cup, to chill the surface by plung- ing it into cold water, and then introduce it quickly into the mouth. When the impression is secured, and the gutta percha has become somewhat rigid in the mouth, it should be removed and filled immediately with plaster, as it contracts rapidly in cooling. Plaster of Paris, or gypsum, or technically, sulphate of lime, has been long employed in taking impressions of the mouth for entire dentures, and more recently and to a limited extent, for partial or broken sets. For entire pieces, it has almost wholly superseded the use of wax, and is better adapted for receiving an accurate impression of the mouth, whenever it is desired to secure a copy of all its parts in 120 MECHANICAL DENTISTRY. their undisturbed relation to each other, than any material that has yet been employed. When used for this purpose, it should be of the best quality, finely pulverized and well sifted, and should always be kept in a closed vessel, as the moisture which it attracts from the atmosphere impairs its property of hardening quickly when prepared for use. If impregnated with mois- ture, it should be first dried in a shallow vessel over a mode- rate heat before being used. It is prepared for use by mixing with it a sufficient quantity of water to form a batter of about the consistence of very thick molasses, in which condition it hardens by a species of crystallization in from three to five minutes. The condensation of the plaster mixture is hastened somewhat by the admixture of a small quantity of the chloride of soda or common salt. The best method of preparing plaster, how- ever, for the purpose under consideration, is to combine water with it in sufficient quantity to form, in the first place, a very thin batter, and then to stir or beat it constantly with a small spatula until it becomes sufficiently thickened to admit of its adhering in a body to the vessel in which it is mixed when the latter is inverted, and when one portion will retain nearly its form when heaped upon another. By this process of protracted beating, called sometimes "temper- ing," it is made tough and pasty, without having its plas- ticity impaired, and when introduced into the mouth in this condition, it adapts itself readily to the parts, hardens quickly, and is not liable, with ordinary care, to incommode the patient by running back into the fauces. So quickly, indeed, does it condense, that unless expeditiously introduced into the mouth, it will begin to " set " before the parts are fairly imbedded. When preparing it for use, therefore, the plaster should be mixed at the chair with the cup con- veniently at hand, while the patient should be in proper position and in immediate readiness for the operation. In view of the liability of the plaster to run back into the OBTAINING IMPRESSIONS OF THE MOUTH. 121 fauces when the cup is pressed to its place in the mouth, pro- ducing nausea and involuntary retching and which is very liable to occur whenever the mixture is too thin or is im- properly manipulated, it is recommended to instruct the patient to avoid swallowing while the plaster is in the mouth. Patients are also advised to breathe through the nostrils, but we see no good reason for this injunction. It should be re- membered that, in the act of breathing through the nose, the velum palati or soft palate is depressed to cut off the passage of air through the mouth, and that it is thus brought more immediately in contact with any portions of plaster that may be protruding from the heel of the cup. The stimulus of contact will tend to produce involuntary contractions of the muscles of the soft palate and fauces, and thus portions of soft, or fragments of hard, plaster will be worked or drawn back into the fauces producing the very evils it is designed to avoid. If, therefore, patients are instructed at all in this respect, they should be advised to breathe naturally through the mouth, this channel affording less obstruction to respi- ration than that through the nostrils in the act of taking an impression. Manner of Obtaining an Impression of the Mouth in Plas- ter for Partial Upper Dentures.—There are conditions of the mouth, incident to the presence of natural teeth within it, which would seem to preclude the use of plaster in taking an impression of the parts. Thus, if any number of the teeth remaining are small at the necks with enlarging crowns, or if they are irregularly arranged in the circle, having either an anterior, posterior or lateral obliquity, it would not only be difficult to detach hardened plaster from teeth so circumstanced, but the force necessary to remove it would inevitably break away portions of plaster from around the teeth. Another apparent objection to the use of plaster in these cases consists in the difficulty with which the impression is separated from the plaster model, it being necessary to cut away the former 9 122 MECHANICAL DENTISTRY. by piecemeal, as it would be impossible to separate the two in the ordinary way. The difficulties incident to the detachment of the plaster from the teeth in the mouth may be obviated in either of the following ways : 1. Take an impression first in wax, and with a metallic die and counter, obtained from a model of the parts, swage a plate of tin, brass, or silver, of the size and form of the intended base; coat the palatal surface of this temporary holder with a thin coating of plaster mixture, and apply it to the mouth in the manner usually employed in obtaining an impression. 2. Take an impression of the parts in wax and cut away from the latter all those portions indented by the teeth, leaving only so much of the wax surface as corresponds with the palate and interspaces in the ridge; use this as a holder, and secure the impression by coating its surface, as before, with a thin layer of plaster batter. By either of the above methods an impression of those parts, only, on which the substitute is designed to rest, can be taken; the form and position of the teeth must be secured in a separate impression either with wax or gutta percha. Notwithstanding the obvious objections already stated, many operators prefer, in taking impressions for partial cases, to imbed all the parts in plaster in the same manner as when wax is used, being careful to remove the plaster from the mouth before it has acquired the usual hardness. If the remaining teeth have contracted necks, or are placed irregu- larly in the arch, they may be partially encased in wax before applying the plaster; this will facilitate the withdrawal of the impression and preserve the form of the teeth; the wax, receiving the impress of the teeth, coming away with the plaster. The form of the cup used in these cases is the same as that represented in Fig. 21, the edge of which may be turned in a little at points to prevent the plaster from being dragged from the cup. The cup being filled with the plaster mixture is introduced into the mouth and carefully pressed OBTAINING IMPRESSIONS OF THE MOUTH. 123 up until all the parts are fully imbedded. When partial hardening of the plaster has occurred, sufficient tractive force should be judiciously applied to the cup to separate the plaster from the teeth and soft parts, when it should be care- fully removed from the mouth. If any portion of the plaster, essential to the form of the impression, should break away, the fragment or fragments may be secured and afterwards applied to the fractured surfaces. Considerable force is sometimes necessary to separate the plaster from the teeth, and in the effort to remove the former, it is liable to part from the cup and remain fixed in the mouth; in which case it will be necessary to cut it away by piecemeal. This casualty may be effectually prevented by employing the form of cup shown in Fig. 24, contrived by Dr. Samuel War- die, of Cincinnati, and used by him with entire suc- cess. It will be seen to consist of an ordinary hold- er, the cup por- tion perforated in the centre through which a small rod passes with a screw cut on one end, and the other surmounted with a concave flange, around and un- derneath which the plaster collects. The rod is formed with a shoulder resting on the palatal face of the cup, and is fixed in position by screwing the tap against the lower surface of the holder. A number of these centre pieces with shafts of various lengths, should be provided, in order that any desired elevation may be given to the cap or flange; for the latter is designed not only to confine the plaster, but also to carry it up into the roof of the mouth where the latter is very deep. Manner of Obtaining an Impression of the Mouth in Plaster for Entire Upper Dentures.—The form of cup used in securing 124 MECHANICAL DENTISTRY. an impression of the upper jaw for entire sets of teeth, differs in no essential respect from that recommended when wax is used for similar purposes. If the external border of the alveolar ridge is very deep, or there is considerable space intervening between the heel of the cup and the floor of the palate, a rim of wax may be placed along the outer margin of the cup, and extended across its posterior border, in order, more effectually, to confine the plaster within the cup and prevent its escape into the back part of the mouth before it has fairly reached the palatal vault. If the latter is very deep, with a marked excavation in its central and anterior portion, or if it presents somewhat the form of a deep fissure, the plaster may fail to be carried perfectly against the floor of the palate, or the air becoming confined within the central portion of the arch, when the plaster is pressed up, may dis- place the latter and form corresponding chambers in the im- pression. If these imperfections are but slight, they may be •subsequently remedied either by filling up the cavity or cav- ities in the impression, or by trimming away at these points from the model. The better plan, however, where these con- ditions of the vault prevail, is to take up a small portion of plaster on the end of a spatula and apply it to the deeper portions of the arch just before introducing the cup. The patient being seated as nearly upright in the chair as possible, with the head inclined slightly forward, the cup is filled with the plaster mixture and introduced quickly into the mouth, when it is pressed up slowly and gently until the parts are completely encased and portions of plaster are seen to protrude from all parts of the margins of the cup, other- wise the impression is liable to be imperfect either on its outer borders or on its palatal face. Immediately after introducing and pressing up the cup, the lip in front should be extended and drawn down over the cup, when gentle pressure, as the plaster is hardening, may be made upon the outside of the lip in front and at either side of the mesial line, to force the plaster more perfectly into the fosses which exist at these points. OBTAINING IMPRESSIONS OF THE MOUTH. 125 It is essential to perfect success in this operation, that the cup, after the parts are once imbedded, should be held per- fectly stationary until the plaster becomes fixed, as the slightest movement, when the plaster is in the act of consoli- dating, will derange the impression and render it faulty. Again, if after the parts are imbedded, the operator discovers that they are not sufficiently encased, and the plaster has partially set, no further effort should be made to press the plaster up upon the parts, but the cup should be withdrawn and the operation repeated with fresh plaster. If the operation has been successfully conducted, the plaster will adhere to the mouth, in most instances, with great tenacity, and it will be necessary to observe some caution in removing it, for, if forcibly detached, injury may be inflicted upon the soft parts by tearing away portions of mucous membrane; or the impression may be fractured or otherwise impaired. In addition to the means already ad- verted to in connection with the method of separating wax impressions from the mouth, resort is sometimes had to the following expedient:—The central portion of the cup being pierced with two or three small holes, a blunt-pointed probe is passed at these points through the plaster, before the latter has hardened perfectly, to the roof of the mouth. Into these passages the external air passes and diffuses itself between the surface of the plaster and the palate, when the impres- sion may be readily detached. The author has succeeded best in detaching impressions in such cases, by upward and interrupted traction upon the handle of the cup, which, by depressing the heel of the same, more readily permits the introduction of air than by either of the methods commonly employed. Manner of Obtaining an Impression of the Mouth in Plas- ter for Entire Lower Dentures.—Until recently, wax has been almost invariably used in taking impressions of the lower jaw. Plaster, however, may be used for the same purpose, and, by some, is esteemed superior to the former. The ordi- 126 MECHANICAL DENTISTRY. nary wax holder as shown in Fig. 23, may be used, and which, being filled with the plaster batter thoroughly beaten until quite tough and pasty, is inverted and quickly intro- duced into the mouth and pressed down upon the ridge until the latter is completely imbedded; when sufficiently hard it should be removed in the ordinary way. A better form of cup, however, contrived expressly for the purpose by Dr. B. W. Franklin, is exhibited in Fig. 25. It consists of two chambers, or a double groove, com- municating with each other by a fissure running from heel to heel of the cup. The groove corres- ponding with the curvature of the lower jaw is filled with plaster pro- perly prepared, inverted, passed into the mouth, and pressed down upon the parts. As the cup is pressed down, portions of plaster will be forced through the fissure into the upper chamber,—this should be pressed down at all points along the groove with the finger, securing more perfectly, in this manner, the intrusion of the plaster into any irregularities or depressions that may occur in the ridge. Or, the empty cup may be placed in its proper position over the jaw and the plaster introduced into the upper groove and pressed down with the fingers through the fissure on to the ridge, filling the depending chamber. CHAPTER III. PLASTER MODELS. After an impression of the mouth has been secured in either of the ways mentioned in the preceding chapter, the next step in the process of constructing an artificial denture, is, to procure from the impression a representation of the parts in plaster. The copy thus secured is called a model, and, if correctly obtained, is a true counterpart or fac-simile of all parts of the mouth represented in the impression. Manner of Obtaining a Plaster Model from an Impression in Wax for Partial Dentures.—The impression in wax should be first trimmed by cutting away superfluous portions that overhang the borders of the cup, care being taken not to mar any essential part of the impression. The surface of the wax imprinted should then be uniformly smeared with a thin coating of oil applied with a camel's-hair brush. The oil should not be of too thick a consistence, nor applied in too large quantities, as it will collect in the more depending por- tions of the impression, and, failing to be displaced by the plaster, will leave the model imperfect at these points, espe- cially at the coronal extremities of the plaster teeth. The cup is now surrounded by some substance that will confine the plaster and give proper form to the body of the model. For this purpose any material that is easily shaped may be used, as a thin sheet of lead or wax, paper, strips of oil or wax cloth, &c. Before pouring the plaster, if it is desired to strengthen any of the plaster teeth,—as those adjoining the vacuities in the jaw, or such as are to be used in adjusting clasps, and thus secure them against accident in handling,—adequate 128 MECHANICAL DENTISTRY. support may be imparted to them by placing short pieces of stiff wire vertically in the depressions made in the wax by the teeth, and which may be supported in an upright position by imbedding one end in the wax in the centre of the bottom of each cavity. When the cup is properly enclosed, a batter of plaster, of somewhat thinner consistence than that used for impressions, is poured in upon the surface of the wax in sufficient quan- tity to give to the body of the model a depth of from one to three inches according to the particular requirements of the case. The plaster should not be poured directly or hastily into the cavities formed by the teeth, but upon points con- tiguous to them, and from which it should be allowed to run slowly into the depressions expelling the contained oil or air and filling them perfectly. When the plaster has become sufficiently hard, any portions overlapping the borders of the wax, and not essential to the form of the model, should be cut away and the two separated either by immersion in warm water, or by placing the model over the flame of a spirit lamp or upon a heated surface until the warmth imparted to the model renders the wax sufficiently soft to allow the former to be removed without fracturing the plaster teeth. The latter methods should be adopted whenever it is desired to obtain duplicate copies from the same impression, as by the use of hot water the impression is destroyed, the latter, however, being generally used when gutta percha is em- ployed. When separated from the impression, the model should be properly trimmed and shaped with a knife-blade. The general form of the body of a model is shown in Fig. 26. The walls, as it will be seen, are made as nearly vertical or parallel as will admit of the model being readily detached from the sand in the process of molding; for if made too flar- ing or divergent, the metallic die obtained from it will be more liable to crack or spread apart under the repeated strokes of a heavy hammer, or to rock under one-sided blows. During the process of stamping or forcing a metallic base PLASTER MODELS. 129 into adaptation to the die,—which is a metallic counterpart of the model—the plate, when cut to the exact pattern of the parts to be covered by it, is frequent- ly forced or dragged back toward the heel of the die, and is thus drawn from the teeth at the sides and in front. This displacement of the plate may be prevented by cutting away all of the plaster teeth from the model, leaving, how- ever, enough of them remaining where they unite with the body of the model to form a shoulder to each tooth, as in Fig. 26. In this case, the plate should be sufficiently ample in its dimensions to partially overlap the border, when, as it is forced into adaptation, distinct indentations will be made in it corresponding exactly with the palatal curvatures of the teeth ; the portions of the plate covering the cut ends of the teeth are then cut away with plate forceps or other instru- ments. If, however, the plate is of the exact size required before stamping, one or two plaster teeth upon each side of the model may be allowed to remain, against the anterior face of which the plate is made to rest, holding it stationary. Manner of Obtaining a Plaster Model from an Impression in Wax for Entire Dentures.—The same general method is pursued in obtaining a plaster model from an impression in wax of either the upper or lower jaw for entire dentures, as that employed in partial cases. The general form of these pieces is represented in Figs. 27 and 28. If it is desired to swage a rim to the plate, forming a groove or socket into which the plate extremities of the teeth are received, the model should be formed in the manner repre- sented in the annexed cuts; in which it will be seen that an abrupt shoulder is formed on the external border of the model of the upper jaw (Fig. 27), but which on the lower (Fig. 28), is extended round the inner border also, as it is de- 130 MECHANICAL DENTISTRY. sirable, in the latter case, to give a rounded edge to the lingual border of the plate, and which is accomplished in part by swaging in the first instance and afterwards by turning Fig. 27. Fig. 28. the edge down upon the plate with pliers or by other means. The model is prepared by adjusting a strip of softened wax around the border and cutting away from its upper surface in such a way as to form a groove, the bottom of which shall be on a line with the extreme edge of the base or plate and which should be indicated upon the model with a pencil mark before applying the roll of wax. Plaster may be substitued for wax, and should always be used whenever heat is applied to the model in the process of obtaining a metallic swage, as by the " dipping " method. If the model is to be used in molding, the groove should be sufficiently open to permit the ready withdrawal of the sand, otherwise the die at this part will be imperfect; if, however, the face of the model is to be immersed in molten metal, securing first the counter-die, any form may be given to the groove that will best facilitate the operation of over- turning the margins of the plate. Rimmed plates are only required when single gum teeth or sectional or entire blocks are employed, or when plate teeth are mounted on a platinum base with continuous gum. Whenever an air chamber is to be stamped in the base, the model should be prepared for the purpose before casting the PLASTER MODELS. 131 metallic swages. The general form and position of the central cavity or chamber in the arch is represented in Fig. 27. The model may be prepared in either of the following ways: 1. The form of the chamber may be cut from the wax or plaster impression; in which case the plaster will be raised at a corresponding point or points upon the model, and will have exactly the same form and depth as the cavity in the impression. 2. Cover the palatal face of the model with a sheet of wax equal in thickness to the required depth of the chamber, and cut out from this, at the desired point, the form of the cavity; fill the latter with plaster, and when hard remove the wax and trim the raised portion to the proper form. 3. Cut a pattern chamber, of the required form and thickness, from sheet wax or lead; place it in proper position in the arch and press down with the fingers or burnisher until it conforms to the contour of the palate; it is then fixed in place either by confining it with a small piece of wire or tack driven through it into the plaster, or by interposing softened wax or other adhesive material between the chamber and model. A small brush loaded with a varnish mixture passed round the edge of the chamber will insure sufficient adhesion of the latter. The same general method as that when central chambers are formed is pursued in the preparation of the model when it is desired to construct lateral cavities in the plate. The form and position of these on the model will be indicated by inspection of the form of "lateral cavity " plates as exhibited in the chapter on " Entire Dentures." There are other modifications in the form of cavity plates, some of which are obsolete; that known as " Cleveland's chamber," is still in limited use and will be described in a subsequent chapter, but does not require a model differing in form from the one described in connection with full dentures with central chambers. Manner of Obtaining a Plaster Model from an Impression in Plaster for Partial Dentures.—The surface of the impression 132 MECHANICAL DENTISTRY. in plaster should first be rendered hard by applying to it, with a camel's hair-brush, a uniform coating of varnish to prevent adhesion of the model. Two kinds of varnish are in common use,—a transparent and colored. The former is preferred for the reason that it penetrates the plaster more thoroughly, giving to it a greater depth of surface hardness, while the latter, if not sufficiently fluid, forms a somewhat % superficial incrustation, which is liable to peel off in handling, leaving portions of the model unprotected. Either, however, if properly prepared and applied, may be employed. FORMULA NO. I. FORMULA NO. II. Transparent Varnish. Colored Varnish. Gum sandarach, . . 5 oz. Gum shellac, . . . 5 oz. Alcohol, .... 1 quart. Alcohol, .... 1 quart The sandarach and shellac should first be freed from all impurities by careful picking and washing; they are then added to the alcohol and digested over a moderate heat until thoroughly dissolved. Other substances, as gum elemi, Venice turpentine, &c, have been recommended as addi- tional ingredients, but they are not indispensable, and may be omitted without sensibly impairing the properties of the varnish. After glazing the surface of the plaster impression with varnish, a thin and uniform coat of oil should be applied; it is then enveloped and the model procured in the same manner as when wax is used. In separating the model from a plaster impression for par- tial cases, it will be necessary to cut the latter away in pieces, as any attempt to separate the two in the ordinary way would inevitably break away the plaster teeth from the model. The impression should be chipped away with great care to avoid defacing the model. To provide more perfectly against this accident, it is better to coat the impression with the colored varnish, as this will indicate, with greater cer- tainty, the line of contact or union between the two pieces. PLASTER MODELS. 133 When separated, the model should be trimmed and formed in the manner heretofore described. Manner of Obtaining a Plaster Model from an Impression in Plaster for Entire Dentures.—The preparation of a plaster impression of either the upper or lower jaw for full dentures, and the method of procuring a model therefrom, differ in no essential respect, except in the mode of separation, from the manipulations required when the impression has been taken in plaster for partial pieces. A model can, ordinarily, be readily detached from an impression of the lower jaw, but is not always so easily effected in the case of the upper. To accomplish this in the latter case, the model may be taken in the hand and the back of the handle of the cup tapped lightly with an instrument; or a small wedged-shaped instru- ment may be carefully forced between the model and impres- sion at the posterior border of the latter until they part slightly, when they may be easily detached ; before doing which, however, any overlapping portions of the model which may tend to bind the two pieces together, should be trimmed away. If any portion of the heel of the model is defaced by the introduction of the wedge, it may be afterwards remedied by restoring the contour of the parts with either wax or plaster. After obtaining a model in either of the ways mentioned, the entire body of it should be glazed and hardened by ap- plying to it a uniform coat of varnish. This protective cov- ering will prevent the surface from wearing, render it more pleasant to the touch, facilitate its withdrawal from the sand, and give a more perfect mold. A model may be better pre- pared for permanent preservation by immersing it for a short time in a solution of carbonate of soda, by which its surface is converted into carbonate of lime and thereby rendered hard and durable; care must be taken not to introduce any of the bicarbonate of soda into the solution. CHAPTER IV. METALLIC DIES AND COUNTER-DIES. A metallic die is a fac-simile or transcript of the mouth in metal, and is also a copy or likeness of the plaster model. A metallic counter-die is a copy of the impression and is a reversed image of the die and plaster model. Manner of Obtaining a Metallic Die.—Two general me- thods are employed in procuring a metallic counterpart of the model: first, by molding,—secondly by a process termed " dipping." Molding.—For this purpose sand is usually employed, though other substances, as Spanish whiting, etc., have been recommended. When sand is used, it should be fine and even grained, the best for the purpose being that used by brass founders. It is prepared by mixing with it sufficient water to render its particles somewhat adherent, so that when portions of it are pressed in the hand and then parted with the fingers, it will break away in well defined' fragments. Excess of water should be avoided, as the vapor formed by the molten metal, when poured upon it, will displace portions of the latter and form cavities or blisters in the face of the die; nor should the sand be used too dry, as in that case it will crumble away in detaching the model. Oil has been proposed as a substitute for water, in which case it is recommended to add one quart of the former to a peck of sand. It is claimed that the sand so prepared is always in immediate readiness for use. The sand properly prepared, the model is next placed with its face uppermost on the molding board and surrounded with METALLIC DIES AND COUNTER-DIES. 135 a metallic ring. A common "wagon box," of which two or three sizes should be had, will answer every purpose. The sand should first be well sifted to remove the coarser particles, and then filled into the ring, packing it closely with the fingers around and over the model until even with the upper edge of the box. Some care must be observed in the manage- ment of the sand when packing it, for if made too compact, the vapor, formed in pouring hot metal, failing to pass out readily through the sand, will be confined within the cavity and form imperfections in the face of the die; or, if too loosely packed, the fluid metal when poured into the mold will, to some extent, permeate the pores of the sand and ren- der the face of the die rough and inperfect. The box, with the model encased, is then lifted above the board and the model dislodged by tapping it gently under- neath with a small mallet or hammer until it parts from the mold. The uncertain and hazardous method, sometimes re- sorted to, of detaching the model by pressing it alternately backward and forward until loosened, and then lifting it from the sand, should never be practiced, as more or less deformity of the mold is unavoidably produced thereby. It not unfrequently happens that the ridge on the plaster model of the upper jaw overhangs, forming corresponding de- pressions above, the excavations occurring more commonly in front and on each side of the mesial line. Whenever this form of the model exists, it will be impracticable to obtain a correct mold in the manner just described, since the sand becoming impacted in these excavations will be broken away and remain with the model when the latter is dislodged. The difficulty mentioned, however, may be readily over- come in all cases by employing the sectional molding flask invented by Dr. G. W. Hawes, the several parts of which are represented in the accompanying cuts. Fig. 29 represents the lower ring, composed of three mov- able pieces with flange extensions, which project in toward the centre. When used, this portion of the flask is closed 136 MECHANICAL DENTISTRY. and the sections kept in place by pins passing through the joints. Inside of this ring the model is placed face upward, the ridge extending a little above the upper plane of the Fig. 29. Fig. 30. ring. Sand, well sifted, is then packed in around the model on a level with the most projecting points on the outside of the ridge as indicated by the dotted line in Fig. 31. The surface of the sand should be trimmed smoothly, and should be cut squarely and at right angles with the ridge, to prevent the sand from breaking away when the model is withdrawn. Very finely pulverized charcoal contained in a loose muslin bag, is now sifted over the exposed surface of the sand, to prevent the next portion contained in the upper ring from adhering. The plain ring (Fig. 30) is then placed over the one containing the model, and is filled with sand well packed over the face of the die. The upper ring is now careiully lifted from the lower one on a line with the pins, thus separating the two portions of sand, and again exposing the uncovered face of the model. One of the pins should then be drawn from the lower ring; the sections of the latter carefully unfolded, and the model withdrawn ; when the ring may be again closed and confined by replacing the pin. The upper ring is then re-adjusted in its proper relation to the lower one, and the flask inverted; when the mold, if the process has been accurately conducted, will be found perfect. In obtaining a mold from the model of a lower jaw, but METALLIC DIES AND COUNTER-DIES. 137 little difficulty will ordinarily be experienced in obtaining it perfect in the manner first described. The depressions at the posterior and inner border of the ridge are the points most liable to drag or displace the sand, and when the latter occurs, the surplus metal in the die at such points must be cut away with suitable instruments; or the cavities in the model may be so filled out with wax before molding as to permit the ready separation of the model without displacing the sand ; in which case, also, it will be necessary, afterward, to trim the redundant metal from the die. A die is more readily and accurately obtained from a model for partial dentures by cutting away the plaster teeth as before described. The displacement of sand where the ridge overhangs will, as a general thing, be unimportant in these cases, as the base but rarely more than partially over- laps the border. When whiting is used in molding, it is unnecessary to mix water with it, as the moisture which it absorbs from the at- mosphere will give to it the proper consistence. Having obtained a mold in either of the ways mentioned, the metal designed for the die should be melted and poured carefully in upon the more prominent portions on the face of the former. If the metal is raised much above its fusing; point, or the sand is quite damp, the former should be poured very slowly into the mold. It is better, however, that the sand should be partially dried before pouring the metal, and the die cast on the instant of the metal becoming sufficiently fluid. An observance of these precautions will protect the sand from the over-action of heat, prevent ebullition of the fused metal from the too rapid decomposition of the water of the sand, and will give a smoother face to the die, and secure the metal or metals from undue waste by oxydation. The opinion is entertained by some that greater skrinkage of the die occurs when the metallic substance of which it is composed is poured at a temperature much above its fusing point; the fallacy of this is made obvious by a moment's 10 138 MECHANICAL DENTISTRY. reflection, as a simple example will show that any change af- fecting the face of the die, as a consequence of contraction, can only occur in the metal between its point of solidifica- tion or liquifaction—for they are identical—and its working temperature. Zinc, for example, melts at 773°. Now if its temperature be raised to 1200°, it will remain fluid until it reaches 773°, and in passing through the intermediate de- grees of heat, it will, in obedience to gravity, adapt itself perfectly to all parts of the more depending portions of the mold; and this perfect continuity of the two surfaces will remain unaffected by the contraction of the metal until the latter commences to "set," or solidify, (773°,) after which, and not until then, the zinc begins to part from the face of the mold by contracting upon itself between 773° and the mean temperature of the air. So far as any change, by con- traction, in the face of the die is concerned, therefore, it is obviously immaterial whether the zinc be poured on the in- stant of melting or at 1200°; the result will be the same in either case. The author is indebted to Dr. B. W. Franklin for the fol- lowing method of securing metallic dies and counters by a process which greatly facilitates the operation and insures accurate and satisfactory results:—"I take all impressions, full and partial, in plaster. A small hole, less than TJg inch, is drilled through the highest point of the palatal surface of the impression, through cup and all; into this place two or three broom splints, cutting them off even with the sur- face of the plaster, to allow any vapors to pass off. I some- times smoke the surface of the impression. Around the im- pression place sufficient putty to form a ring the size and height required for the die. Into this pour, at as low heat as consistent with the mobility required for sharp castings, the bismuth alloy known as Sir Isaac Newton metal, or, which is better in some respects, 8 parts bismuth and 4 parts each of tin and lead—the latter composition being a little harder.. If. a. little judgment is exercised in pouring either METALLIC DIES AND COUNTER-DIES. 139 of the above alloys, a perfect die will be secured from moist plaster impressions without any drying. As the bismuth is expansive, and the alloy is hard and somewhat brittle, I run only a thin casting, not more than \ inch in thickness, over the highest portion of the impression. I have cast iron or brass heads made 3£ inches in length, 3 inches in diameter at large end, and 2 inches at the other; the large end is flat and well coated with common tinman's solder. This head is heated until the solder begins to soften; it is then placed in a pan or other convenient vessel, and the die, face side up, is placed upon the tinned surface. When the die begins to melt, and perfect union is secured, cold water is dashed upon the die and head; and thus we have a sharp die, with an iron head, to sustain the force of the blow in stamp- ing the plate, and by this means preventing any spreading of the face of the die or liability of breaking in the process of swaging. " I now take sheet lead of the thickness of about No. 24 standard gauge, and adapt it to the face of the die by means of a wooden mallet or burnisher, or other convenient means. Trim the lead plate to the size required for the plate to be stamped; when the lead plate is nicely fitted, remove it care- fully from the die, and place it in a ring or narrow molding flask, the palatal side up; now gently stamp molding sand into the plate and flask up level with the edges of the flask; then reverse the flask and cut the sand away clean for J inch or more down to the edge of the lead plate all around. Around the plate place a common molding ring sufficiently large to form the counter, which is made by pouring melted tin or lead (or any alloys of these metals) on to the lead plate, being careful not to run the metal so hot as to melt the lead plate. When the counter is cool enough to handle, the adhering sand is brushed or washed away; the die is then placed into the bed or counter, and, with a moderate sized hammer, give one or two sharp blows to bring the die and counter together. In swaging gold plates, two or three or 140 MECHANICAL DENTISTRY. more dies may be required; these may be made either by running the die-metal into the impression (if not broken), or by running into lead plates, gotten up as before described, reserving, of course, the first die and counter for the final swaging of the plate. I have gotten up a die and counter from the impression, with the aid of an assistant, in the fore- going manner in twelve minutes. I usually get out my die immediately after taking the impression; adapt a wax or gutta percha plate to the die, and get the articulation before the patient leaves the office." Dipping,—By this process the counter-die is first obtained, and from this the die. For the purpose, two or three sheet- iron pans varying in size should be provided, measuring from three to five inches in diameter, and from two to three inches in depth, the open ends of which should be somewhat larger than the bottoms. The metal for the counter is melted and poured into the pan, and immediately before "setting," the model, being unvarnished and previously well dried, is im- mersed, face downward, until all parts of the palatal arch and ridge are imbedded in the metal. The chamber, as well as the groove around the outside of the ridge concerned in the formation of a rim to the plate, should, in this case, be formed in plaster. Ordinarily, the conformation of the ridge above and below is such as to render it impracticable to remove the model from the metallic matrix without injury; hence two or more models will be required whenever this method is practiced. After the counter-die and model are separated, all traces of plaster should be carefully washed from the matrix, and the latter surrounded with a sheet-iron ring forced slightly into the counter immediately outside of the cavity formed by the model; into this the metal for the die is poured, filling the matrix and ring. If the metal or alloy forming the die fuses at the same, or a higher, heat than that composing the counter, the matrix should be protected from adhering to the die by coating its METALLIC DIES AND COUNTER-DIES. 141 surface uniformly either with lamp smoke, or a thin mixture of whiting and water or alcohol. Counter-Die.—A counter to the die is generally obtained directly from the latter, and may be procured in either of two ways. 1. The die is placed, face upward, upon the molding-board, and sand, prepared as in molding, built up around it, leaving only the ridge and palatal face exposed. It is then encircled with a cast or sheet-iron ring two or three inches deep, its edge imbedded in the sand to prevent the escape of the fluid metal; into this the metal for the counter is poured until nearly or quite full. 2. The metal for the counter may first be poured into a sheet iron vessel of proper size, and, immediately before setting, the face of the die is immersed in the liquid mass, and held perfectly stationary until solidification of the counter takes place. The method of procuring a counter die directly from the plaster model, as in the process of dipping, has already been des- cribed. The metal commonly employed for the counter is lead, al- though other substances, as tin, type metal, some of the more fusible alloys hereafter to be mentioned, &c, are sometimes employed. When the counter is taken by pouring the metal or metals composing it upon a die fusing at a low heat, some caution should be observed lest the two pieces adhere by par- tial fusion of the die. In such cases, the surface of the die should be well protected with lamp smoke or whiting; the lead should be poured at the lowest practicable temperature, and the conduction of heat facilitated by surrounding the die with a heavy cast-iron box or ring. To avoid incurring any risk, however, the counter-die, if composed of a less fusible metal or compound than the die, may be first obtained di- rectly from the model, and the die obtained from this as in the process of dipping; or, a counter of lead, previously taken from a zinc or other more infusible die, may be used. During the process of forcing a plate into adaptation to the form of the mouth with swages, it not unfrequently happens 142 MECHANICAL DENTISTRY. that the marginal portions of the former become wedged or immovably fixed between the outer border of the die and corresponding portions of the counter before its central por- tion is forced into contact with the palatal surface of the former, thus rendering it difficult to conform the plate accur- ately to the parts without the application of sufficient force to deface or otherwise mar the form of the die. In such cases, the central portion of the plate may be first swaged with a partial counter, which is made to receive only the palatal portion and upper surface of the ridge of the die. After forcing the central part of the plate into adaptation with the partial counter, the process may afterwards be completed with a full counter after having turned the edges of the plate down upon the outer border of the ridge with a mallet and pliers. As before remarked, preference is usually given to lead in the formation of a counter die, mainly on account of its greater softness. This property in a counter is practically important. In the process of forcing a metallic plate into adaptation to the mouth, partial displacement or yielding of either the die or counter, or of both, necessarily occurs, and it is scarcely necessary to remark that whatever change of form is produced should take place wholly in the counter, otherwise deformity of the die must ensue. Essential Properties of a Die.—There are certain properties which it is indispensable that a metallic die should possess, more or less perfectly, in order to answer fully the require- ments of the dentist. 1. A die should be sufficiently hard to resist any necessary force applied to it in stamping the plate without suffering any material change in the form of its face, by which latter term is meant that portion of the die with which the plate is brought into contact. This property is most indispensable in those cases where the arch of the mouth is very deep, the ruga prominent and sharply defined, and where the alveolar ridge is marked by angular and abrupt prominences and de- METALLIC DIES AND COUNTER-DIES. 143 pressions. In such cases, if the die is not sufficiently resis- tant, the points most prominent upon its face will be bruised or battered down, while the plate will fail to be forced perfectly into the cavities or depressions, and its co-aptation to the mouth, to that extent, rendered faulty. The cases in which a less degree of hardness is admissible is, where the arch of the mouth is broad and shallow, the ruga imperfectly defined, and the ridge regular and symmetrical. The con- formation of the mouth, therefore, will, in respect to the property of hardness, admit of some latitude in the choice of the metal or alloy employed in the formation of a die. 2. Another important property of a metallic die is non-1 contraction, so far, at least, as this is attainable. Inasmuch as the successful adaptation of the plate depends, in a great measure, upon an accurate representation of the precise form of the mouth in the die, it is of the first importance that the latter, other essential requisites being secured, should be composed of some metal or metals having the least possible contraction in cooling. Contraction is, in varying degrees, common to all metals exposed to a decreasing temperature, and it is impossible, therefore, to obtain a perfectly faultless copy of the mouth in metal. Fortunately, as well for the expert as the unskilled manipulator, the unavoidable shrink- age incurred is partially or wholly compensated for by the expansion of the plaster and the yielding condition of the soft tissues of the mouth, but under no circumstances should the accommodation afforded by the condition last mentioned encourage negligence or unskillfulness in the use of all available means necessary to secure the most accurate adap- tation of the base. Ordinarily, a moderate degree of con- traction will not materially impair the fit of a plate; on the contrary, in the case of the upper jaw, it sometimes favors its adhesion and retention in the mouth. Cases, on the other hand, frequently occur where the least practicable amount of shrinkage, even at the partial sacrifice of other properties, becomes indispensable in the die. 144 MECHANICAL DENTISTRY. 3. A third important requisite of a die is fusibility'. Aside from the convenience incident to the use of metals which fuse at a low heat, there is another consideration favoring this property of more practical importance. It is well known that all metals expand by heat and contract by cold. In obedience to this law, metals fusilig at a high heat suffer a greater aggregate contraction than those melting at a lower temperature, and, as between two metallic bodies of equal dimensions, liquifying at different temperatures, the difference in contraction will correspond exactly with the difference in the number of degrees through which each passes from the point of solidification to the mean temperature of the air, allowance being made for the difference in their ratios of con- traction. Two dies, one composed of copper and the other of zinc will serve to illustrate. Fused copper solidifies at 1900°; in cooling, therefore, it contracts through over 1800° to reach a working temperature; while zinc, fusing at 773° contracts through only about 700° to reach the same temperature. As before stated, the difference in the contraction of metals will be somewhat modified by that in their ratio of contraction, but it will always be found that the more fusible metals have the least aggregate shrinkage whenever any considerable disparity exists between their fusing points. It is in accor- dance with the principles here set forth, that the more fusi- ble alloys, some of which melt at remarkably low tempera- tures, are employed whenever it is important to obtain a die as nearly the exact counterpart of the model as possible. 4. Finally, a die should be sufficiently cohesive to resist the repeated blows of a heavy hammer without parting or cracking. Many metals, as antimony, bismuth, &c, in other respects suitable for dies, are objectionable on account of brittleness. But it must not, therefore, be inferred that all metals that are denominated brittle are inadmissible for this purpose; for zinc, which, in its ordinary condition is ranked as a brittle metal, and type metal, which is always so, are in no danger of being forced asunder or of suffering displace- METALLIC DIES AND COUNTER-DIES. 145 ment when in the compact form of a die, provided the force used in swaging is judiciously applied or proper form and sufficient depth are given to the body of the die. To recapitulate briefly: a die should be formed of some metal or alloy that has a surface hardness sufficient to resist compression ; that fuses at a low temperature; that does not, in any material degree, contract in the act of cooling; and whose particles adhere with sufficient cohesive force to maintain perfectly its integrity of form under the hammer. Any one or two of these properties are readily attainable in the same die, but no one known metal or alloy combines all of them perfectly. Thus either cast-iron, brass, bronze, or cannon metal, would form an excellent material in respect to surface hardness, and, in the compact form of a die, would be sufficiently cohesive, but few enjoy convenient facilities for melting them; besides, their great contraction consequent upon their high fusing point, would render their employment entirely inadmissible. Again, certain alloys, as those com- posed of lead, tin, and antimony or bismuth, are eminently suitable on account of their extreme fusibility and compara- tive exemption from shrinkage, but they gain these properties at the expense of that degree of hardness necessary to resist compression. Tin, in its uncombined state, is ordinarily, sufficiently fusible, tenacious, and non-contractile, but is too soft and yielding when forcibly compressed. Antimony and bismuth are sufficiently hard, fusible and non-contractile, but are objectionable on the score of extreme brittleness. Any metallic substance that combines most perfectly the several properties referred to, is, therefore, best adapted to the necessities of the mechanical operator, and experience has universally accorded pre-eminence in this respect to zinc. It presents a more resistant surface to the blow of a hammer than either copper or brass; three times more so than that of tin; and more than double that of type metal. As it usually occurs in commerce, it may be classed as a brittle metal, but when annealed, it is tough and malleable. It melts 146 MECHANICAL DENTISTRY. at a heat (773°) which may be readily commanded and con- tracts but little in cooling. Prof. Austin has demonstrated by actual experiments that an average sized zinc die, measuring two inches transversely, contracts 27'1000ths of an inch from outside to outside of the alveolar ridge, being equivalent in thickness to three ordinary leaves of a journal. Prof. A. remarks: "In the first case, (upper jaw,) the plate would ' bind' and if the ridge were covered by an unyielding mucous membrane, it would prevent accuracy of adaption. In the second case, (under jaw,) the plate would have too much lateral ' play' and consequently lack stability. Again, in a moderately deep arch, say a half inch in depth, the shrinkage between the level of the ridge and the floor of the palate will be nearly 7.1000ths—rather more than one leaf of the journal. In the deepest arches, this shrinkage becomes a serious difficulty; in the shallower cases, it is not of much moment, as there is no mouth so hard as not to yield the 1 or 2*1000ths of an inch." As before stated, a moderate degree of shrinkage in the die may, in certain conditions of the mouth, practically favor the adhesion and permanent retention of a plate applied to the upper jaw. The conditions alluded to, and which prevail in a greater or less degree in all cases, are soft and yielding ridge and comparatively hard and unimpressible palate. Now, if, in the first instance, the plate is swaged into uniform contact with all parts of the jaw, it will be readily perceived that, if pressure is made over the ridge on one side, the latter will yield, while the central portion of the plate, meeting with a fixed point of resistance at the floor of the palate, will "ride" upon the latter and thus throw the plate from the ridge on the opposite side of the jaw. If, however, a space equal to one or two thicknesses of the plate exists between the latter and the roof of the mouth as a consequence of contraction in the die, the plate, as it is carried against the palate in the act of exhausting the air from beneath it, will, at the same time, forcibly compress the ridge, securing METALLIC DIES AND COUNTER-DIES. 147 thereby a more resistant basis along the border, and provid- ing more certainly against displacement of the base on the application of forces brought to bear upon it in mastication. The extent to which the shrinkage of a die may be ad- mitted in any given case, will depend partly upon difference in the conditions heretofore mentioned in the soft parts of the mouth, and in part, also, upon the general configuration of the jaw. In a medium-sized mouth, with a depth of say half an inch to the arch, a moderately soft ridge and resisting palate, the shrinkage incident to zinc will be unimportant, and in many cases will be advantageous. If, however, the vault is very deep, even though there be a yielding ridge, the unavoidable contraction of a zinc die will throw the plate so far from the arch as to render it difficult for the patient to exhaust the atmosphere from between it and the floor of the palate, and even when the latter is practicable, the plate will bind with such force upon the outer border of the ridge as not only to produce pain and irritation of the compressed parts, but the resistance afforded at these points will be sufficient, in many cases, to break up the adhesion and force the plate from the palate. Again, as an extreme case, if the ridge and palate are somewhat uniformly un- yielding, and the palatal vault is, at the same time, very deep, a zinc die can only be made available in bringing the base as nearly into adaptation as possible, after which the operation may be completed with a swage having a less de- gree of shrinkage, and which, as a mere finishing die, need not necessarily be so hard as zinc. In conforming a plate to the lower jaw, the die should be as free as possible from contraction in all cases. The great- est shrinkage in such cases will be between the posterior ex- tremities of the ridge, giving too much lateral play to the plate; in addition to which the posterior and inner edge of the base, projecting out from the ridge, will obstruct the free action of the tongue, while the latter will tend to lift it from the ridge and render it unstable. These conditions may be partially remedied by turning the edge of the plate in 148 MECHANICAL DENTISTRY. against the ridge with pliers; but this expedient should never be resorted to in any case whenever it is practicable to secure a correct adaptation by swaging. In all cases in which a zinc die fails to bring the plate into proper adaptation to the parts, either of the following me- tallic compounds may be used to complete the process after partial stamping with zinc. Type Metal.—Lead, 5 parts; antimony, 1 part. Fuses at 500°; contraction less than one-half that of zinc; more compressible than the latter, and very brittle. Babett, or Anti-friction Metal.—Copper, 3 parts; anti- mony, 1 part; tin, 3 parts. First fuse the copper, and then add the antimony and tin. Melts at a moderately low heat; contracts but little; is brittle, but may be rendered less so by adding tin. Zinc, 4 parts; tin, 1 part. Fuses at a lower heat, con- tracts less in cooling, and has a less surface hardness than zinc. Tin, 5 parts; antimony, 1 part. Melts at a lower heat than either of the preceding alloys; contracts but slightly in cooling; is harder than tin, and sufficiently cohesive. It is readily oxydized, and should be poured as soon as melted. Fusible Alloys.—The following tabular view of the more fusible alloys, the respective properties of which are deduced from actual experiments, is given by Professor Austen, in a paper on "Metallic Dies."* Zinc is introduced into the table for the purpose of comparison. Melting Contrac- Hard- Point. tility. ness. 770° 01366 •018 440° 00633 •050 340° 00500 040 420° 00433 026 320° 00566 035 300° 00266 030 250° 00066 042 200° 00200 ■045 200° 1 00133 •048 * American Journal of Dental Science, vol. vi., p. 367. 1. Zinc......... 2. Lead, 2, Tin, 1....... 3. Lead, 1, Tin, 2,...... 4. Lead, 2, Tin, 3, Antimony, 1, . . . 5. Lead, 5, Tin, 6, Antimony, 1, . . . 6. Lead, 5, Tin, 6, Antimony, 1, Bismuth, 3, . 7. Lead, 1, Tin, 1, Bismuth, 1,. . 8. Lead, 5, Tin, 3, Bismuth, 8,. . 9. Lead, 2, Tin, 1, Bismuth, 3,. . METALLIC DIES AND COUNTER-DIES. 149 Professor A., in commenting on the preceding table, ob- serves :." The last column contains an approximate estimate of the relative brittleness of the samples given. As in the other columns, the low numbers represent the metals, so far as this property is concerned, most desirable. Those marked below 5 are malleable metals; those above 5 are brittle; zinc, marked 5, separates these two classes, and belongs to one or the other, according to the way in which it is managed." Allusion is here made to the process of annealing zinc, and which has already been adverted to when considering the latter metal in a former part of the work. The special method employed is thus described by the author already quoted. " The simplest way to anneal a zinc die is to place it in the melting ladle with about a tablespoonful of water, removing it in thirty seconds after the water has boiled away. If the fire is a very hot one, remove it immediately on the disappearance of the water. It will often happen that the die is annealed in the process of taking the counter-die. This will more certainly occur when Nos. 7, 8, or 9, (see table,) are used for the counter. For example, take tin : using 'a mass twice the size of the die, should it be heated to 540°, (100° above melting point,) it would not, allowing for loss of heat by radiation and contact with the cast-iron ring, (if one be used,) heat the zinc beyond 330°. Lead, cast as cool as it could possibly be poured, unless in a very heavy ring, (such as a ' cart wheel box,') or in quantity too small for a well-shaped counter, would be apt to raise the zinc at least to 400°, and so impair its malleability, whilst, if poured as hot as many are in the habit of doing, the zinc will remain as brittle as when first cast."* * To Professor P. H. Austen, whose various contributions relating to the mechani- cal department of practical dentistry have done much to unfold and elucidate the principles involved in the practice of this important specialty, the author would acknowledge his indebtedness for many of the valuable data and practical suggestions that may he found embodied in the foregoing chapter; and had not the limited space assigned us, compelled a condensation of his views on the subject of metallic dies and counters, we might have done the reader an essential service by transferring to our pages many of his eminently practical observations on the subject entire. CHAPTER V. PARTIAL DENTURES. The almost unlimited modifications in the form of substi- tutes designed to supply the loss of a portion only of the natural teeth, and the difficulties oftentimes incident to a harmonious arrangement of the teeth of replacement, as well, also, as the impracticability of always securing a perfectly satisfactory and efficient antagonism or closure of the artificial with the natural organs, frequently surround this process with peculiar embarrassments, and often render their successful application extremely difficult. They will, accor- dingly, be found to demand of the operator the exercise of greater skill, ingenuity, and discrimination than are usually required of him in the construction and application o f entire dentures. The various means employed in fixing or retaining partial sets of teeth in the mouth will be first considered. Either of the following methods may be adopted according to the pre- ferences of the operator, or the requirements of individual cases. 1. Pivoting an artificial crown to the root of a natural tooth. 2. Clasping to the natural teeth. 3. Wood pivots adjusted to tubed plates. 4. Pivoting plate to the roots of the natural teeth. 5. Atmospheric pressure. PIVOT TEETH. The process of pivoting or grafting an artificial crown upon the root of a natural tooth has been long practiced, and when skillfully performed with intelligent views of the various con- PIVOT TEETH. 151 ditions which recommend and justify the operation, it affords a valuable and unobjectionable means of substitution. The success of the operation will be greatly modified by the fol- lowing circumstances: 1. The Condition of the Root, its Appendages, and Sur- rounding Structures.—If the root to be pivoted is strong, well formed, and securely attached to the jaw, a living, healthy nerve present, and the peridental membrane free from disease, the operation will be attended, in a large pre- ponderance of cases, with the happiest results, and in respect to utility, comfort, and appearance, is superior to any other mode of substitution. If, however, the nerve of the tooth has been previously destroyed by disease, and inflammation and suppuration of the adjacent tissues have supervened, the probabilities of complete success will be greatly diminished ; for although these latter conditions may have been subdued for the time being by appropriate treatment, yet that a latent predisposition favoring their recurrence exists is manifest from the frequency with which unfavorable results follow the operation of pivoting even under circumstances so apparently favorable. In no case, we are convinced, unless under circumstances of peculiar exigency, should an artificial crown be attached to a root whenever the latter is complicated with incurable disease of the investing membrane or alveolar abscess. It may be safely affirmed that the failures so common to this method, and the consequent disrepute into which it has de- servedly fallen, as ordinarily performed, is fairly chargeable , not so much to unskillful manipulation as to a want of proper appreciation of the pathological conditions which clearly contra-indicate its employment. The facilities enjoyed by the dentist of the present day in the employment of the various approved methods of replacement, other than the one under consideration, no longer make it either necessary or pardonable to subject the patient to a course of treatment which unavoidably necessitates a perpetual drainage of de- 152 MECHANICAL DENTISTRY. praved and offensive pus either through fistulous openings in the gum or through channels provided by art. As respects the surrounding structures, it is well, with a view of removing any disturbing causes, to institute a careful examination of the mouth before inserting pivot teeth, and if any of the remaining teeth are found carious or incrusted with tartar, or the mucous membrane and gums are inflamed or otherwise diseased, they should be treated in accordance with the indications furnished by the particular morbid con- ditions present. 2. Diathesis of the Patient.—Cases frequently occur where there exists a marked constitutional predisposition to inflam- mation, in some of its various forms, from the operation of very slight causes, and although it is not always practi- cable to determine the exact degree of this tendency in all cases, yet some evidence of its existence may be acquired by inquiry of the patient as to the usual termination of injuries inflicted upon any portion of the body, as, whether they heal kindly by the ordinary process of reparation or tend to inflam- mation and suppuration. Whenever this predisposition exists in any marked degree, the operation, though other con- ditions may favor success, is liable at all times to terminate unfavorably, and, therefore, if performed at all, it should be done in the most careful manner and only under circumstan- ces that promise the best chances of success. It is well, in cases characterized by a phlogistic habit of body, to first re- duce the system somewhat by appropriate antiphlogistic treat- ment, and in no case should more than one tooth be inserted at a single sitting, and it will be prudent, in many cases, after having prepared the root, to defer the completion of the operation for a few days, or to adjust a tooth temporarily with a pivot loosely fitted, until the irritation, unavoidably- produced by filing, drilling, &c, has completely subsided. 3. Manner of Performing the Operation.—The healthy condition of the root and contiguous parts, and the ultimate utility of the substitute, may be very materially prejudiced PIVOT TEETH. 153 by careless, hurried, or injudicious manipulation; as where the remaining portions of the natural crown of the tooth are violently removed with excising forceps, or by the unskillful use of files in dressing the root, or drills in enlarging the central cavity, or by undue or misapplied force in the final adjustment of the artificial crown, or finally, by a faulty position of the tooth of replacement by which the root is sub- jected to injurious strain either by lateral pressure or prema- ture closure against those of the opposite jaw. By the operation of either or all of these causes, disease of a more or less intractable character may be induced which will impair the usefulness of the artificial organ and subject the patient to much present and future distress and annoyance. The Roots of Teeth to which Artificial Crowns are usually Attached.—The operation of pivoting artificial substitutes is necessarily limited in its application, being confined to the front or single-rooted teeth,—usually to the incisors and cus- pidati of the upper jaw. The same class of teeth of the in- ferior maxilla, though sometimes used for the purpose, are not favorably formed or situated, having a posterior incli- nation in the arch which renders ready access to them with the drill difficult or impossible, while the small size and flat- tened condition of the roots preclude the use of pivots of sufficient strength. Nor are the roots of the bicuspids of either jaw ordinarily admissible, as the lateral compres- sion of these fangs do not admit of the use of pivots of adequate size and strength to secure them against accident in mastication. Comparatively speaking, the roots best adapted to this process are those of the superior central incisors, and cuspidati or canine teeth. Preparation of the Root.—In the process of preparing the root for the attachment of an artificial tooth, all remaining portions of the natural crown should first be removed with suitable instruments. If the cervical portion of the tooth is comparatively sound and unbroken, this may be most expe- ditiously accomplished, and with less risk of injury to the 11 154 MECHANICAL DENTISTRY. root, by employing a very fine saw attached to a steel frame or carrier as shown in Fig. 32. The saw should be narrow enough to enable it to take a curvilinear direction in cutting, as this will enable the operator to separate the crown nearly • or quite on a line with the arched margin of the gum, thus Fig. 32. dispensing, in a great measure, with the use of the file in the final dressing of the cut end of the root. The saw should be passed along the side of the tooth to the gum and the crown separated either by cutting directly across to the opposite side, or as nearly on a line with the curvature of the anterior and posterior margins of the gum as possible without wounding the latter. During the opera- tion, the saw should be kept constantly wet, and the crown should be supported by the fingers. If the remains of the crown are friable, or if they consist of but fragmentary portions of enamel, they may be readily cut away with excising forceps, two forms of which are ex- hibited in Fig. 33. The one having square transverse cutting Fig. 33. edges, closing at right angles with the shaft, is generally em- ployed. Serious, if not irreparable injury may be inflicted by the careless or unskillful use of this instrument, either by producing so violent a concussion as to induce, in some in- PIVOT TEETH. 155 stances, incurable disease or absolute necrosis of the root; or by fracturing the latter in such a way as to unfit it for the reception of the pivot-crown. The forceps, therefore, should never be used to excise the crown with a single cut whenever any considerable portion of the root at the gum remains unaffected by disease, and even when the latter is friable or partially destroyed by decay, they should be used with great caution, cutting or chipping away small portions at a time, until as much of its substance is removed as is practicable with the forceps. After the use of the saw or excising forceps, any remain- ing portions projecting beyond the free margins of the gum should then be removed and proper shape given to the end of the fang with a file. A half-oval file, with a sharp and tolerably fine cut running obliquely across its convex sur- face, is the best for the purpose, and when in use, it should be kept constantly wet and free from clogging particles of bone. The end of the root should be filed down, anteriorly at least, a little below the free margin of the gum, care being taken not to lacerate its peridental attachment; in this way the artificial crown, when adjusted to the root, will unite so intimately with the gum in front as to render ex- posure impossible. The surface of the root, prepared in this manner, will present a concavity corresponding with the fes- toon of the gum. If a living nerve remains in the root it will not ordinarily be practicable, unless there is partial obliteration and conse- quent recession of the pulp cavity as the result of ossific de- posits, either to saw off the root on a line with the gum or even transversely, or to file the root even with the gum without inflicting insufferable pain. It will be necessary, therefore, either to remove the nerve through the carious opening in the crown before the latter is removed, or, if not exposed by the operation of sawing or filing, through an opening into the pulp made with a drill or cutting instru- ment after excision. The use of arsenic for the destruction of the nerve pre- 156 MECHANICAL DENTISTRY. paratory to its removal, though sometimes employed, is liable, under the most cautious management, to result in periodontitis and ultimate suppuration of the surrounding tis- sues, and should, therefore, never be used if circumstances aMmit of its removal by direct operation with instruments. By the latter method, the vitality of the root, through its peridental circulation, will be better preserved. Its extirpa- tion may generally be readily effected with the use of a three or four-sided, barbed, untempered broach, which, being small enough to penetrate freely to the apex of the fang, is thrust quickly to the bottom of the canal, rotated, and with- drawn; when, if the entire nerve does not come away adhe- rent to the broach, the operation may be repeated, with com- paratively little pain, until all portions of it are removed. A method more especially recommended by Prof. Taft, in his recent work on " Operative Dentistry," page 234, is the fol- lowing : " Take a very fine untempered steel wire, round and smooth, not larger than 34 to 36 of Stub's gauge-plate; flatten the extreme point, and turn it to an angle of from thirty to forty degrees; place the edge of this against one wall of the canal at the point of exposure of the pulp; press it steadily up the canal, with its edge bearing against the wall, as far as it will go, and then twirl it suddenly round: thus an excision is effected near the point of the fang, when the pulp with the instrument may be drawn away together; or, if not thus drawn, it may be caught with some fine point, and removed without pain. This manner of introducing the instrument, too, causes less pain than either of the others, for there are no • sharp edges or points presented in passing the instrument up the canal, to cut or lacerate the pulp. In the removal of the pulp from the teeth of young persons, care should be taken lest the instrument pass entirely through the foramen, at the apex of the fang; but with adults, there is little or no danger of such an accident." The nerve being removed, the canal of the root should be enlarged for the reception of a pivot. This is effected with PIVOT TEETH. 157 a suitable broach, or with drills of various forms. When the canal presents the form of a cleft or fissure, a spherical or cone-shaped bur-drill should be used; if, however, the nerve cavity approaches a cylindrical form, the operation will be more speedily performed with a four-sided broach, or what is still better, a spear-pointed or spiral drill; all of which are exhibited in Fig. 34. The natural opening in the fang Fig. 34. t should be enlarged to the depth of from one and a half to two or more lines, according to the length of the root; and the orifice should be made large enough to admit a pivot of sufficient size to secure the crown firmly in position. The direction of the drill in cutting should follow closely that of the natural canal in the root, since but a slight deviation in this respect may endanger the integrity of the latter by too great a thinning, or actual perforation, of its walls. In all cases, however, where the direction of the canal will admit of it, the shaft of the instrument should be held steadily on a line with the circle formed by the cutting edges of the ad- joining teeth, and either equi-distant between the latter or with such a lateral inclination as will give to the tooth of re- placement a symmetrical arrangement in the arch. During the operation, the drill should be kept constantly wet, and loose particles of bone should be washed from the cavity by occasional injections of water. The unenlarged portion of the nerve canal between the bottom of the pivot cavity and the apex of the root should 158 MECHANICAL DENTISTRY. next be thoroughly closed with gold in the manner usually practiced in fang filling,—all diseased conditions associated with the root, if such exist, having been previously subdued by appropriate treatment. , Fitting the Crown.—The pivot crown selected for any given case should correspond, as nearly as possible, in size and general configuration with its fellow of the opposite side, or, where several are inserted, with the form and size of the natural organs which they represent. The tooth or teeth of replacement should also harmonize in color with those imme- diately adjoining. The cervioal*portion of the crown applied to the root should correspond, as nearly as possible, in di- mensions with the filed surface of the root, and the adapta- tion of the two surfaces should be sufficiently accurate at all points to afford a firm basis for the crown, and, at the same time, to exclude perfectly all particles of alimentary or other solid substances, the decomposition of which would tend to the decay or disease of the root, or become offensive in the mouth. To secure such a coaptation of the articulating sur- faces, more or less filing of the root and grinding from the base of the crown will be required, so that while uniform contact of the surfaces is secured, the artificial crown will be made to occupy its proper relative position in the arch. The articulation of the crown and root may be very accu- rately obtained in the following manner: Attach to the crown a temporary pivot of wood that may be easily applied and removed; coat the surface of the root uniformly with some pigment, as carmine, rouge, or rose pink, and apply the crown, with pivot attached, to the root; the points of contact will be indicated upon the base of the crown by the adherent coloring matter; the colored portions are then ground down somewhat on an emery wheel, and this process is repeated until the entire surface of the base of the crown exposed to the pigment becomes uniformly coated. Whenever it is thought important to preserve the form of the porcelain crown unchanged, the operation may be reversed by coloring PIVOT TEETH. 159 the base of the latter and filing from the extremity of the fang until a perfect adjustment of the parts is secured. Another method, sometimes employed, may be adopted, and will secure an equally accurate bearing of the crown without subjecting the patient to the annoyance of repeated trials of the pivot tooth in the mouth. After having pre- pared the root in the manner already described, an impres- sion of the root and contiguous teeth is taken, and from this a plaster model is obtained. The drilled cavity in the root will be indicated on the model by a corresponding depression ; this may be extended into the body of the latter with an in- strument fitting the orifice and held in the exact position as when enlarging the canal in the fang. The model is then varnished, and a pivot being temporarily attached to the crown, the base of the latter may be ground, with or with- out the use of coloring matter applied to the model, until the articulating surfaces close uniformly, and the crown occupies the required position indicated by the adjoining teeth repre- sented on the mod^l. If the impression is correct, and the manipulations are' ^accurately conducted, a pivot tooth pre- pared in this manner will be found to fit the root and occupy a proper position in the circle with but little, if any, addi- tional filing or grinding. Attaching the Crown by means of Wood Pivots.—The usual method of fixing artificial crowns to the roots of teeth is by means of pivots of wood. Thoroughly seasoned white hickory of small growth, fine grained, and straight, compact fibers, istesteemed the best for the purpose. This substance is often used in its natural condition, but it will be much im- proved, both in respect to strength and durability, if pre- viously well condensed by forcing it through the holes of an ordinary draw-plate, or, what is better, through apertures of various sizes formed with smooth bevelled edges in a piece of ivory, steel, or porcelain of sufficient thickness. Cylinders of wood of uniform thickness throughout are most conveni- ently formed by splitting the timber into rods five or six 160 MECHANICAL DENTISTRY. inches in length, and from one-eighth to a quarter of an inch in diameter, trimming them with suitable instruments to a size one-third larger than required when condensed, and then passing them through the holes of a draw-plate, on the side where they form a cutting edge, commencing with the larger and passing to those of diminished calibre, until a cylinder of the size mentioned is obtained. These are then compressed in the manner before mentioned. When the draw-plate is used to compress the pivot, the latter should be passed through from the side opposite to the one used in forming the pivot. One end of the pivot, dressed to the proper size, is made to fit accurately the hole in the crown, care being taken not to fracture the latter when forcing the pivot into place. The depth of the enlarged opening in the root will determine the length to be given to the end of the pivot projecting from the crown, and the former may be readily determined by means of a gauge, (Fig. 35,) consisting of a rod of wire of a Fig. 35. size to enter freely the canal in the root, with a movable slide, to the end of which is attached a circular collar or flange. The end of the wire being pressed to the bottom of the canal, the flange, resting against the end of the root, will force the slide back upon the rod, and thus indicate with certainty the depth of the canal. The pivot being cut off at a point distant from the crown equal to the length of the uncovered end of the wire, is then dressed to the size of the orifice in the root. The pivot should be accurately fitted to the canal in the fang, but not so tightly as to require any greater force in adjusting the crown to the root than may be readily applied with the fingers. A pivot thus easily applied, will, when enlarged by the absorption of fluids, be so firmly PIVOT TEETH. 161 retained as to render its removal difficult; and even when moisture is excluded, adequate stability will be imparted to the attachment, provided sufficient depth is given to the cavity in the fang. Before adjusting the crown permanently, the pivot may be wrapt with one or two thicknesses of gold foil, and a thin layer of the same may also be placed between the crown and root. This is done with a view of protecting the pivot and inner walls of the fang from the action of fluids of the mouth. Other and more plastic substances, which are impermeable and not soluble in the secretions of the. mouth, are sometimes interposed between the root and crown, as Hill's stopping, collodion, mastic dissolved in ether, &c. The following ex- pedient recommended by Dr. S. D. Muse,* will answer the same purpose perfectly, but as it involves the use of " amal- gam," it may be regarded as, in some degree, objectionable. The root being prepared in the ordinary way for the recep- tion of a pivot, the central portion of its articulating surface is counter-sunk to the depth of half a line, leaving but a thin border on the outside to support the crown. The counter- sink is made with a bur drill, like that represented in Fig. 36. A pivot previously fitted to the root and crown, and of Fig. 36. the required length when the latter is adjusted to the root,. is then fitted to the canal in the fang, when the excavation around the pivot is packed with carefully prepared amalgam in sufficient quantities to completely fill the counter-sink. The crown is now applied to the pivot projecting from the root and pressed firmly to its place, the amalgam, as an im- permeable cement, luting the joint perfectly. It not unfrequently happens that considerable enlargement exists at the orifice of the canal, as the result of decay, and, * Dental Register of the West, vol. vi. p. 154. 162 MECHANICAL DENTISTRY. which, if not filled, will not only render the attachment of the crown insecure, but form a chamber for the accumulation of the secretions of the mouth and other substances, which decomposing, will render the substitute, in time, exceedingly offensive and pernicious in the mouth. In such cases it will be difficult to conform a wooden pivot accurately to the cavity, but the decayed portion and enlarged canal of the root may first be filled compactly with gold, and a central passage made through this with a drill for the admission of a pivot; or, in lieu of this, a highly polished and uniformly cylindrical steel wire of the same size as the crown-pivot, may be introduced into the prepared oanal of the root and gold packed around it and into the excavation in the end of the root even with the surface of the latter; after which the steel wire is carefully withdrawn. In either of the above cases, the gold should be securely fixed in place by forming small pits, or transverse grooves, in the walls of the fangs. Another method is to apply to the base of the crown, and round the pivot, a sufficient quantity of Hill's stopping, or other analogous material to fill perfectly the carious exca- vation ; this is then warmed until sufficiently plastic, by passing it through the flame of a spirit lamp, when it is ap- plied to the root and pressed up with sufficient force to expel any excess of material. Pivots of Metal and Wood.—Pivots are sometimes formed of gold wire encased in wood. These impart additional strength to the attachment, and at the same time enable the operator to change the direction of the crown by bending the pivot whenever the root stands irregularly in the arch. A hole, somewhat smaller than that in the fang, is drilled into a block of pivot wood, and into this is forced a gold wire__ that formed of gold and platinum being the best, as it pos- sesses greater stiffness and elasticity. The wood is then dressed down to a size a little larger than the canal in the rootj and then compressed. One end being fitted to the hole in the crown, the projecting portion of the pivot, cut to the PIVOT TEETH. 163 proper length, is trimmed to fit the opening in the root and applied in the manner before described. Another method is to close the hole in the crown with a cylinder of pivot wood; trim it even with the base of the crown; perforate its centre with a drill; and introduce into this one end of the wire, the surface of which is cut up into small barbs, or otherwise roughened to prevent it from drawing. A similar piece of wood is fitted to the orifice in the fang, and trimmed and drilled in like manner for the re- ception of the wire pivot—the latter being barbed and filed square to render it stationary when forced into place. Metal Pivot.—The best and most approved method of at- taching the crown to the root consists in adjusting a metallic pivot to a.gold tube attached to the root in such a manner that the substitute may be readily applied and removed by the patient. The tube which lines the enlarged canal in the root is constructed and applied in the following manner. A thin strip of ordinary gold plate, No. 28 or 30, and five or six inches in length, is first bent round a polished, cylindrical steel wire, the size of the intended pivot; these are both drawn together through a draw-plate until the gold tube is accurately conformed to the steel rod. The wire is then withdrawn, and the joint or seam in the tube soldered; before doing which, however, the joint should be coated on the inside with a mixture of whiting, to prevent the solder from flowing in upon the inner walls of the tube. A fine thread is then cut with a screw-plate on the tube, and having intro- duced into the latter a piece of the steel wire on which the tube is formed, the tube is seized with pliers or a small hand- vice, and screwed gently and carefully into the fang. The steel wire is then withdrawn, and the protruding portion of the tube removed with a file, or cut off with a fine saw, like the one represented in Fig. 32. Another method of fixing the tube securely in place, as recommended by Dr. F. H. Clark, consists in closing the lower end of it with a spherical-shaped cap, and attaching to 164 MECHANICAL DENTISTRY. the centre of this a small screw, which passes into the canal of the root, the latter being sufficiently enlarged and " tapped" for the reception of the screw. In either case, if the filed end of the root is hollowed out by decay, a collar or flange should be soldered to the end of the tube on which the crown rests; underneath and around which gold is packed, filling completely the carious excava- tion. The metal pivot may be fastened to the crown in either of the following ways: 1. A gold pivot fitting the orifice in the crown loosely is placed either in the centre or to one side of the latter, as the case may require, and around this solder filings or scrap are packed, filling in perfectly between the pivot and crown; the latter, with its cutting edge down, is then partly imbedded in plaster, and sufficient heat applied to flow the solder. 2. The use of a platinum pivot has been suggested by Dr. Frank Fuller, in which case he recommends the employment of jewellers' " soft enamel" as the uniting medium. The pivot is adjusted to the crown in the same manner as just described, and the enamel closely packed with suitable instruments around the pivot; the pieces are then invested, and sufficient heat applied to fuse the enamel. The latter may be obtained from jewellers or by pulverizing frag- ments of an old watch face. The pivot being permanently fixed to the crown, the pro- jecting portion is cut to the proper length and dressed to fit accurately the hollow wire lining the fang, but not so tightly as to prevent it from being easily introduced and withdrawn with the fingers. To render the crown stationary when ap- plied to the root, the pivot should be slightly flexed that it may press upon the walls of the tube when introduced; or a small bar may be soldered on one side and near the bottom of the tube, so that when the pivot is forced up it will be compressed between the bar and opposite side of the tube, the portion of the pivot facing the bar being somewhat flat- tened. By far the best method of giving permanence to the PIVOT TEETH. 165 pivot, however, is that recommended by Dr. Dwinelle, in which he directs one-half or more of the pivot to be split into two equal parts with a fine watch-spring saw. The sur- face is then cut up into numerous small barbs, opening down- ward, and the end of the pivot filed somewhat to a point, so that when the two sections are spread apart, it will readily enter the orifice in the fang (Fig. 37). Pivots so formed should be constructed of gold alloyed with sufficient platinum to render them stiff and elastic. The di- verging sections of the pivot when pressed into the canal will be found to bear with sufficient force against the sides of the tube to retain the toofh se- curely in place, and, at the same time, enable the patient to remove it at any time for the purpose of cleansing the substitute. Pivot Plate.—It sometimes occurs that the root to be used occupies a position in the arch inconsistent with a harmoni- ous arrangement of the tooth of replacement by the usual method of attaching an ordinary piVot-crown. Thus it may lie closely against one or other of the adjoining teeth, distant from the centre of the space; or it may have too great an anterior or posterior obliquity, or too great a lateral inclina- tion; or, again, it may range with the other teeth with re- spect to its direction, but may occupy a position entirely within the circle; in either case it will be difficult or im- practicable to give a proper relative position to the crown in the usual way. It is true, that any slight deviation from a just position or inclination of the fang may be compensated for by a corresponding inflection of the pivot, or by forming an abrupt angle to it where the crown and .root unite, or by placing the pivot on one side of the hole in the crown; but when the irregularities spoken of exist to any considerable extent, it will become necessary to adjust a pivot-plate to the root, and attach to the base an ordinary plate tooth, to which any desired position may be assigned. The root, in such case, should always be provided with a gold tube, constructed 166 MECHANICAL DENTISTRY. and applied in the manner already described, in order that the substitute may be easily removed and cleansed. The form of plate, with pivot attached, for the replace- ment of a single tooth, is shown in Fig. 38. The ' method of constructing the plate and attaching the 4 pivot will be fully described in a subsequent part of the work. Whenever an edentated space exists contiguous to the root to be pivoted, the plate may be made to extend into it, and two or more teeth, as the case may be, mounted on the same base, provided the root, employed as a means of attachment, is strong, firmly socketed, and in a healthy condition. (Fig. 39.) It will be sufficient to indi- cate, briefly, the means em- ployed to favor the escape of purulent secretions through the fang, either where the discharge exists at the time of the operation, or is subsequently induced by it. It is customary to afford a passage for it by cutting a groove along the wall of the root or on the side of the pivot, through which pus is conducted from the bottom of the canal through the opening between the root and crown. A plan recommended many years ago by Dr. W. H. Elliot, consists in passing a small gold tube through the centre of the pivot, and in having an opening continuous with it through the crown of the artifi- cial tooth. A modification of this practice was subsequently introduced by Dr. Coghlan, of Ireland, who substitutes, for the solid metal pivot, what he terms a capillary tube, con- sisting of a gold wire with its centre traversed by a very minute tube or canal, through which contained pus is per- mitted to escape from the apex of the root. In any case, where either of the above expedients is resorted to, the natu- PIVOT TEETH. 167 ral canal of the fang beyond the end of the pivot should, of course, be left open. Since the publication of the first edition of this work, the author has made some applications of vulcanized rubber to the process of pivoting teeth, which are deemed sufficiently practicable and important to justify, in this connection, a de- scription of the methods employed. Substitution of Rubber for Wood Pivot.—Take small gold wire (alloyed with platinum), say about the dimensions of a medium sized knitting needle, or about one-third the diame- ter of the enlarged opening< in the root, encase closely in a single layer of rubber, and vulcanize, but not so hard but that it shall possess some degree of flexibility, to admit of any necessary slight change in the direction of the pivot when permanently adjusting the crown. The relative ad- vantages of the rubber pivot traversed with wire are obvious. It is wholly impervious to the fluids of the mouth,—it will present greater resistance to the forces applied in mastication than either the crown or root, and will sustain the pivot- crown immovably in its place. Wood readily absorbs and becomes thoroughly saturated with the secretions, and it is chiefly from this source that the offensive odor so frequently associated with pivot teeth arises. Besides, wood being flexible, its fibres -are soon broken up or detached by the mobility of the crown, whose base is seldom accurately fitted to the root, thus necessitating a frequent renewal of the pivot. Wired Rubber Pivot, with Rubber Base.—The object of this method is to secure a faultless adaptation of the base of the crown to the filed extremity of the root, and by inter- posing one or two thicknesses of gold foil, to render the joint impervious to the fluids of the mouth. Having prepared the root in the ordinary manner, select and fit the pivot-crown to the vacuity, leaving something of a space posteriorly between the base of the crown and the root. Fit a wood pivot to the enlarged opening in the root, accurately, but in 168 MECHANICAL DENTISTRY. such manner that it may be easily withdrawn—the end pro- jecting from the root should be trimmed down to say half the diameter of the hole in the crown, so that when the latter is applied in the manner to be mentioned directly, some margin will be left, admitting of a proper adjustment of the crown when applied to the space. With the wood pivot in the root, fill the hole in the crown with stiffened wax, warm the pivot crown sufficiently to soften the wax, and apply it to the root in the desired position with respect to the other teeth. When the wax has hardened somewhat, withdraw the crown carefully on a line with the pivot, bringing the latter with it; then add sufficient softened wax to the base of the crown to fill in the gap or space between the crown and root; replace the tooth with the pivot attached, and press up until the crown again takes its proper position in the vacuity. By this rneans we get an impression of the filed extremity of the root, and at the same time secure an accurate relation of all the parts. The crown and pivot are then carefully re- moved. Now, take plaster and pour a small quantity on a piece of paper or card and sink the pivot into it until the surface of the wax at the base of the crown rests upon the plaster, and then build the latter up upon the front part of the crown to the cutting edge, thus forming a shallow bed for its anterior face. When the plaster has hardened, warm the model to soften the wax, and then remove the crown and wax,—the pivot will be found remaining in the model, but which, if previously oiled, can be readily drawn. We have now in the model an accurate representation of the end of the root, the size and direction of the fang canal, and, when the crown is replaced in its shallow bed, also the space between the crown and root to be filled in with rubber. The hole in the model, corresponding with that in the root, should be enlarged somewhat, and this may be done with the same drill used in enlarging the orifice of the fang. After varnish- ing the hole in the model, pack in softened rubber until full, and insert into this the gold wire previously heated, one end PIVOT TEETH. 169 projecting a line or so. Next fill the hole in the crown with rubber, heat the crown, and press it down upon the model and over the gold wire until the crown goes accurately into the depression made for it in the model. Then pack softened rubber into the space between the base of the crown and the model until it is filled in compactly. The whole is then en- cased in plaster, confined in a flask and vulcanized. If the foregoing manipulations are carefully conducted, the crown will go to its place in the mouth with unerring accuracy, and by placing one or two thicknesses of gold foil between the rubber and root, the joint will be rendered impervious. There is still an additional advantage in this method. All are aware of the liability of a wood pivot drawing from the crown where the latter is short and the hole in it shallow. The rubber pivot, vulcanized in the crown, cannot be drawn from the shallowest cavity without fracturing the crown. This test was submitted to members of the Cincinnati Association, none of whom, holding the crown in the fingers, could with- draw the pivot with pliers, the hole being a fraction over a line in depth. The same tooth was subsequently fixed in a vice, and in drawing the pivot with pliers, the crown was fractured in the attempt. In very many cases this circum- stance is of great value. Canal of Fang lined with Rubber Tube for the reception of Gold Pivot Vulcanized into a Crown with a Rubber Base.— The manner of forming a rubber tube for the fang is exceed- ingly simple. Draw a wire of gold alloyed with platinum, say half the ordinary diameter of enlarged opening in the root, and from two to three inches in length; encase this in a single lamina of thin tin leaf to prevent the rubber from adhering to the wire, which will enable the latter to be readily withdrawn after induration of the gum. Then wrap the wire tightly in a single layer of rubber, encase in plaster and vulcanize; place the vulcanized tube in a vice, and with- draw the wire with pliers. Digest the tube in solution of muriatic acid to remove tin. The same wire is used for pivot, 12 170 MECHANICAL DENTISTRY. and will enter the tube readily, but accurately. Dress down a portion of the tube to fit the orifice in the fang, and force the tube up with wire encased, and then remove the latter, and file off the tube to the end of the root. The tube may be fixed more securely in the fang by smearing its surface with a little thin osteoplastic material before introducing. The crown being fitted to the space, and the gold pivot placed in the tube with a line or more projecting, the hole in the crown is filled with wax, and applied to the base of the crown, and the same manipulations practiced to secure an impression of the filed extremity of the root, as previously described. Re- move the crown, with wire pivot attached, carefully from the mouth, and imbed in plaster precisely in the manner as before related. In this case, however, after the crown is removed from the model, the wire pivot is allowed to remain at- tached to the model, when the hole in the crown is filled with rubber, the latter softened with heat, and the crown forced back into its place upon the model, the projecting end of the wire pivot penetrating the rubber in the crown. The inter- space is then packed as before, the whole encased and vulcan- ized. In re-applying the tooth to the fang in the mouth, the pivot will be found to enter the tube too freely to give it the proper support,—the latter may be secured either by flexing the pivot upon itself slightly, or by splitting it part away with a fine watch-spring saw, and spreading the ends apart, as recommended by Dr. Dwinelle. By this method, the patient will be enabled to remove the crown at will, for the purpose of cleansing the parts, though if the crown can be secured in such manner >that there shall be no movement of it, or change of relation with the root, under any force that may be applied to it, (a practicable thing with this method,) and the joint can be rendered perfectly impervious, it is preferable to make the crown stationary by fixing it immova- bly in the first instance. Substitution of a Plate for a Pivot Tooth.—Line the canal with rubber tube as before. Adjust a plate tooth to the PIVOT TEETH. 171 space, and then solder on a gold backing. Place the wire pivot in the tube, with the end projecting. Hold the crown firmly to its position in the vacuity, and press softened wax or plaster around the end of the pivot and down upon the filed surface of the root and against the gold backing. Re- move crown and pivot carefully, and imbed in plaster and sand; remove the wax or plaster, heat up and solder the pivot to the backing. Reapply to the mouth, press in softened wax posteriorly down upon the filed extremity of the root; remove carefully; imbed as before in plaster; remove wax, and pack softened rubber down upon the portion of the model representing the filed end of the root, around the pivot and against the gold backing, adding sufficient gum to enable you to dress it, when vulcanized, to the form of the adjoining natural organs. If, after the operation of engrafting an artificial crown, in- flammation of the peridental membrane and surrounding structures ensues, active measures should be immediately in- stituted for its reduction. In a majority of cases, active suppurative inflammation is induced either by shutting up an habitual discharge from secreting surfaces at the apex of the root, or by a forcible injection of the air contained in the nerve canal into the sensitive tissues beyond the apex in the act of pressing up a tightly fitting pivot. These two cir- cumstances, in conjunction, sometimes, with rough and un- skillful manipulation, afford a rational explanation of the needless failures so common to this method, and enforce the necessity of first radically treating any existing disease in the appendages of the root, and of afterward filling to the apex before introducing a pivot. Where inflammation re- sults from the injudicious application of the pivot as just stated, it will ordinarily be sufficient to remove the latter if timely application is made by the patient for relief; and it is important in all cases to instruct the latter in reference to the necessity of early attention to any disturbance that may accrue from the operation. In addition to the withdrawal of 172 MECHANICAL DENTISTRY. the pivot, it will be prudent at the same time to direct the application of some active counter-irritant to the gum di- rectly over the affected root, until resolution, if practicable, is effected. The topical remedies will sometimes be rendered more effective, especially when the local trouble is associated with an inflammatory diathesis or febrile condition of the system, by such constitutional treatment as will tend to equalize or diminish the force of the circulation, as general blood-letting, emetics, saline aperients, nauseants, arterial sedatives, &c. CHAPTER VI. PARTIAL DENTURES RETAINED IN THE MOUTH BY MEANS OF CLASPS ATTACHED TO THE NATURAL TEETH. Remarks on the Use of Clasps.—Clasps, or metallic bands, have been long and very generally employed as a means of retaining parts of sets of teeth in the mouth, and are still almost exclusively used for that purpose by a large class of practitioners. When these appliances are skillfully adjusted, and all the conditions pertaining to the mouth and remain- ing natural teeth are favorable to their application, they afford a certain, permanent and satisfactory means of sup- porting partial dentures, and may be employed, under such circumstances, with comparative safety to the natural or- gans. When it is remembered, however, that in a lament- ably large proportion of cases, clasps are carelessly or un- skillfully formed and fitted to the teeth; that the organs of support are often indiscriminately selected, and are neither adapted in form, situation or structure for such uses; and that they are frequently diseased and insecurely attached to the jaw, or are mutilated for the reception of clasps; we can readily understand to what unlimited extent this method is subject to abuses. In fact, no other special process in me- chanical practice has been so fruitful of evil as that under con- sideration, and the opprobrium which but too justly attaches to it in professional as well as popular estimation, is chargeable more properly to bad faith and unskillfulness on the part of the operator, and to want of attention in respect to the clean- liness of the substitute and the organs of the mouth on the part of the patient, than to any inherent unsuitableness of the method itself. Nevertheless, it must be admitted that, 174 MECHANICAL DENTISTRY. under the most favorable circumstances, the teeth clasped are not wholly exempt from liability to injury, and this cir- cumstance in itself renders it the more imperative that the process should be surrounded by all the safeguards that skill and ingenuity can devise. The opinion, at one time current, that the injury inflicted upon the teeth by clasps was mainly the result of mechanical action, has given place to the more defensible view that the causes concerned in its production are chiefly of chemical origin. Thus, the secretions of the mouth with particles of alimentary and other substances being retained between the clasp and tooth for a sufficient period of time, and exposed to the favoring conditions of warmth and immobility, suffer a process of putrefactive decomposition by which acids are eliminated, and which, in their nascent state, act with perceptible energy upon the bone constituents of the tooth, producing decay. The rapidity and extent of this action will depend much upon the nature and quantity of the acids libe- rated ; the structural characteristics and vital resistance of the teeth; the mechanical execution, adaptation, and compo- sition of the plate; and the personal habits of the patient with respect to cleanliness. The most usual seat of decay in these cases is at the neck of the tooth where the enamel is thinest, and is sometimes limited to a circumscribed spot, but oftener extends on a line with the gum involving nearly or quite all of that part of the neck of the tooth embraced by the clasp. At first the enamel becomes bleached and softened as though macerated, and is ordinarily very sensitive to both chemical and me- chanical irritants. With a continuance of the cause, the superficial portions of the affected parts become more and more thoroughly disintegrated, and sooner or later assume the open form and characteristics of ordinary decay. If, as was formerly supposed, decay or solution of tooth-bone in these cases resulted from mechanical attrition, or wearing away of the enamel, the injury would be inflicted at points CLASPS. 175 distant from the neck of the tooth, where the clasp lies in more direct and immediate contact with the protuberant portions of the crown; but we find that decay, from this cause, is not only of infrequent occurrence at such points, but, on the contrary, the enamel here is frequently found con- densed and polished by the mechanical action of the clasp. Certain conditions of the plate and clasp undoubtedly favor chemical action and accelerate the destruction of the tooth; as where the clasp bears unequally with sharp and unfinished edges upon the tooth, or where the base is faulty in its adaptation to the mouth, admitting by its mobility, of ir- regular traction or pressure upon the organs of support. Whenever the artificial appliance is thus unskillfully con- structed and applied, and free interspaces are furnished for the lodgment and retention of particles of food, and the teeth clasped are defective in structure, and we have conjoined with these an utter disregard of cleanliness in respect to the substitute and remaining natural teeth, the destruction of the latter is certain, rapid, and generally irretrievable. The Teeth to which it is most proper to attach Clasps.—The utility, comfort, and appearance of a partial set of artificial teeth in the mouth, will depend much upon the fitness of the natural organs selected for the purpose of support. " A clasp," says Professor Harris, " should never be applied to a loose tooth, or to one situated in a diseased socket, or which is so much affected by caries as to render its perfect resto- ration and permanent preservation impracticable, and when none but such can be had, the proper course to pursue is to extract every tooth in the jaw, and replace the loss of the whole with an entire upper set. The application of clasps to diseased or loose teeth, always aggravates the morbid con- ditions of the parts, and causes the substitute which they sustain, to become a source of annoyance to the patient. Besides, such teeth can be retained in the mouth only for a short time, and when they give way, the artificial appliance 176 MECHANICAL DENTISTRY. becomes useless, and even while it is worn, it is not held firmly in place, but is moved up and down by the action of the lips and tongue, so that its presence can hardly escape observation from the most careless observer."* Teeth, also, that are too short to admit of sufficient breadth to the clasp to impart stability to the substitute, and those that stand very irregularly in the arch, rendering it difficult for the patient to apply and remove the appliance, are unsuitable as organs of support. In respect to the individual classes of teeth, it may be ob- served that the incisors, both as regards form and situation, are inadmissible for clasping, and are, therefore never used for this purpose. The cuspidati, likewise, being placed con- spicuously in the front part of the mouth, cannot be securely embraced without manifest exposure of the clasp; besides, the conical form of these teeth makes the use of a very slender clasp indispensable; hence, these teeth are rarely em- ployed, and may only be used when, in the judgment of the operator, the necessities of the patient for the time being seem to require it. Either the anterior or posterior molars, when sound and firm, offer, in respect to their general conformation and po- sition in the arch, the most desirable and efficient support for parts of sets of teeth. The crowns of these teeth generally afford ample breadth to the clasp ; have nearly parallel walls; and furnish, by the strength and immobility of their attach- ments to the jaw, the greatest security to the artificial ap- pliance. The anterior molars are preferable where these are remaining in good condition, or are susceptible of being properly restored and preserved if diseased or carious. Of the bicuspids, the posterior are to be selected, if prac- ticable, as these better favor the concealment of the clasps; to effect which more perfectly, in the use of either the first or second bicuspids, it will be sufficient in many cases to em- brace only the posterior half of the crown. * Principles and Practice of Dental Surgery, p. 717. CLASPS. 177 The dentes sapientise, or wisdom teeth, will seldom admit of the application of clasps, as the crowns of these teeth are usually very short and cone-shaped, the walls converging abruptly from the gum; besides, the retractive forces applied to the anterior teeth of the substitute, would, on account of the increased leverage consequent upon the extension of the plate back to these teeth, tend either to disengage the clasps or produce displacement of the teeth to which they are ap- plied. In supplying the loss of one or more of the inferior in- cisors, the appliance should, as a general thing, be attached either to the anterior or posterior bicuspids, as these teeth stand more nearly vertical in the arch. In fixing partial lower dentures, it will be sufficient to simply provide against mobility of the base, as they are favored rather than op- posed, as above, by gravitation. The replacement of the in- ferior teeth posterior to one or both bicuspids, however, are more frequently demanded; in which case it is customary to attach the clasps to the teeth immediately in front of and adjoining the vacuities on each side. It will not, however, be necessary to attach clasps in these cases whenever the edentated portions of the jaw present a distinctly scooped form, or marked concavity of outline, forming a kind of bed for the plate. If, on the other hand, the ridge falls back with a tolerably uniform inclination from the teeth in front, with no sufficient elevation at the base of the coronoid pro- cess, it may become necessary to provide against backward displacement of the substitute by attaching clasps, as before stated, to the teeth immediately in front. In any case, if the dentes sapientiae remain, partial or stay clasps may be attached to each heel of the plate, and so adjusted as to rest against the anterior face of these teeth, obviating entirely the necessity of clasps in front. Separation of the Teeth, by Filing, for the Reception of Clasps.—The practice of separating the teeth with the file to provide for the application of clasps should always be 178 MECHANICAL DENTISTRY. avoided if practicable, since the liability of the teeth thus denuded of enamel to decay is greatly increased under cir- cumstances so favorable to their disintegration. In the case of young subjects, especially, where the teeth are but im- perfectly consolidated, and in adults whose teeth are defec- tively organized, presenting but a feeble resistance to the disorganizing agents usually present in the mouth, the use of the file, for the purpose indicated, is eminently pernicious, and should never be resorted to until every other means of supporting the artificial appliance have been fairly and pa- tiently tried. Whenever a plain necessity for this operation exists, a careful examination of all the teeth, to which it is proper to apply clasps, should be made, and if decay is found upon their proximate surfaces, the separation should be made be- tween the teeth so affected; and this circumstance should, in most cases, determine the selection, though the affected tooth or the one adjoining may not be esteemed, in other respects, the best for the purposes of support. If decay exists on the proximate surface of only one of the teeth to be separated, a safe-sided file should be employed, and the filing confined en- tirely to the carious tooth, leaving the enamel of the one ad- joining unbroken. The cavity of decay should be well filled, and the filed surface thoroughly condensed and polished with a burnisher. Modifications in the Form of Clasps.—1. Plain Band.— The most usual form of clasp is that shown in Fig. 40. It consists of a plain metallic band of greater or less width and thickness, and is made to embrace the larger portion of the circumference of the tooth. In respect to the general properties of metal- lic clasps, it may be said that they should be, as nearly as practicable, of the same CLASPS. 179 quality or fineness as the plate or base to which they are united; they should be heavy enough to impart adequate se- curity to the attachment, say twice the thickness of the base, and exceeding this in some cases; and sufficiently elastic to embrace accurately the more contracted parts of the teeth after having been temporarily forced apart in passing over the enlarged portions of the crowns. In constructing a plain band or clasp, a strip of sheet lead or other pliable substance may first be fitted accurately to the plaster tooth, making it of the required width, and shaping the edge next the gum in conformity with the irregularities in the latter around the neck of the tooth; the exact counterpart of the pattern thus obtained is then cut from the plate to be used in the formation of the clasp. The strip thus obtained is then bent with round-nosed or grooved pliers, (Fig. 41,) until Fig. 41. conformed as perfectly as possible to every portion of the surface of the tooth embraced by it. This coaptation should be sufficiently accurate to exclude perfectly all solid sub- stances from between the clasp and the tooth. A more accu- rate adaptation of the clasp may be secured in the following manner: First secure a pattern, as before described, and by this cut from a thin strip of platinum, say No. 30 or 32 of the gauge-plate, a band of the required size. and form, and press or burnish it accurately to the form of the plaster 180 MECHANICAL DENTISTRY. tooth. The soft and pliant condition of this metal will admit of its being easily adapted to any irregularities upon the lateral walls of the tooth. The band thus molded to the tooth is then carefully removed from the model, or the mouth, if fitted to the tooth in the latter, and its central portion filled with a mixture of plaster and sand with a small metallic wire or bar passing through the centre to support it while soldering. The outer or exposed surface is then smeared with a mixture of borax, and small scraps or fragments of gold plate of equal fineness with the mam plate, are placed at intervals and fused with the blow-pipe until diffused uniformly over the surface. Small pieces may be added from time to time, until the required thickness of the clasp is obtained. The piece should be heated uniformly throughout to induce an even flow of the gold over the ex- terior surface of the platinum ring. By this method a fault- less adaptation of the clasp to the tooth may be secured, provided the form of the latter is correctly represented on the model. In all cases where the plain band is used, it should be made as broad as the tooth will admit of, as a clasp so formed gives greater stability to the plate, and does not endanger the tooth clasped in any greater degree than a narrow one. 2. Standard Clasp.—To guard more perfectly against the retention of vitiated secretions and particles of food around the neck of the tooth, a method of constructing clasps has been devised and introduced to the notice of the profession by Dr. C. W. Spalding; which, by leaving the cervical portion of the tooth in a great degree uncovered, permits the action of the tongue and the natural circulation of the fluids of the mouth to wash or cleanse that portion of the tooth most liable to be injuriously affected. In commenting on this method, Dr. S. remarks: " The writer has for many years been in the habit of employing narrow clasps for the purpo- ses of support, making them of sufficient thickness to give the required strength, and attaching them to the plate by CLASPS. 181 means of standards, so arranged as to induce the removal of accumulations between the clasp and tooth, by the circulation of the saliva, (Fig. 42.) The use of one or more standards as a means of attachment, also provides, by a variation of their length, for the grasping of the tooth at any desired point. If the tooth is long, and particularly if it is at the same time bell- crowned, the point selected should be toward the grind- ing surface, as far from the gum as is found practicable. If the tooth is short and of such form that it can be successfully clasped at no other point than that near the gum, the plate should be cut away at least one or one and a half lines from the tooth, and standards introduced for the purpose of pro- moting circulation, by affording a free passage for the ingress and egress of fluids. These standards should also be narrow no wider than the clasp itself, and should constitute the only point of union between clasp and plate. Half-round wire will be found to be a very convenient article for making clasps. The particular form of the clasp is, however, imma- terial, if it is both narrow and strong."* 3. Scalloped Clasp.—Somewhat analogous in form to the clasp just described, and constructed with a similar design, is the one recommended by Dr. B. T. Whitney. A plain band of gold is fitted to the tooth in the manner first described, when that portion of it next the gum on the lingual side of the tooth is scalloped or cut away in the form of a semi-circle or arch, the ends of the clasp being in like manner narrowed sufficiently to relieve them from contact with the neck of the tooth. The intermediate points of the clasp which serve to unite the latter to the base may be two or more in number, and should be wide enough to impart adequate strength to * American Dental Review, vol. i., p. 12. 182 MECHANICAL DENTISTRY. the attachment. A clasp so formed and applied to the base will present very nearly the appearance of the standard clasp as represented in Fig. 42. Dr. W. recommends solder- ing but a single point at first, and then having tried the plate in the mouth and adjusted the clasp properly to the tooth, remove and solder the remaining point or points. 4. Partial or Stay Clasp.—This form of clasp, instead of embracing the tooth, is designed to steady or fix the substi- tute in place by simply resting against one side of the tooth to which it is applied. (Fig. 43.) They should be so con- nected to the plate that when pressed over the enlarged portions of the crowns of the teeth, they will spring readily into place and adapt them- selves closely to the more contracted parts near the gum. In cases where there is no adequate opposing force to that exerted by the clasp, care should be taken that no more pressure is produced than is necessary to keep the substitute in place, as, without this precaution out- ward displacement of the teeth is liable to occur, and the ap- pliance, losing its bearing upon the teeth, soon becomes loosened and insecure in the mouth. The result alluded to should be particularly guarded against in the case of young subjects whose teeth are easily moved by the application of very slight forces. Modifications in the form of Plates for Partial Dentures supported in the Mouth by Clasps.—The particular form and dimensions of a plate, when clasps are used, will be mainly determined by the number and position of the teeth to be replaced, and by the location of the natural organs to which the clasps are attached. It will be sufficient in this place to indicate the leading forms as they relate to the substitution of the several classes of teeth. In supplying the loss of a superior central or lateral incisior, it will be sufficient in many cases to attach the plate to either a bicuspid or CLASPS. 183 molar on the same side, as in Fig. 44. If two or more of the front .teeth, however, are to be replaced, it is better to extend the plate on each side of the palatal arch, and attach to a bicuspid or molar (Fig. 45); or to a bicuspid on one side, and a molar on the other; unless two firm and well-form- ed teeth on the same or the op- posite side can be commanded, (Fig. 46,) while those upon the other could not be employed without a separation. In all cases where it is necessary to extend a narrow plate from the extreme front part of the Fig. 45. Fig. 46. mouth to a single tooth situated posteriorly in the arch, the former should be strengthened by soldering a narrow rim of plate or half-round wire along the border next the teeth, and the clasp should, whenever practicable, pass in front of, and embrace, the anterior face of the tooth to which it is applied. If an anterior bicuspid is to be replaced, the plate may be attached to the adjoining bicuspid ; (Fig 47, right side,) or if both are absent, then to the first molar; (Fig. 47 left side,) or the clasp may embrace both of the latter if remaining Fig. 44. 11532 184 MECHANICAL DENTISTRY. and no separation between them exists. Fig 48 represents the form of a plate supplying the loss of teeth at intervals ; Fig. 47. Fig. 48. the clasp on one side embracing the posterior bicuspid in front and extending round the back part of the adjoining molar. Fig. 49 represents the form of plate supplying the loss of the two bicuspids on one side, and the anterior bicuspid and molar on the opposite, the plate being attached to an anterior . molar and second bicuspid. The antero-posterior extension of the plate, as exhibited in connection with the bicuspid tooth, greatly favors the stability of the substitute, and, pro- vided the plate and clasp are accurately fitted to the parts, the support afforded by a bicuspid tooth under such circum- stances is equivalent to that furnished by a firm and well formed molar clasped as shown on the opposite side. A base so supported may be made to sustain any number of teeth with the greatest security. Either the anterior or posterior molars, if firm and se- curely attached to the jaw, will afford adequate support to a plate replacing all of the teeth anterior to them. (Fig. 50.) Even a single molar situated on either side of the arch, if similarly circumstanced, may be made to sustain, with toler- able firmness, a base supplying the loss of all the remaining teeth,—though, ordinarily, it is better to extract such a tooth and substitute an entire upper denture. In all cases, CLASPS. 185 where any considerable number of teeth anterior to those clasped are to be replaced, and a vacuity on the ridge exists Fig- 49. Fig. 50. posterior to the latter, the plate should be extended back and overlap the ridge, (Fig. 51,) the latter affording a Fig. 51. Fig. 52 counter-point of resistance when traction is made upon the anterior teeth, thus directing the forces applied more on a line with the long axes of the teeth that sustain the ap- pliance. In supplying the loss of the inferior molars and bicuspids, or any number of these teeth, the form of plate represented in Fig. 52 is generally employed. The parts of the plate overlapping and resting upon the ridge behind, are connected with each other by a narrow strip of plate extending round 13 186 MECHANICAL DENTISTRY. the ridge in front on the lingual side of the anterior teeth. This latter portion of the plate should be accurately swaged to the form of the gum on which it rests, and should be made narrow enough to avoid encroaching upon the reflected portion of mucous membrane, the glands beneath the tongue or the frsenum linguae. To avoid wounding these parts, and to allow them unobstructed play, it will be necessary to make this portion of the plate quite narrow; and as a single thickness of plate would not impart adequate strength, it is customary to double this connecting band—the duplicate band extending back to the lateral wings of the plate, and crossing them obliquely, as indicated by the dotted lines in Fig. 52. Additional strength will be given by doubling the entire plate, but this is not generally required. The outer borders of those portions of the plate overlapping the ridge may be turned up to the depth of from half a line to a line to form a groove or socket for the reception of the ends of gum teeth, or blocks, if such are used; while the inner margins should terminate in a rounded edge, extending from heel to heel of the plate, this form being given to it either by turn- ing the edge over and filling in the groove with solder, or by soldering a narrow strip of plate or half-round wire along the border. The circumstances or conditions which make the use of clasps necessary in these cases, as well as those, also, which contra-indicate their employment, have already been noticed. The practice of extending a narrow band or wire from the sides of the plate round the outer border of the ridge in front of the anterior teeth, to prevent a back- ward displacement of the base, is liable to produce irritation and tenderness of the mucous membrane immediately over the roots of the anterior teeth, and should, therefore, never be resorted to, unless there are no teeth remaining to which clasps may be applied. If the appliance is designed to restore the loss of teeth re- cently extracted, and where but little or no change has oc- curred from absorption of the parts, the portions of the plate CLASPS. 187 which pass in between the adjoining teeth should terminate a line or more within the outer circle of the remaining teeth; and where the space, if it happens in the front part of the mouth, admits of two or more teeth, the edges of the extended portion of plate should be scalloped in correspond- ence with the festoons of the gum, as seen in Fig. 53. In such cases, plain or plate teeth, by which is meant those which repre- sent only the crowns of the natural organs, should be employed; these, resting on the edge of the plate, will overlap somewhat, with their an- terior edges resting directly upon the gum in front, taking the place occupied by the crowns of the ex- tracted teeth. On the other hand, if sufficient time has elapsed after the extraction of the teeth to permit the changes in the form of the ridge to occur incident to partial or complete absorption of the parts, and a greater or less concavity exists between and above the teeth on the outside of the jaw, the plate, where it passes into the interspace, should extend some distance over the border of the ridge. Swaging or Stamping the Plate.—Having determined upon the proper form and dimensions of the plate for any given case, its outlines may first be traced upon the model; from this an exact pattern in lead may be obtained, or the pattern may be sufficiently ample to partially overlap the cut extremities of the teeth when the latter are not repre- sented upon the die, having been previously cut from the model. The outlines of the pattern are then traced upon the plate of gold or other metal to be used for the base. The redundant portions of plate are then cut away with plate shears and forceps, and the edges trimmed smooth with a file. A very convenient and almost indispensable instru- ment for cutting away the plate where it describes the palatal 1S8 MECHANICAL DENTISTRY. curvatures of the teeth, is a plate forceps, as exhibited in Fig. 54. Fig. 54. The plate cut to the proper form is now placed upon the die and brought as nearly as possible into adaptation with a wooden ©r horn mallet; it is then placed between the die and counter, the latter resting on an anvil or other equally resist- ing surface, when the two metallic pieces are brought forcibly together with a few -steady and well directed blows of a heavy hammer. Tilting of the die, resulting sometimes unavoidably from a one-sided blow, may be obviated by placing a cone- shaped piece of cast-iron, brass, or zinc over the die, the base of the cone resting on the back of the die; by this expedient the force of the blow is equalized and concentrated more directly over the die. The metallic swages should, at first be brought cautiously together, and should be separated after the first blow or two to enable the manipulator to detect and remedy any malposition of the plate, before it becomes in- tractable from continued swaging. If, in the pro'cess of stamping, any portion of the plate is found cracking or part- ing, its further extension at that point may be prevented by flowing a little solder at the termination of the fissure. During the progress of swaging, the plate should be fre- quently annealed, which is done by bringing it to a full red heat under the blowpipe, or by placing it in the furnace; the plate is thus rendered more pliant and can be more readily CLASPS. 189 and perfectly forced into adaptation to the irregularities on the face of the die. If, after somewhat protracted swaging, the plate is not con- formed perfectly to the face of the die, another and unused counter should be substituted for that in use; and, indeed, it is better in all cases to have duplicate copies both of the die and counter in reserve with which to complete the swaging, inasmuch as more or less deformity of both swages unavoid- ably occurs before the plate is brought into very accurate coaptation with the die. The stamping conducted thus far, the plate may be applied to the plaster model, and if found too full at any points, it should be trimmed with a file to the exact dimensions required. The margins of the plate adjoin- ing the necks of the teeth should be permitted either to lie closely to them, or should be cut away, leaving a space equal to a line or more between the plate and the teeth; for if but a very narrow line of uncovered gum remains at these points, injury to the parts immediately surrounding the necks of the teeth is more liable to occur from strangulation of the inter- posed gum than if the plate were further removed from the teeth or rested directly against them. If the portion of plate which passes in between the re- maining teeth is quite narrow, as where but a single tooth is to be supplied, it should be strengthened by wiring the edges or doubling the plate at such point. It is also advisable in many cases, in order to provide more perfectly against frac- ture or distortion of the base in mastication, to wire or double the entire border of the plate adjoining the necks of the teeth. Narrow bands of gold resting against the necks of the teeth, constructed and adjusted after the manner of stay clasps, are sometimes soldered to the edge of the plate next the teeth; but unless the substitute is frequently re- moved from the mouth and cleansed, as well, also, as the teeth to which they are applied, serious injury is likely to be inflicted upon the teeth implicated. The edges of those parts of the plate occupying the vacui- 190 MECHANICAL DENTISTRY. ties on the ridge should be filed thin to admit of a more ac- curate adaptation of the artificial with the natural gum, and should not, as before observed, ordinarily extend beyond the outer circle of the contiguous teeth, allowing the gum extremity of the artificial tooth to overlap and rest directly on the natural gum above. If, however, the concavity be- tween and above the teeth on the external border of the ridge is considerable, the interdentinal portions of plate should overlap the border completely and underlie the porcelain gum. Uniting the Plate and Clasps.—Having proceeded thus far in the operation, the plate and clasps should next be united to each other, and the utility and comfort of the ap- pliance in the mouth, as well as the safety of the natural organs used for the purpose of support, will depend, in a great measure, upon the accurateness of the relation of the several parts of the appliance to the organs of the mouth; it being a matter of primary importance that the various parts of the substitute should be so adjusted to the remaining teeth,—especially those to which the clasps are applied,—and the ridge and palate, that it shall not, in any material degree, act as a retractor upon the:organs of support, or furnish interspaces for the lodgment of food, while at the same time it should be so fitted as to be easily removed and applied by the patient. The clasps having been fitted to the plaster teeth and the base swaged to the form of the palatal arch and ridge, the plate is placed in its proper position in the mouth and an im- pression in wax taken of the latter with the plate in place. The impression with the plate adhering, is then removed from the mouth, its surface oiled and a model obtained in the manner heretofore described. If, in separating the model and impression, the plate adheres to the latter, it should be detached and adjusted to the model and the clasps arranged upon the plaster teeth. The plate and clasps may now be bound to the model with annealed wire, and united to each CLASPS. 191 other with solder; but the better way is to attach them to each other temporarily, with adhesive wax, in the relation they occupy on the model, and then remove them carefully and imbed the clasps and palatal face of the plate in a mix- ture of nearly equal parts of plaster, sand and asbestos. Before uniting the two pieces, on the model with wax, how- ever, the ends of the clasps should be straightened out or spread apart, in order that they' may part readily from the plaster teeth, without, in any degree, changing their exact relation to the plate; in doing which, it should be observed that all parts of the clasps which are to be united to the plate should remain in close contact with the plaster teeth. After the plaster mixture, in which the plate and clasps are imbed- ded, has become sufficiently hard, the portions of wax which temporarily united the latter should be removed, and the surfaces of the clasps and plate, where they unite with each other, smeared with borax ground in water to the consistence of cream; small pieces of solder are then placed along the lines of contact, the investient heated in the furnace until the plate acquires a full red heat, when it is removed, placed upon a suitable holder, and the solder fused with the blow- pipe. Whenever the form and inclination of the teeth to be clasped are not fairly represented on the model, owing to dragging or displacement of the wax in withdrawing the im- pression, the difficulties of securing a proper relative adjust- ment of the several parts of the appliance will be increased; but either of the following methods, if carefully and accu- rately manipulated, will secure accurate results. 1. Gutta percha may be substituted for wax when taking an impression with the plate in the mouth. With the proper use of this material, the exact form and inclination of the teeth will be preserved; and when employed, it should be filled in with plaster for the model immediately after re- moving it from the mouth. The subsequent steps in the operation are precisely similar to those described when wax is used. 192 MECHANICAL DENTISTRY. 2. Another method is to adjust the clasps and plate to the parts in the mouth, attach them temporarily in their proper relation, and remove, invest, and solder in the usual way. This may be accomplished in the following manner: First spread apart the ends of the clasp somewhat to permit it to be easily removed from the tooth; place this upon the tooth in the mouth to be clasped; then adjust the plate in the mouth, and attach the two to each other by pressing a piece of stiff, adhesive wax in against the clasp and plate where they unite; harden the wax by placing against it, for a few minutes, the end of a napkin moist with cold water; then remove the plate and clasp carefully from the mouth, and invest and solder as before. The plate, with one clasp per- manently attached, is now placed back in the mouth, and the second clasp adjusted to the tooth on the opposite side in the manner before alluded to; this is then temporarily fas- tened to the plate and otherwise treated in like manner as the one first described. If the teeth to be clasped are favor- ably formed and regularly arranged in the arch, both clasps may, at the same time, be temporarily attached to the plate in the first instance; if not, it will be impracticable to re- move them from the teeth without disturbing the wax and changing their relation to the base and the teeth clasped. The additional labor and consumption of time incident to a separate attachment of the clasps, will, in proportion as they secure better results, amply reward the operator for his pains-taking. Plaster is sometimes substituted for wax in this process; in which case it is introduced into the mouth on a small piece of wax or sheet lead and pressed gently against the uniting portions of the plate and clasp, and allowed to re- main until sufficiently hard. Any superfluous portions around the tooth that may hinder the easy removal of the clasp should now be cut away, when the pieces so attached to each other are removed from the mouth. A separation of the plaster from the clasp or plate, or both, may occur when CLASPS. 193 removing the latter; in this case, the several parts may be readily and accurately adjusted to each other again in their exact relation when out of the mouth; as the latter will be plainly indicated by the impression made by the plate and clasp in the plaster. Being re-adjusted, they may be fur- ther secured by sticking them together with a little softened wax, when they are invested, the temporary fastenings of plaster removed, and the pieces united by soldering. The use of plaster in these cases is due to Dr. Lester Noble, and unquestionably possesses many advantages over wax for the purpose, as the latter is liable even with the most skillful manipulation, to become displaced in removing it from the mouth; and this change, when it occurs, not being indicated by inspection of the wax, is incapable of timely correction. 3. Still another method is that contrived by Dr. Fogle, and described by Dr. Cushman in the tenth volume of the American Journal of Dental Science. It consists in securing the proper relation of the clasps to the teeth in the mouth by the use, in the first instance, of what are termed "tem- porary fastenings." The plate and clasps are first applied to the model, and are then connected by a narrow strip of plate or piece of wire bent in the form of a bow, the concavity facing the model, one end of which is soldered to the palatal side of the clasp, and the other to a contiguous point upon the plate. The pieces thus temporarily united are removed from the model and adjusted to the parts in the mouth. If the position of the clasps is found in any respect faulty, they can be easily and accurately adapted to the walls of the teeth by bending or twisting the connecting strip in any de- sired direction with pliers or other instruments suitable for the purpose. This accomplished, the plate and clasps are removed, and the operation of permanently uniting the clasps to the plate performed in the usual manner. CHAPTER VII. PARTIAL DENTURES SUPPORTED IN THE MOUTH BY MEANS OF CYLINDERS OF WOOD ATTACHED TO TUBED PLATES. The following description of a method of supporting par- tial sets of teeth in the mouth by means of wood cylinders attached to the plate is copied from an article contributed by Dr. W. M. Hunter to the fourth volume of the American Journal of Dental Science. The same principle had long been made available in Europe in attaching artificial substi- tutes constructed of the hippopotamus ivory, but the credit of its application to metallic plates is alike due to Drs. Hunter and Charles Stokes of London. "After swaging, the plate, as usual, is tried in the mouth, and an accurate impression of the teeth to be used, is taken over the plate, as recommended by Dr. Arthur, in the Ame- rican Journal, which will show the exact position of the tooth in its relation to the plate; after which the edge of the plate surrounding the teeth to be made use of, should be doubled or wired, when the tubes may be soldered at their proper points, taking care never to apply pressure to one side of a tooth without some means of counteracting the effect; the means being either a sufficient number of natural teeth con- tiguous to the tooth to be used, a counter tube, an arm of metal, or an artificial tooth, depending entirely upon the nature of the case. " At times, it is well to tube but one side of the plate and clasp the other; in cases where the crown of the tooth is much larger than the neck, a beautiful application may be thus made. " The tubes should be from one-eighth of an inch to one CYLINDERS OF WOOD ATTACHED TO TUBED PLATES. 195 line in diameter, and sjiould be filled with whiting before ap- plying heat, to prevent them from filling with solder at the time of soldering to the plate. They should be placed upon the plate so carefully, that the mouth of the tube will come in contact with the natural tooth, as it is desirable to have the wood protrude but very slightly beyond the orifice. " When it can be properly done, the tubes are soldered at the same time the teeth are, as it saves much trouble in fitting; it cannot, however, be very well done where it is de- signed to fit a tooth over a tube, but can very readily be done where the tube is designed to fill the angle caused by the meeting of the stay and plate, in the incisors and canine teeth, and where a canine is used for a bicuspid, building over the tube with metal to form the inner cusp." The accompanying cuts show clearly the form and appli- cation of the tubes referred to. In Fig. 55, showing on one side but a single tube, the counter-force is obtained by the artificial tooth which rests against the anterior face of the one to which the wood cylinder is applied. Fig. 56 exhibits Fig. 55. Fig. 56. tubes arranged on one side, and a clasp on the other; and shows the substitution of a canine for a bicuspid, with an inner cusp built up over the tube, practically converting a cuspid into a bicuspid tooth. In commenting on the application of this principle to partial sets of teeth, Dr. H. remarks: "The advantages in many cases must be apparent to the thinking dentist, but, perhaps, it might not be amiss to enumerate a few. 196 MECHANICAL DENTISTRY. " The fixture is held in place with greater firmness than by means of clasps. " In some instances where I have used clasps, I have also used the tube in combination, to give stability for masticating purposes. " The injury to the natural teeth must be much less, owing to the smaller amount of surface in contact. " If decay should take place, it would require but an ordi- nary filling to restore the tooth. " It prevents that peculiarly disagreeable sensation experi- enced, particularly in fruit season, upon removing and replacing artificial teeth. " After having tested it for more than a year, I am satis- fied that it greatly lessens the chances of decay in those cases where it can be applied, and I have removed the clasps in some old cases with great satisfaction to my patients." CHAPTER VIII. PARTIAL DENTURES SUPPORTED IN THE MOUTH BY PIVOTING THE PLATE TO THE ROOTS OF THE NATU- RAL TEETH. A limited number of teeth may be mounted on a plate pivoted to the roots of two or more of the front teeth; and provided the latter are firm, well formed, and in a healthy condition at the time of the operation, an appliance so ad- justed may be worn by the patient with comparative comfort and efficiency for from five to eight years. Ordinarily, the roots of the cuspidati afford the most secure means of attachment, and will furnish adequate support to a substitute supplying the loss of a part or all of the teeth anterior to the bi- cuspids, (Fig. 57,) and, in some cases, one or two of the latter on each side. The roots of the incisors, also, may be used, or one of the latter and a cus- pidatus. In some cases the plate may be secured in the mouth by pivoting to a root on one side, or in front, and clasping to a tooth on the opposite side, (Fig. 58,) provided the crown of the latter and the pivot root stand nearly or quite parallel with each other, as any con- siderable deviation from this relation will render it difficult or impossible to apply and remove the substitute. The roots of the teeth to be used as a means of support should be prepared in the manner described under the head of "Pivot teeth." 198 MECHANICAL DENTISTRY. In all cases, the enlarged canal of the root should be provided with a gold tube, as this method is the only one which will protect the root from the mechanical action of the pivot, or permit a ready and frequent removal of the appliance for the purpose of cleansing it and the parts associated with it in the mouth. The roots being prepared in the manner indicated, an im- pression of the mouth is taken, and with a die and counter obtained from a plaster model of the parts, a plate of the re- quired form is swaged covering the filed extremities of the roots to be pivoted, and extending anteriorly very nearly or quite to the free margins of the gum in front. The plate at those points corresponding with the openings into the roots, is then perforated and enlarged sufficiently to admit of the passage of the metallic pivots; the form and position of the orifices in the roots being transferred to the metallic die, the corresponding depressions in the plate at these points when the latter is swaged, will serve as a sufficient guide in perfo- rating the plate for the pivots. The plate is now *applied to the mouth, and the metallic pivot, one-half longer than that ultimately required and formed to fit the tube accurately but not tightly, is passed through the opening in the plate and pressed to the bottom of the tube, leaving the surplus portion of the pivot projecting on the lingual side of the plate. The plate and pivot are now secured in this precise relation by imbedding the projecting portion of the latter and the parts of the plate immediately surrounding it, in a batter of plaster. When the plaster has hardened, the plate and pivot with the plaster attached, are removed in their undisturbed relation from the mouth. To preserve the several pieces in situ more perfectly, the projecting end of the pivot may be flexed, or a head formed on it with the file before applying the plaster; the pivot thus secured will bring all parts together if traction is made on the plate in the act of with- drawing the pivot. The plate being removed from the mouth, its palatal por- TIVOTING TO THE ROOTS OF THE NATURAL TEETH. 199 tion is imbedded in the plaster mixture, and when the latter is hard, the plaster is removed from around the pivot on the opposite side of the plate, and the pivot permanently united by flowing solder at its point of contact with the plate. The redundant portion of the pivot on the lingual side of the base is then cut and filed away even with the surface of the latter. If the manipulations have been accurately con- ducted, the plate and pivot, on being reapplied to the parts in the mouth, will be found to adapt themselves perfectly to the palatal arch and roots. It is better, unless the roots to be pivoted stand nearly or quite parallel, to adjust and solder but a single pivot at a time, as but a very slight variation in the direction of the roots would render the withdrawal of both pivots at the same time difficult or impracticable without more or less change of relation. The same may be remarked of those cases where a clasp is used in conjunction with the pivot. The most efficient method of rendering the appliance sta- tionary when applied to the roots, and at the same time of enabling the patient to readily apply and remove it at will, is that recommended by Dr. Dwindle, and described in a previous chapter. Gold used for pivots in these cases should be alloyed with platinum, as that ordinarily employed for plate is too inelas- tic for the purpose. CHAPTER IX. PARTIAL DENTURES SUPPORTED IN THE MOUTH BY ATMOSPHERIC PRESSURE. The method of attaching partial sets of teeth to the su- perior jaw by means of atmospheric pressure, is much more generally practiced now than formerly, and whenever the condition of the soft parts of the mouth, the general configu- ration of the palatal arch, and the antagonism or occlusion of the artificial with the natural teeth favor its adoption, there are good and sufficient reasons why it should be pre- ferred, in all practicable cases, to either of the other methods heretofore described. Modifications in the Form of the Base.—If vacuities exist at various points on the ridge, the plate on which the teeth of replacement are mounted, should be ample in its dimen- sions, covering nearly or quite all of the hard palate. The general form of the base where several teeth scattered through- out the arch are required, is shown in Fig. 59. In most cases, whether but one or a greater number of teeth are to be replaced, increased adhe- rence and stability of the sub- stitute will be better secured by permitting the plate to cover the larger portion of the roof of the mouth; though in cases that present the best form of the vault, a diminished surface may be given to the base with equally satisfactory results. In the substitution ATMOSPHERIC PRESSURE. 201 of a single incisor, for example, it will frequently be suffi- cient to employ a very small plate, covering only a part of the anterior sloping wall of the palate. (Fig. 60.) In the Fig. 60. Fig. 61. latter case, the plate used may be very thin, say No. 30 of the gauge; it will thus impede the movements of the tongue less, and may be swaged more accurately to the parts. If constructed with an air-chamber, the latter should be quite shallow. A somewhat anomalous form of atmospheric pressure plate employed in the substitution of one or two bicuspid teeth on each side is described by Professor Taft,* the design of which is to secure, in such cases, increased stability of the substi- tute, while much of the palatal arch is left uncovered. It consists, as will be seen by reference to Fig. 61, of two lateral cavity-plates accurately adjusted to the sloping walls of the palate on each side, immediately adjoining and partly occu- pying the spaces to be supplied. These lateral plates may be made as large as a dime, or somewhat larger, and of an eliptical shape, if both bicuspids on the same side are to be replaced, and are connected with each other by a narrow band of gold plate, two lines or more in width, having an anterior curvature, and resting on the front wall of the palate, two or three lines behind the anterior teeth. The * Dental Register of the West, vol. xiii. p. 112. 14 202 MECHANICAL DENTISTRY. entire appliance may be constructed from a single piece of gold plate swaged accurately to the parts; or the lateral plates and connecting band may be separately swaged and secured in their proper relation to each other in the mouth with wax or plaster, when they are carefully removed, in- vested and soldered together; it should then be re-swaged.to correct any change of relation that may have happened during the concluding manipulations. The liability of the plate to ride upon the central and raised portion of the palate, when pressure is made upon one side, throwing the plate off from the ridge on the other, as in the case of a base extending across the arch, is in a great degree obviated by the method just described. Manner of Forming an Air-chamber.—Atmospheric pres- sure plates for partial cases are usually constructed with a central air-chamber; in which case, the part of the model representing the chamber may be formed in either of the ways mentioned in the chapter on "Plaster Models." The model prepared, the form of the plate to be used is first indi- cated thereon, and from this a pattern in sheet lead is obtained, which is placed on the plate of gold or other metal, and its outlines traced with a pointed instrument; the re- dundant portions are then cut away with plate shears and forceps. The plate is now placed on the die, and brought as nearly as possible into adaptation to the latter with the mal- let and pliers ; it is then interposed between the die and counter and swaged until it conforms perfectly to the face of the former; annealing the plate frequently to render it more pliant and manageable under the hammer. Unless the plate used is purer and thinner than is generally employed, or than is consistent with the required strength, it will fail to be forced perfectly into the groove around the chamber by the process of swaging alone; a more definite border, however, may be formed by forcing the plate in at this place with a small, smoothed-faced stamp, shaped to the angle of the groove, passing round the chamber and carefully forcing the ATMOSPHERIC PRESSURE. 203 plate in with the stamp and a small hammer or mallet until a somewhat sharp and abrupt angle is obtained to the palatal edge of the chamber. After the chamber is as perfectly formed as possible in this way, the plate should be well an- nealed and again swaged to correct any partial deformity occasioned by stamping the chamber. A still more perfectly defined angle may be given to the borders of the chamber in the following manner: After swaging the plate sufficiently to indicate the exact position and form of the chamber, the portion forming the latter should be separated from the main plate by completely di- viding it with a small, sharp, chisel-shaped instrument, cutting on a line with the groove around the chamber until the latter is entirely separated. The cut portion of the main plate is then trimmed evenly with a file, being careful not to enlarge the opening more than is required to remove the ir- regularities of the edge formed in cutting. The plate, with its central portion removed, is then placed upon the die, when a separate piece of gold cut to the general form of a chamber, but somewhat larger than the opening in the main plate, is adjusted over the chamber and struck up with the plate until the overlapping portions of the central piece are forced down upon the plate around the margins of the cham- ber. It is not, however, always necessary to employ a separate piece of gold for the chamber, as the central portion cut from the plate in the first instance may be sufficiently enlarged for the purpose. This is accomplished by first flat- tening out the detached portion, annealing it, and then passing successive portions of its edges a sixteenth of an inch or more between the rollers, the latter being sufficiently ap- proximated to produce a perceptible thinning of the margins. When the entire border of the chamber piece has been thus attenuated and extended, it will be found so much enlarged that when adjusted to the die and swaged in connection with the main plate, its borders will overlap and rest upon the margins of the opening in the base, as in the other case. 204 MECHANICAL DENTISTRY. The portions of the plate and cut chamber lying in contact are now coated with borax and pieces of solder placed along the line of union on the lingual side of the plate, when the two pieces, being transferred to a bed of charcoal, are perma- nently united by flowing the solder with a blowpipe. Sufficient heat should be applied to induce an extension of the solder between the two portions of plate, filling up com- pletely the gap between them to the edge of the orifice in the main plate, forming, at this point, a square and well defined angle to the margins of the chamber. CHAPTER X. METHOD OF OBTAINING AN ANTAGONIZING MODEL FOR PARTIAL DENTURES; SELECTING, ARRANGING, AND ANTAGONIZING THE TEETH; INVESTING, ADJUSTING STAYS, SOLDERING, ETC. Having constructed the plate or base to be used as a sup- port for partial sets of teeth in either of the ways described in the preceding chapters, it will be necessary, before arrang- ing the teeth on the plate, to secure an accurate representa- tion of all the remaining natural teeth of both jaws in plaster, preserving accurately the relation which these organs bear to each other in the mouth. This is effected by what is called an antagonizing model, and may be secured in the following manner. A roll or. strip of adhesive wax is first attached to the lin- gual border of the plate, and its adhesion secured by holding the opposite side of the plate for a moment over the flame of a spirit lamp. The wax used for articulating purposes should be harder and more tenacious than plain beeswax, and may be compounded from the following formula : Beeswax......1 pound Gum mastich, . . . . 2 oz. Spanish whiting, . . . . 1 oz. The wax is first melted in a shallow vessel, and the mastich, finely pulverized, gradually added, and then the whiting, stirring constantly until thoroughly incorporated. The rim of wax being arranged on the plate, all superfluous portions overhanging the margins occupied by the remaining teeth are cut away; the plate may then be placed on the model 206 MECHANICAL DENTISTRY. and the wax again trimmed, leaving it somewhat fuller than the outer circle of the teeth, and from one to three lines longer than those immediately adjoining the spaces. The plate, with the wax attached, is then placed in its proper position in the mouth and the patient instructed to close the jaws naturally until the remaining teeth meet; one-third or more of the crowns of the opposing teeth opposite the spaces will thus be imbedded in the wax. A still fuller impression of the opposing teeth may be obtained, if desired, by pressing the edges of the wax down upon the crowns with the finger. If a series of anterior teeth are to be replaced, the mesial line of the mouth in front should be indicated upon the wax by drawing a line vertically across the latter to serve as a guide in the arrangement of the central incisors and adjoining teeth. The plate and wax are then carefully removed from the mouth and again placed upon the plaster model, the latter having been previously obtained from an impression of the parts with the plate in the mouth. The model is then placed on a slip of paper with the plate and wax upward, and the heel of the model extended from one to two inches posteriorly to form an articulating surface for the remaining portion of the antagonizing model. The added portion of plaster may be confined by a narrow strip of wax or sheet-lead extending back upon each side of the model, into which a batter of plaster is poured to the depth of half or three-fourths of an inch. When hard, the edges and upper surface of the added plaster should be trimmed smooth, and a crucial groove, or two or three conical-shaped holes, cut in the surface of the latter to secure a fixed and definite relation of the two parts of the model. The articulating surface is then varnished and oiled to prevent the next portion of plaster from adhering; the imprints of the teeth in the wax are also oiled. This portion of the antagonizing model, with the plate and wax attached, is exhibited in Fig. 62. The open space looking into the palatal vault should be closed with a sheet of softened wax to prevent the next portion of plaster from SELECTING AND ARRANGING THE TEETH. 207 flowing into the cavity underneath. A batter of plaster is now poured carefully upon the exposed surface of the wax, filling the im- prints of the teeth per- fectly, and extending back upon the heel of the model until it acquires a depth of half an inch or more. When sufficiently hard, the two sections of the model are separat- ed; superfluous portions trimmed away; and the entire surface of both pieces glazed with varnish. The model complete, with the plate in place, and the wax (retained as a temporary support whilst adjusting the artificial teeth,) trimmed as requir- ed, are shown in Fig. 63, and if the manipulations have been accurate, this simple contrivance will exhibit all the parts rep- resented in plaster in pre- cisely the same relative position which they occupy in the mouth when the teeth are closed upon each other. It will be seen, by reference to Fig. 59, that only those teeth of the opposing jaw which present to the spaces, are represented in plaster, as these are all that are required in arranging the teeth of replacement. Selecting, Arranging, and Antagonizing the Teeth.—The teeth of replacement should harmonize, as nearly as possible, in size, configuration, and color, with those remaining in the mouth; and when selecting teeth for any given case, the operator should be provided with a sufficient number of sam- Fig. 62. 208 MECHANICAL DENTISTRY. pie teeth to meet every requirement, by comparison, in respect to the various tints or delicate shades of color char- acteristic of the natural teeth and gums. The required size and form of the artificial teeth may be determined with tol- erable accuracy by a comparison with those on the plaster model, but the form or figure more certainly by a careful in- spection of those in the mouth. A greater or less change in the form of porcelain teeth will be required, in nearly all cases, in arranging and fitting them to the vacuities in the jaw; and this is more particularly so in those cases requiring the use of gum teeth. This altera- tion of form is effected by grinding away portions of the tooth upon an emery or corundum wheel, attached, as will be seen in Fig 64, to a foot-lathe. If the edentated portions of the ridge have suffered but little change of form by absorption, as where the teeth have been recently extracted, and plate' teeth (those representing only the crowns of the natural organs) are used, the posterior portions of the base of the latter resting upon the margins of the plate will only require to be conformed to the irregularities on the surface of the base, grinding sufficiently to give to'them the proper length and relative position, while their anterior cervical portion is permitted to overlap the edge of the plate and rest directly upon the gum in front on a line with the adjoin- ing teeth. When, however, a consider- able concavity exists in the ridge and external border, and single gum teeth are employed to restore the customary fullness and contour of the parts, the gum portion of the tooth should be ground away on its posterior face sufficiently to restore the circle of the gum on the external border of the alveolus, and from the SELECTING AND ARRANGING THE TEETH. 209 base of the tooth where it rests upon the plate, to admit of a proper relative position of the artificial crown; while those portions of the porcelain gum terminating at, and adjoining the remaining teeth, next the spaces, should be formed with a thin, retreating edge, where it laps upon the natural gum, giving to the parts, when the substitute is adjusted to the mouth, the appearance of an unbroken denture and a con- tinuous gum. When the space to be supplied requires a series of two or more single gum teeth, the latter should be united to each other with the greatest care and exactness by grinding the proximate edges of the gum portions until the coaptation is such as to render the seams imperceptible in the mouth. In adjusting the porcelain teeth to the plate, the base of each tooth should be ground to rest as directly and uniformly on the plate as possible; for if thrown, in any degree, from the plate, the whole strain in mastication will come upon the platinum rivets, and, in a comparatively short time, the latter will either be entirely worn or cut off, or the artificial crown will be fractured on a line with the pins. In antagonizing partial sets of teeth, the indications pointed out by the customary closure of the natural organs should be followed as nearly as the form and position of the opposing teeth will permit. A changed or abnormal relation of the teeth of both jaws, however, frequently renders it difficult to effect a satisfactory adjustment of the teeth of replacement. If, in the case of the bicuspids, for example, one or more teeth in the under jaw project into a vacuity above to the extent of one-third or more of its depth, a direct closure of the sub- stituted organs upon these, in the ordinary manner, would be impracticable without a corresponding shortening of the porcelain teeth, enforcing, in such cases, an inharmonious arrangement, entirely inconsistent with the just requirements of the case. The difficulty cited, or any of the various modi- fications of it, may be overcome wholly or in part in one of two or three ways. If the teeth encroaching upon the op- posite space are very loose, as is frequently the case with 210 MECHANICAL DENTISTRY. those that have become elongated from the long-continued want of an adequate opposing force, or are hopelessly carious or otherwise diseased, they should be at once removed. If they remain firm and sound, and stand slightly within the circle of the teeth of the opposite jaw, or if they have some- what of an inward inclination in the arch, the vacuity op- posite may be filled with non-masticating teeth, as a canine, on the lingual side of which an antagonizing cusp of gold may be constructed, allowing the point of the cuspid to lap over upon the labial face of the encroaching tooth or teeth; or a bicuspid, manufactured for the purpose, with the inner cusp near the base of the tooth, may be used instead. Addi- tional room may be provided, in such cases, for the overlap- ping portion by filing away from a corresponding point on the opposing tooth. If, however, taking the most impracti- cable case, the intruding teeth are sound and firm, and stand vertically in the arch, closing between the opposing teeth on a line with, or somewhat outside of, the outer circle of the latter, (the elongation of such teeth being rather relative than absolute, as where it results from a mechanical wearing away of the remaining antagonizing teeth and a correspond- ing approximation of the jaws,) the practitioner will be com- pelled either to submit to a mal-arrangement of the teeth of replacement by grinding away sufficiently from their grinding surfaces to permit an unobstructed closure of the natural organs, or decline the operation altogether. The undue projection of the teeth of one jaw into a vacuity occurring in the one opposite more frequently happens, how- ever, in connection with the loss of the superior incisors. In such cases, the points of the lower incisors very frequently encroach upon the circle of the upper teeth, so that when the artificial teeth are arranged above in correspondence with the circle of the adjoining teeth, and the jaws are approxi- mated, the points of the inferior teeth will strike prematurely either upon the cutting edges of those above or will close upon their inner surfaces,—impeding thereby, or entirely SELECTING AND ARRANGING THE TEETH. 211 preventing, the occlusion of the teeth posterior to them. For such cases, thin teeth should be selected, and whenever necessary, the lower teeth may be filed away sufficiently, while those of replacement should, at the same time, be arranged as prominently as the circle will admit of. If these expedients fail, and a sufficient number of teeth posterior to the incisors require to be substituted in connection with the same appliance, it will be better, in cases not susceptible of satisfactory correction by the means already suggested, to change the bite by substituting an entirely new antagonism with the artificial teeth,—spreading the jaws sufficiently apart to relieve the artificial incisors in front. In no case, except that last described, should the artificial teeth come in contact with the opposing teeth before the oc- clusion of the remaining natural organs when the jaws are closed. The contact of all the teeth of one jaw, artificial and natural, with those of the opposite, should either be simultaneous, or the natural teeth should be permitted to strike first. In view of the difficulties which so frequently present themselves in connection with the arrangement of artificial teeth in partial cases, it may not be amiss to observe that, however essential to the natural and agreeable expression of the individual an exact and harmonious arrangement of the teeth of replacement may be, this requirement should, in some degree, be disregarded whenever the necessities of the patient, in respect to the comfort and utility of the appliance or the safety of the natural organs demand it;—to what ex- tent appearances should be sacrificed to these considerations, will depend upon the peculiar exigencies of the case, and cannot, therefore, be specifically stated. On the other hand, it may be observed that, if a sufficient number of the natural teeth are remaining in both jaws to enable the patient to perform, with tolerable efficiency, the act of mastication, the mere utility of the substitute in regard to the performance of this function may be partly or wholly disregarded when- ever there is sufficient reason to apprehend that the substi- 212 MECHANICAL DENTISTRY. tuted organs cannot be antagonized with a view to the com- minution of food without endangering the permanency and usefulness of the appliance by necessitating the application of forces unfavorably directed. Investing, Adjusting Stays, Soldering and Finishing.— Having arranged and antagonized the teeth as accurately as possible on the plaster model, the piece should be placed in the mouth to detect and remedy any faultiness that may be found to exist either in the adaptation, position or antagon- ism of the artificial teeth. It is then removed and imbedded in a mixture of plaster, sand and asbestos, in the proportion of about two parts of the former and one part each of the latter. The body of the investient may be surrounded by a copper or sheet-iron band to prevent the plaster from break- ing away whilst adjusting the stays or linings to the teeth. All parts of the plate and teeth, except the lingual side of the former and the backs of the latter, should be encased in plaster to the depth of half an inch or more, and when the latter is sufficiently hard, all traces of wax from the inside should be carefully detached with suitable instruments. The piece is now ready for the adjustment of stays or backings, which, when permanently united by soldering to the base and teeth, are designed to sustain the latter in po- sition. These supports are formed from plate somewhat thicker than that used for the base; a heavier and stronger stay being necessary when they are not united laterally, as when plate teeth are used. If, however, single gum or block teeth are employed, and the stays are joined, forming a con- tinuous band, plate one-half thicker than that used for the base will, ordinarily, impart adequate security to the attach- ment. A plain strip, corresponding in width with the tooth to be lined, is cut, and the end resting on the main plate conformed accurately with the file to the irregularities on the surface of the latter, and in such a manner as to permit the strip to take the direction of the tooth. The general form of the stay may, in the first place, be obtained by cut- ting a strip from a piece of gold with a pair of plate forceps. INVESTING AND FINISHING. 213 (Fig. 65.) The points upon the stay to be pierced for the admission of the .platinum pins, may be ascertained by coat- Fig. 65. ing the surface of the former with wax softened in the flame of a spirit lamp, and pressing it first against the lower pin, the point of which will be indicated by an indentation of the wax. The backing is then perforated at this point with a plate punch, two forms of which are exhibited in Fig. 66, Fig. 66. one armed with a tongue, which, when the plate is pierced, forces the latter from the punch. The strip is then reapplied 214 MECHANICAL DENTISTRY. to the upper pin and the second hole obtained in like manner as the first. Instead of using wax, the ends of the rivets may be stained with some pigment, which will show the points to be pierced in the lining. Fig. 67 represents an ingeniously contrived instrument, \ Fig. 67. invented by Dr. Samuel Mallet, and designed to secure an accurate relation of the two holes. After straightening the pins, one is placed in the hole i, at the head of the punch, the other pin pressing out the movable punch e, (which works by the spring g,) until it slips into the slot h; the two punches /, e, then make the holes at the exact distances apart to receive the pins. The stay should be adapted accurately to the face of the tooth; it is then cut to the proper length, reaching nearly or quite to the point of the tooth, and then shaped with a file to the general form of the crown. When the stays are to be united they should be formed with a shoulder at a point cor- responding with the neck of the tooth, and the proximate edges below united closely by square edges, or the latter may be beveled and made to lap upon each other. The process of soldering will be greatly facilitated and the piece will be more easily and artistically finished, by securing, in the first instance, a perfect coaptation of all the parts which are ulti- mately to be united. The sides of the holes in the stays facing the plate should now be enlarged or counter-sunk with INVESTING AND FINISHING. 215 a spear-shaped or conical bur drill, and when applied to the teeth, the projecting ends of the platinum pins are cut off even with the backings and then split and spread apart with a small chisel-shaped instrument; a head will thus be formed to the rivets when solder is fused upon them, and which will prevent them from drawing from the linings. All the lines of union between the several pieces should next be well scraped, exposing a clean, bright metallic surface to the solder; the seams are then smeared with borax, ground or rubbed in clean, soft water to about the consistence of cream;* after which small pieces of solder are placed along the joints and over the points of the platinum pins. The piece thus pre- pared is now placed in the furnace or ordinary fireplace in order to heat the entire mass throughout preparatory to soldering. The fuel most proper for this purpose is charcoal, either alone or combined with coke; the latter being prefera- ble for the reason that charcoal alone is more quickly con- sumed, and burning away more rapidly underneath, the piece is liable to drop to the bottom of the furnace. The fuel should be broken into small pieces and built up around the borders of the investient in order that all parts of the latter may be uniformly heated. The heating process should be conducted gradually, for if the piece to be soldered is subjected suddenly to a high heat, the plaster will be displaced by the too rapid evolution of vapor, and the integrity of the porcelain teeth will be endangered. The piece may be allowed to remain in the fire until the plate acquires a visible red heat, when it should be removed, placed on a suitable holder, and the solder fused with the blowpipe. A broad, spreading flame should first be thrown over the entire surface of the plate and border of the plaster until the temperature of the entire mass is nearly that required to fuse the solder, and which is indicated by the * Slate is often used for this purpose, but is unfit, as in rubbing the borax, loosened particles of the former become mixed with the latter and impede the flow of the solder, and becoming entangled render it unclean and porous. Ground glass or a porcelain slab is the best for the purpose. 216 MECHANICAL DENTISTRY. I latter settling and contracting upon itself; the flame may then be concentrated upon a particular point, as at the heel of the plate on one side, passing round from tooth to tooth, until all parts are completely united and the solder is well and uniformly diffused. Having united the teeth to the plate, the piece may be al- lowed to cool gradually, or it may be plunged after the lapse of a few minutes into boiling water without risk of injury to the teeth. When cool, the plaster is removed and the plate placed in a solution of equal parts of sulphuric acid and water, where it may be allowed to remain until the discolo- ration of the plate and the remains of the vitrified borax, incident to soldering, are removed; or it may be put into a small copper vessel, partly filled with the same solution and boiled for a few minutes. After removing the plate from the acid, it should be boiled for five or ten minutes in a solution of chloride of soda or common salt and water to remove thoroughly all traces of the former. Superfluous portions of solder are now to be removed, and this at first may be more quickly accomplished by the use of burs of various forms and sizes attached to a lathe. After the rougher and more re- dundant parts are thus cut away, any remaining irregulari- ties upon the surface may be further reduced with properly formed files, scrapers and cutting instruments. Flat and half-round curved files, and scrapers having a right and left curvature to their cutting edges, and chisel-shaped cutting instruments for paring or chipping away, (Fig. 68,) are the implements usually employed for this purpose, and with which a comparatively smooth surface may be obtained; after which, the filed portions should be well rubbed with scotch stone until all traces of file-marks or other scratches are completely removed. With a rapidly revolving brush attached to a foot- lathe, the final polish or lustre may be imparted by the use first, of Spanish whiting, or prepared chalk, and then rouge mixed with water or alcohol. The following method of finishing plate-work, communi- INVESTING AND FINISHING. 217 cated by Prof. J. L. Suesserott,* embodies some practical suggestions in reference to this process : " The first step is to Fig. 68. procure, and attach to the lathe, a three or four-pronged fork, or a screw such as is used for withdrawing a load from a gun; upon this a good smooth cork is fixed, and with a sharp knife, turned to any desired shape. The cork is satu- rated with water as well as it can be, and powdered pumice placed upon it. If we have been careful to remove all excess of solder from our work, which can easily be done by a bur attached to the lathe,—we can, with the cork and pumice, make a very smooth surface, and this can be still more per- fectly accomplished by substituting a very finely powdered spar for the pumice, after we have removed the largest scratches with the latter. By continuing the cork for a little while after the above named powders have been used off, we avoid the use of the scotch stone; and finally we dispense with the burnisher, by taking a new cork with a piece of chamois or buck skin stretched upon it, and going over the plate in the same manner as before, with the lathe revolving very rapidly. "A higher color can be given to the plate by the use of the burnisher after the above proceeding, but we can certainly not produce a smoother surface. * Erroneously ascribed in the former edition to Prof T. L. Buckingham. 15 218 MECHANICAL DENTISTRY. " Some precaution is necessary by those who have never used the lathe in finishing the plate: in the first place the careless use of the bur, in removing the excess of solder, might result in the weakening of the piece by removing more than necessary, or, what would be still worse, holes might be cut entirely through the plate. Again, in polishing, if a little care is not taken, the fork or screw, whichever is used, may pass through the cork, and before the operator is aware, he will have inflicted an injury that will be difficult to repair. A small amount of experience—that which is essential in the proper performance of every nice operation—will enable almost any one, even those, to use a common expression, 'whose fingers are all thumbs,' to finish their work in about one-eighth of the time that the most expert workman would require for the accomplishment of the same by the old method."* In the final adjustment of the finished piece to the mouth, and after any additional change in the form of the teeth necessary to secure the most perfect antagonism has been made, the patient should, in all cases of partial dentures, re- ceive explicit directions in regard to the general care and management of the appliance and the remaining natural teeth. Ordinarily, there will be but little difficulty experi- enced by the patient in the immediate and successful use of a substitute supported in the mouth by clasps, or any equiva- lent means, but in the case of atmospheric pressure plates, the patient should be candidly advised of the probable want of stability incident to the first use of the appliance, and the consequent annoyance which, in many cases, follows its occa- sional displacement in mastication until such time as the adaptation of the several parts to each other are perfected, and the patient has.acquired a habit of controlling and di- recting the forces applied to the substitute. The time neces- sary to accomplish these results will depend much upon the form and condition of the mouth, a favorable or unfavorable * Dental Cosmos, vol. i. p. 330. INVESTING AND FINISHING. 219 antagonism, the adaptation of the plate, and the aptitude and temper of the patient. It will be prudent and but just to the patient, to state, that the complete utility of an appliance sustained by atmospheric pressure, will not, probably, be real- ized in less time than from four to six weeks ; and this estimate of time, in a majority of cases, will be fully justified by ex- perience in the cases under consideration. The importance of thorough and absolute cleanliness of the substitute and natural teeth, and the reasons therefor, should be clearly stated; and the comfort, utility, and durability of the artificial fixture, as well as the safety of all the remaining natural organs will depend, in a great measure, upon the fidelity of the patient with respect to the observance of these injunctions. In those cases especially, where clasps are used, the substitute should invariably be removed after each meal and cleansed, while the teeth clasped, should, at the same time, be freed from deposits of food or other foreign substances with a brush, or any of the means usually recommended for the purpose. CHAPTER XI. ENTIRE DENTURES. Method of Constructing a Plate Base for an Entire Den- ture for the Upper Jaw.—The general form and dimensions of the required base to be used as a support for a complete denture for the upper jaw may first be indicated by drawn lines upon the plaster model, and a sheet-lead pattern obtained from this to serve as a guide in securing the form of the plate to be swaged. Whenever the substitute for the upper jaw is designed to be retained in situ by the external pressure of the atmosphere, and especially where a central air chamber is employed, the plate should be made sufficiently ample in its dimensions to cover all the hard palate, the alveolar ridge, and all portions of the external borders of the latter not di- rectly encroached upon by the muscles and reflected portions of the mucous membrane of the lips and cheeks. Before swaging, the plate should be well annealed, and its central portion brought as nearly as possible to the form of the palatal face of the die with the mallet, forcing the heel of the plate down in advance of the portion covering the more anterior concavity of the arch, preventing thereby a doubling of the posterior edge of the plate upon itself. This central portion may also be forced more perfectly into adaptation with a partial counter before swaging in the ordinary manner, and this is advisable in all cases when the palatal arch is very deep ; but as this is very liable to be drawn from the arch in the process of turning the borders of the plate over upon the ridge, a useful contrivance has been invented by Dr. Rurras, ENTIRE DENTURES. 221 of New York, to prevent the displacement. Fig. 69 shows the form of this instrument. The die and plate are placed near the edge of the bench, and the upper part of the clamp adjusted over the central portion of the plate; the two pieces are then bound firmly to the bench by tightening the screw underneath. A protective piece of buckskin, cloth, or paper, should be placed between the plate and clamp to prevent former from being bruised or indented. The margins of the plate are now turned over upon the ridge, and if the external borders of the latter are undercut or stand even vertically, the edges of the former will tend to double upon themselves at such points, and hence it will be necessary, before swaging, to split the plate in front, and, in some cases, one each side, and wherever divided, a V-shaped piece may be cut out of sufficient width to allow the divided edges to overlap slightly when approximated in the process of swaging. The proximate edges of the divided sections should be filed to a thin edge before swaging, so that when brought together and soldered, there will be but little addi- tional thickness of the plate at such points. The cut portions should not be soldered until after a partial or complete swaging. Having conformed the plate as nearly as practicable to the die with the mallet and pliers (Fig. 70,) or with plate forceps constructed for the purpose, (Fig. 71,) it should be placed be- tween the die and counter, and the latter forced together with a heavy hammer until a tolerably accurate coaptation of the plate is obtained, the latter being frequently annealed during the process of stamping, to render it more pliable. At first, considerable yielding and consequent deformity of the counter-die will occur; hence, after partial swaging, another should be substituted and the process continued 222 MECHANICAL DENTISTRY. until the greatest possible accuracy of adaptation is secured. If the face of the die is marked by prominent and sharply Fig. 70. defined ruga, or other irregularities, such points will, to some extent, be bruised or flattened; it will therefore be ex- pedient in such cases, and better perhaps in all, to finish the Fig. 71. swaging with a new and unused die and counter, in which case two or three moderate, steady, and well-directed blows of the hammer will be sufficient. If the plate is brought into uniform contact with all parts of the face of the die, this conformity is the only reliable test of its adaptation out of the mouth. In no case will the swaged plate fit the plaster model perfectly, inasmuch as the unavoidable contraction of the die, however slight, will, es- pecially in deep-arched mouths, cause the plate to bind on ENTIRE DENTURES. 223 the posterior and external borders of the ridge, preventing it from touching the floor of the palate; while the bruising, though inconsiderable, of the more prominent points upon the die, and a corresponding flattening of the plate at such points, will prevent uniform contact of the latter with the unchanged surface of the plaster model. After final swaging, the plate should be again annealed with a heat nearly or quite equal to that which will be ulti- mately required in soldering; after this any additional swag- ing should be avoided, unless the plate warps in the heat and which may be determined by applying it to the die; if any change has occurred, it should be re-swaged and again an- nealed at a high heat, and the operation should be repeated, if necessary, until the plate retains its integrity of form after the last annealing. This process of final heating does not apply to silver if in the form of a swaged plate, as this metal invariably suffers some change of form when subjected to an annealing heat. Modifications in the Form of Plates for Entire Upper Dentures.—Whenever a central air chamber is employed, it may be constructed in either of the, ways described when treating of partial atmospheric pressure plates. The gene- ral form of a plate for an entire upper denture, with a cen- tral chamber, is exhibited in Fig. 72, other modifications in Fig. 72. Fig. 73. the form of cavity plates for full upper sets are in limited use, as where chambers are arranged one on each side of the sloping walls of the palate, or directly over that portion of 224 MECHANICAL DENTISTRY. the ridge previously occupied by the anterior molar and the bicuspids on each side, as seen in Fig. 73, called " Lateral Cavity Plates." Dr. M. Levett, of New York, has recently introduced another modification of cavity plate, consisting of a number of small air chambers arranged directly over the ridge and placed at short intervals throughout the entire border. It is claimed that plates constructed in either of the ways last mentioned cohere with equal firmness to the jaw and are less liable to "rock" in the mouth than when formed with a central chamber. Whatever their general utility may be, cases doubtless occur where they may be ad- vantageously employed, as when any great inequality exists in the hardness of the ridge and palate, and a plate con- structed in the ordinary manner is dislodged by "riding" upon the hard palate when forces are applied to the ridge on either side. It has been recommended, after having constructed a base of the form represented in Fig. 72, to cut through the plate immediately in front of the central chamber, making an opening of a semi-lunar form. It is claimed that, by the more ready application of the tongue to this part, the air contained in the chamber, when the plate is applied to the mouth, may be more easily and thoroughly exhausted. There is great danger, however, of the soft tissues being drawn into the opening, in which case it can scarcely fail to produce injury of the parts implicated; the expedient, there- fore, is seldom resorted to. There is still another form of cavity plate known as Cleve- land's modification of air-chamber, and which is constructed in the following manner: A plate like that exhibited in Fig. 72 is first struck up and the chamber cut out. A thin sheet of wax, or a layer of plaster, is then placed upon the lingual side of the plate, extending from two to three or four lines from the edges of the orifice in the main plate; a thin, retreating edge is given to the wax or plaster at the outer borders, making it continuous with the surface of the plate. ENTIRE DENTURES. 225 The plate with the wax attached may now either be tacked to the model with softened wax along its outer borders, and shaped in such a way as to permit the model and plate to be withdrawn from the sand, and a mold of the parts taken in the ordinary way, and from this a die and counter; or an impression in wax or plaster may be taken of the lingual face of the plate and wax, and afterward a model, die, and counter. With the latter, a second plate, covering nearly or quite all of the palatal concavity is swaged, and when this is applied to the main plate over the cut chamber, and united by soldering, a space, equal to the thickness of the wax or plaster placed on the primary plate, will be found to exist between the two lamina. Fig. 74 exhibits a transverse sec- tion of the two plates, dis- closing the space between them, and also the opening through the gum plate into the cavity. Before solder- ing on the duplicate plate, a half-round wire should be soldered around the open- ing in the palatal plate on its lingual side, to protect the soft tissues of the mouth from injury when drawn in as the air is exhausted from the chamber; or, what is preferable, this form of cavity may be converted, practically, into what is known as " Gilbert's chamber," (which is the central swaged chamber before described,) by filling in the space between the two plates with some impervious substance, as Hill's filling, or an amalgam of gold, the excess of mercury being driven off by heat. In the construction of continuous gum work, the interspace may be filled in with gum body. The advantages of these double plates are, a greatly increased strength imparted to the base, a diminished liability of warp- ing in the process of soldering, a smoother surface presented to the tongue, and a more decidedly angular form of the chamber. 226 MECHANICAL DENTISTRY. In whatever way the plate is formed, a notch or fissure of sufficient depth to receive and permit an unobstructed play of the frsenum of the lip should be formed in the front part of the plate, while the borders of the latter nearly opposite the anterior molars on each side should be narrowed to pre- vent undue contact of its edges with the folds of the mucous membrane stretching obliquely across from the cheeks to the jaw. Care should also be taken to trim away from the heel of the plate any portions that might otherwise encroach upon the soft palate. It is only in the fewest number of cases that a rim can be swaged to form a groove or socket properly situated for the reception of the plate extremities of either single gum or block teeth, as it will usually be found impracticable to adjust the gum extremities to the socket thus formed without necessitating, in some degree, a departure from a just arrange- ment and antagonism of the teeth. Whenever it is thought best, therefore, to rim the plate, it will generally be neces- sary to adjust and solder a separate strip to the plate along the plate ends of the teeth after the arrangement of the latter on the base is completed. After the plate has been worked as nearly as possible into the required form, it should be applied to the mouth of the patient to ascertain the correctness of its adaptation to the parts before proceeding further with the operation. If the adaptation is found imperfect, the fault lies either in the im- pression, or in undue contraction of the die. In the former case, another impression should be taken, and the plate re- swaged ; in the latter, a less contractile metal or compound should be employed in the formation of the die. To deter- mine the practical efficiency of the adaptation and adherence of an atmospheric pressure plate, various tests may be applied. The coaptation of its borders to the external walls of the ridge may be ascertained by inspection, and the patient's sense of contact or non-contact of its central portion with the floor of the palate may, in some degree, be relied on as evidence of ENTIRE DENTURES. 227 the accuracy of its adjustment to parts not visible. The tenacity with which the plate adheres on the application of direct traction, cannot always be relied upon, inasmuch as a well fitting plate will sometimes be readily dislodged in this manner, while, on the contrary, one but illy adapted to the parts may require considerable force to separate it from the jaw when acted on in the same way. The most trustworthy test of actual or practical stability is firm pressure applied alternately over the ridge on each side and in front. If the plate maintains its position and remains fixed under repeated trials of pressure applied in the manner indicated, the adap- tation may be safely relied on; if it slides upon the palate or is easily disengaged from the mouth, the instability of the plate may be referred in many cases, not to a want of coapta- tion, but to a want of uniformity in the condition of the parts on which the plate rests. Thus, for example, if the ridge along the mesial line of the palatal vault is more than usually prominent and incompressible, and the alveolar ridge rela- tively soft and yielding, the plate, meeting with a fixed point of resistance at the floor of the palate, will prevent the ridge from being sufficiently compressed when the atmosphere is exhausted from underneath the plate ; and hence, when forci- ble pressure is made on one side over the ridge, the plate, riding upon the resistant surface in the arch, will be thrown off from the opposite side. Whenever, therefore, the conditions alluded to prevail to any considerable extent, a perfect coap- tation of the plate to the parts, instead of favoring the reten- tion of the former, will impair its stability for all practical purposes. The remedy is found in so constructing the plate that, when adjusted to the mouth, and before the air is ex- hausted, a great er or less space will exist between the central portion of the plate and palate, but which, when a vacuum is formed, will be carried up into contact with the roof of the mouth, and at the same time compress the ridge sufficiently to afford a firm and resisting basis for the plate in mastication. This peculiar adaptation of the plate may be obtained by 228 MECHANICAL DENTISTRY. adjusting a piece of sheet-lead or wax plate over the central ridge on the plaster model, by means of which the corres- ponding portion of the plate, when swaged with a die ob- tained from the model so prepared, will be thrown far enough from the roof of the mouth to answer the purpose before indicated. This will be more particularly necessary in shal- low arches; while, if the arch is very deep, or even mode- rately so, the unavoidable contraction of the die may render the expedient unnecessary. Method of Constructing a Plate Base for an Entire Den- ture for the Under Jaw.—Aside from the differences in the form of the plate, and the manipulations incident thereto, the process of constructing a plate for the under jaw does not differ essentially from that already described in connec- tion with full upper dentures. If the lower plate is constructed from a single lamina of gold, or other metal, it should be somewhat thicker than that used in upper cases, and should also be of finer quality, as the additional thickness of the plate and the peculiar form of the inferior maxilla, renders a greater degree of pliancy necessary in swag- ing it to the form of the ridge. The general form of a base for an entire lower denture is exhibited in Fig. 75. The internal border of the plate should usually be doubled,—either by turning the edge over in swaging. or by soldering on a narrow strip of plate or half-round wire. A more perfect adaptation of the plate to the ridge may be obtained by the use of a double instead of a single plate, in which case a thin basement plate, not exceeding No. 30 of the gauge, should be swaged to the form of the ridge in the first instance, and then a duplicate plate, swaging the two together and uniting them to each other with solder. A plate of the specified thickness may be very readily and ac- curately conformed to any irregularities in the ridge, and ENTIRE DENTURES. 229 when the two are united, the base will be heavier and stronger than a single lamina of the ordinary thickness. In- stead, however, of doubling the entire plate, it will be suffi- cient, in most cases, to adapt the second plate only to the lingual surface of the first, extending it up from the lower edge to a point corresponding as nearly as possible with the posterior portions of the base of the teeth when the latter are ad- justed to the plate, (Fig. 76.) A moderately thin plate may, in this manner, be used for the pri- mary base, while the duplicate band will impart the requisite strength to the plate, and, at the same time, obviate the necessity of wiring its inner edges.' In adopt- ing either of the last named meth- ods, the plates, after they are united to each other, should be again swaged to correct any change of form incident to the use of solder. Antagonizing Model for an Entire Upper and Lower Denture.—Either of the following methods may be adopted in securing an antagonizing model for complete dentures: 1. Attach to the ridge of each plate a roll or strip of ad- hesive wax corresponding in width to the length of the teeth which will be required for each plate respectively; place the plates, with the wax attached, in the mouth, and trim away from the proximate edges of the wax until the two sections close upon each other uniformly throughout the circle; then cut away from the labial surfaces of the rims of wax, above and below, until the proper fullness and required contour of the parts associated with the lips and mouth are secured. The approximation of the two jaws, when the finished substi- tutes are ultimately adjusted to the mouth, will depend alto- gether upon the aggregate width given to the two sections of wax at this stage of the operation, and it is, therefore, 230 MECHANICAL DENTISTRY. important that the "bite" or closure of the jaws secured at this time should be such as will most perfectly fulfill the re- quirements of the case in respect to the utility and comfort of the appliance, and the proper restoration of the required facial proportions. If there is any considerable change pro- duced in the relation of the jaws habitual to them prior to the loss of the natural teeth, the characteristic expression of the individual will, in some degree, be changed or marred; an unaccustomed and restrained action will be imposed upon the muscles concerned in • the movements of the lower jaw, which will render the use of the appliances at least tempo- rarily, if not permanently, uncomfortable and fatiguing, or even painful; while the utility of the fixtures may be im- paired or wholly destroyed, by compelling a particular ap- plication of forces in mastication inconsistent with their sta- bility in the mouth. No specific directions, of course, can be given that will apply to all cases, but it may be observed that, ordinarily, the two sections of wax should be cut away from their approximating surfaces until the jaws close suffi- ciently to permit the edges of the lips to rest easily and naturally upon each other when in a relaxed condition, or the upper rim may extend somewhat below the margin of the upper lip, while the lower section of the wax is cut away on a level with the lower lip, or a little below it. Cases occur, however, where a less exposure of the upper por- tion of wax, even though quite narrow, will be required; as where the alveolar ridge is very deep, and the lip covering it either absolutely or relatively short, or where the latter is retracted, exposing, even when in a state of repose, a greater portion or all of the crowns of the teeth, and in extreme cases, the margins of the gum. Between the latter extreme, and an inordinate extension of the upper lip below the ridge, all intermediate conditions occur, and the practitioner, aim- ing to produce an agreeable, harmonious and truthful expres- sion of all the parts, must rely wholly upon his judgment in reference to the necessary approximation of the jaws, the ENTIRE DENTURES. 231 restoration of the natural fullness and contour of the mouth, and the relative length to be given to the upper and lower teeth. Patients, when requested to close the mouth naturally, are very liable to project the under jaw; hence it is well to have them open and close the jaws frequently, observing, at the same time if the separate portions of wax meet in precisely the same manner at each occlusion. If the bite varies at every approximation of the jaws, the patient should be di- rected to relax and abandon for the moment all control over the muscles of the lower jaw ; the operator should then grasp the chin and press the jaw first directly backward and then upward until the opposing surfaces of the wax meet; in which position it should be steadily held by the patient until the two portions of wax are attached to each other in that par- ticular relation. The latter may be done by drawing lines vertically across the rims of wax at various points which will serve to indicate their relation to each other when out of the mouth ; or a heated knife-blade may be passed between the two sections, the melted wax temporarily uniting them. A very convenient and secure method is to attach them together by means of two strips of metal bent in the form of a staple; these may be warmed in a spirit-flame, and pressed into the wax, one on each side—one end penetrating the upper rim of wax, the other the lower. Before removing the plates, the mesial line of the mouth should be indicated upon the wax by drawing a line vertically across the latter in front to serve as a guide in the arrangement of the central incisors. 2. Another method is to attach to either the upper or lower plate a single rim of wax somewhat wider than will be required for both the upper and lower teeth. The plates are then placed in the mouth, and the jaws brought together, im- bedding the opposite plate in the wax. When this method is adopted, the proper closure of the jaws is best determined by a gauge or guide consisting of a strip of plate or other sub- stance encased in the wax and interposed edgewise between 232 MECHANICAL DENTISTRY. the borders of the two plates in front, in such a manner that when the latter are approximated they will close upon the guide, the desired relation of the jaws to each other having been previously ascertained by trial of the guide with the plates in the mouth before adjusting the wax. The exterior surface of the wax rim is then trimmed away, or additional portions added to it, until the proper fullness and contour are given to the lips; after which the median line of the mouth should be traced upon the wax in front, as before described. The plates, attached to each other in either of the ways mentioned, having been removed from the mouth, a batter of plaster may be poured upon a piece of paper or other sub- stance, forming a layer a fourth or a half an inch thick and two or three inches long, when the under surface of the lower plate may be imbedded in one end of the plaster, and the remaining portion of the latter projecting from the heel of the plate trimmed and formed for articulating with the second piece of the antagonizing model in the same manner as described when considering partial dentures. The en- trance to the cavity between the two plates is now closed up with a sheet of softened wax or otherwise, and the whole surrounded by a piece of oil-cloth, wax, or other substance, and the second part of the model obtained by pouring plaster in upon the exposed surface of the upper plate and the plaster posteriorly to the depth of half an inch or more. When the plaster is sufficiently condensed, the Fig. 77. line across the wax in front should be extended in a direct line across the borders of the plaster model above and below, as, in arranging the teeth, the wax will be cut away, and without this precaution the mesial point of the mouth may be lost. The form of an antagonizing model for an entire upper and lower denture, with the plates and wax attached, the latter being cut away somewhat prepara- tory to adjusting the teeth, is shown in Fig. 77. ENTIRE DENTURES. 233 Various articulators, or antagonizing frames have been de- vised, and may be substituted for the plaster articulator just described. A very excellent adjustable contrivance of the kind, invented by Dr. Hayes, is exhibited in Fig. 78. With this appliance, all the motions of the jaws can be repre- sented, and the relative po- sitions again brought back at pleasure to the starting point. The screw hinge admits lateral motion. The set screw on the side plays into a slot, securing one central position, to which it can at all times, when desired, be brought back. The long screw in the foot produces back and forward motion, the main hinge, up and down motion; and the large nut on the bottom renders all the parts taut and unyielding. Antagonizing Model for an Entire Upper Denture with the Natural Teeth of the Lower Jaw remaining.—In forming an antagonizing model to be used as a guide in arranging and articulating a full upper denture where all or a part of the natural organs of the inferior jaw are remaining, a rim of wax should first be adjusted to the borders of the plate, one or two lines wider than the required length of the artificial teeth. When placed in the mouth, the exterior surface of the wax draft should be cut away or added to, until the proper fullness of the parts is restored. The patient should then close the lower teeth against the wax, imbedding them just sufficiently to indicate the cutting edges and grinding surfaces of the opposing teeth. If a fuller impression of the exterior faces of the lower teeth are required, it may be obtained by pressing in a small strip of softened wax against them and the lower edge of the rim of wax upon the plate; or the projecting borders of the latter may be forced down upon the crowns with the fingers. The median line of the 16 234 MECHANICAL DENTISTRY. mouth is then indicated upon the wax, the plate removed, and its palatal surface im- bedded in one end of a layer of plaster spread upon a strip of paper; the portion of plaster extending from the heel of the plate trimmed, grooved, varnished and oiled; the entire piece enclosed, and plaster poured in upon the exposed surfaces of the wax and plaster to the depth of one-fourth or one half of an inch. The two sections of the model, with the plate and wax attached, the latter being cut away somewhat to receive the porcelain teeth, is exhibited in Fig. 79. Selecting, Arranging, and Antagonizing the Teeth ; Rim- ming the Plate; Attaching Spiral Springs; Investing, Lining, Soldering, and Finishing.—In selecting teeth for an entire upper and lower denture, the special requirements in respect to size, form, and color, will depend, in a great measure, upon the complexion, age, sex, general configuration of the face, &c, of the patient. Every separate denture, therefore, that is constructed in strict conformity with a faithful interpretation of the special requirements of each individual case, will be characterized by shades of differences in the color, form, size, and arrangement of the teeth of re- placement. It will be sufficient in this connection to observe that such selection of the teeth should be made as will most perfectly reproduce the lost proportions of the facial contour, and impart to the individual a natural, harmonious, and agreeable expression. In arranging or adjusting single gum teeth to the plate in those cases where the changes in the form of the alveolar ridge, consequent on absorption, are completed, the portions applied to the base should be ground away sufficiently to restore the required fullness of the parts and to give proper ENTIRE DENTURES. 235 length and inclination to the teeth. The coaptation of the ground surfaces to the base should be accurate enough to exclude perfectly particles of food, and to furnish such a basis to each tooth as will provide most effectually against fracture when acted upon by the forces applied to them in the mouth. The gum extremities of the teeth should also be accurately united to each other laterally by grinding carefully from their proximate edges until the joints or seams will be rendered incapable of ready detection in the mouth,—care being taken that this coaptation of the adjoining surfaces is uniform, for if confined to the outer edge alone, portions of the gum enamel may be broken away in the process of solder- ing. In the construction of substitutes designed to fulfill only a temporary purpose, and where the alveolar processes remain in a great measure unabsorbed, and plain teeth (those repre- senting but the crowns of the natural organs) are used, but little skill will ordinarily be required in adjusting and fitting them to the base. If the ridge in front is prominent and but inadequately concealed by the lip, as where the teeth have been but recently extracted, all those portions of the border of the plate in front anterior to the first or second bicuspid on each side may be cut away on a line a little within the required circle of the anterior teeth, and scolloped, (Fig. 80,) permitting the anterior cervical portions of the artificial incisors and canines, and, in some cases, the anterior bicuspids, to overlap the edge of the plate and rest directly upon the gum in front. This abridgment of the plate will not ordinarily materially affect the adhesion or stability of the substitute. There are cases of a mixed character that render it more difficult to effect a harmonious and symmetrical arrangement of the teeth, as where a limited number of the natural teeth at intervals have been long absent and the excavations in the 236 MECHANICAL DENTISTRY. ridge consequent on absorption alternate with other points upon the ridge in a comparatively unchanged condition. To give uniformity to the denture by restoring perfectly the re- quired circle of the arch in such cases will necessitate the employment of plain and single gum teeth conjointly. When- ever necessary, those portions of the base occupied by the plate teeth may be cut away in such a manner as to permit the latter to be adjusted directly to the unabsorbed gum as before described. In the process of grinding the teeth to the base, above and below, the operator should commence by first arranging the superior central incisors and then the lower, and so passing back from tooth to tooth, grind and adjust an upper and lower tooth alternately, keeping the upper ones in advance of those of the lower jaw. The central incisors above, should be placed parallel with each other, but the cutting edges of the laterals, and the points of the canines, should incline slightly toward the median line of the mouth. In arranging the teeth of the upper jaw, the anterior six may be made to de- scribe, with more or less exactness, the segment of a circle, but a somewhat abrupt angle may be given to the arch on each side by placing the first bicuspids within the circle in such a way that, when standing directly in front of the pa- tient and looking into the mouth, only a narrow line of the exterior face of the crowns of FlG- 8L these teeth will be seen, ( "^ while the remaining teeth \ / posterior to them, should be \ -I arranged nearly on a straight \ / line, diverging as they pass V j backward. When arranged V J in the manner described, the \ / peripheral outline of the arch \w y will exhibit somewhat the -i '---"^ form presented in the above diagram, (Fig. 81.) In regard to the practical efficiency of an upper denture ENTIRE DENTURES. 237 retained in the mouth by atmospheric pressure, it is impor- tant that the teeth engaged in the comminution of food, as the bicuspids and molars, should occupy a position directly over the central line of the ridge, and should either be ar- ranged vertically or with a slight inclination toward the centre of the mouth. The liability to displacement of the substitute in mastication will thus be greatly diminished, whereas, if placed outside of the line indicated, and especially with a diverging inclination, the stability of the appliance will be endangered, and the function of mastication impeded, notwithstanding other conditions necessary to complete suc- cess have been fully secured. In arranging the upper and posterior teeth as described, it will sometimes be necessary to give to the opposing under teeth a decided inward inclination. in order to effect a satisfactory antagonism of the teeth; and cases occur where a practical articulation cannot be secured without departing in some degree, from the arrangement of the upper teeth spoken of,—as where a great disparity exists between the posterior transverse diameters of the two jaws, a medium sized, or even small, arch above being associated with an expanded ridge below. In articulating the upper and lower teeth, the closure or relation of the natural organs should be imitated as nearly as the other essential requirements of the case will admit of. Hence the upper front teeth, describing the segment of a larger circle than the corresponding teeth of the lower jaw, will project beyond and overlap slightly the cutting edges of the latter; and having a greater width of crown, they will extend latterly beyond the opposing teeth, covering one-third of the crowns of those next adjoining, so that when the canines of the upper jaw are reached, they will close between the lower canines and first bicuspids; and, passing back, the anterior superior bicuspids between the first and second bicuspids below; the posterior bicuspids above between the second inferior bicuspids and anterior molars; the first su- perior molars between the first and second molars below ; 238 MECHANICAL DENTISTRY. while the anterior half of the posterior molars above will close upon the posterior half of the inferior second molars, the remaining posterior half of the second molars above ex- tending posteriorly beyond those of the lower jaw. The outer cusps of the superior bicuspids and molars will overlap those of the inferior teeth; while the inner cusps of the teeth of the superior jaw will pass into the depressions in the lower teeth formed by the internal and external cusps, and the external cusps of the inferior teeth will, in like manner, be received into the corresponding excavations of the upper teeth. The relative position and antagonism of the teeth as they appear in a regularly arranged denture for both jaws, are shown in Fig. 82. An abnormal relation of the jaws, as where undue projec- tion, absolutely or relatively, of either maxilla exists, or where the lower jaw closes on one side or other of the upper, will frequently compel a de- parture from the ordinary arrangement of the artificial organs, the extent of which must be determined by the necessities of each individual case. In selecting teeth for a full upper denture in those cases where natural teeth are remaining below, or vice versa, the color, size, and form of the latter, will serve as a guide in the choice of teeth appropriate for the opposite jaw. In fitting and arranging the teeth upon the base, and in antago- nizing them with the opposing natural teeth, the same general principles apply as those already adverted to in connection with full upper and lower dentures. Having adjusted the teeth to the base, they should be placed in the mouth before uniting them permanently to the plate, to detect and remedy any error of arrangement either ENTIRE DENTURES. 239 in respect to prominence, position, inclination, length, or antagonism. Forming a Rim to the Plate.—If the case is one where single gum or block teeth are employed, and it is intended to form a socket or groove upon the borders of the plate for the reception of the plate extremities of the teeth, the riry, forming the groove should be fitted and soldered to the base before investing the piece in plaster. If the alveolar ridge above is shallow, and but imperfectly concealed by the lip, a rim to the plate will be inadmissible, as, when the mouth is opened and the lip retracted, as in laughing, the metallic band will be exposed to view. A rim may be fitted and at- tached to the base in either of the following ways : 1. A strip of plate from one to two lines in width is ad- justed to the plate with one edge resting on the uncovered border of the plate close to the gum extremities of the teeth, and the other overlapping and embracing the latter: The rim may consist of one entire strip extending from heel to heel of the plate, and passing round the posterior molars to unite with the stays; but it may be more conveniently ad- justed by employing two pieces, extending from each heel of the plate, and uniting in front. 2. A half-round wire with the edge beveled where it joins the ends of the teeth, forming a narrow groove, may, in like manner, be fitted to the plate, furnishing a shallow bed for the gum extremities of the teeth. A narrow strip of plate, about the thickness of a heavy clasp material, may be sub- stituted for the half-round wire. In either case, the better plan is first to trace the outlines of the gum portions of the teeth upon the plate with a sharp instrument; remove the wax and teeth from the plate; draw another line a little within the first all round, and solder the rim. to the line last drawn; remove the teeth from the wax, and re-adjust the latter in its proper place upon the plate; then fit each tooth separately to the rim by grinding away sufficiently from the end of the tooth to effect an accurate adjustment of it to the 240 MECHANICAL DENTISTRY. socket. The ends of the teeth may be ground away to the rim until the platinum pins freely re-enter the rivet holes in the wax, .thus restoring them to their proper position in re- lation to the base. 3. Another method of forming a rim consists in swaging a strip of plate accurately to the form of the parts to which it is applied. An impression in wax or plaster is first taken of the gum surfaces of the teeth and exposed border of the plate; but as it will be impossible to detach either wax or plaster in perfect condition, when encircling the entire arch, or to swage perfectly with a die so unfavorably formed for stamp- ing, separate impressions of the two lateral halves of the piece should be taken,—from these plaster models; and from the latter, dies and counters;—with these, two strips of plate of sufficient width are swaged, each extending from the heel of the plate to a little beyond the median line in front, over- lapping slightly at the latter point. The portions of the swaged strips embracing the plate ends of the teeth are then trimmed to the proper width, and scalloped, if desired, in correspondence with the festoons of the artificial gums. An upper denture rimmed in the manner last described is exhibited in Fig. 83. In whatever Fig. 83. way the rim is formed, when it has been fitted to the plate and teeth, it may be held temporarily in place with clamps adjusted at two or three points around the > plate, and then transferred to a piece of charcoal, and secured by first tacking it at two or three points with solder. The groove may then be filled with whiting, mixed with water or alcohol, to prevent the solder from flowing in and filling it up; after which small pieces of solder are placed along the line of union next the edge of the plate, and the rim permanently united through- out with the blowpipe; after which the wax and teeth are re-applied to the plate. ENTIRE DENTURES. 241 Constructing and Attaching Spiral Springs.—The success which has been attained in the use of atmospheric pressure plates has almost entirely superseded the necessity of employ- ing spiral springs as a means of support; nor should the latter be resorted to except under circumstances that pre- clude the use of the former. When applied, they should be attached to the base on each side between the posterior bi- cuspid and first molar below, and opposite the posterior bi- cuspid above. To the border of the plate near the base of the teeth a narrow strip of plate is soldered, extending up and lying closely against the side of the latter—to the end of which near the grinding surfaces of the teeth is adjusted a small circular cap of gold connected with the standard by a small wire on which the looped extremity of the spring plays. To each end of the spring is attached a gold wire, doubled upon itself in such a way as to form a loop, the closed ends being soldered together and filed to enter the Fig. 84. hollow in the wire, A, Fig. 84. B, Fig. 84, copied from Prof. Harris' work on Dental Sur- gery, represents another method of attaching springs, but the former is more readily constructed and will answer every practical purpose. Fig. 85, exhibits the application of springs to an upper and lower denture. Investing, Lining, Soldering and Finishing the Plate.—The plate, with the wax and teeth in place, is next encased in a mixture 242 MECHANICAL DENTISTRY. of plaster preparatory to lining the teeth and uniting them with solder to the base. For this purpose, plaster and sand may be employed, using as little of the former as will serve to hold the investment together during the subsequent manipulations. Asbestos may be added, and is a useful in- gredient. Burnt plaster, or that which has been previously used for investing, may be substituted for the sand and asbestos, adding a sufficient quantity of unused plaster to effect con- solidation. Either of the combinations mentioned will suffer but little change in the fire, if properly managed. It is customary to encase the piece in the plaster mixture to the depth of from one-half to three-fourths of an inch, leaving only the lingual surfaces of the plate and teeth uncovered. However comparatively free from change of form the best combinations of plaster may be, yet some slight contraction of the body of the investient doubtless ensues on the appli- cation of heat, and it is probable that so large and resistant a mass must tend, in some degree, to produce deformity of the plate in soldering; for, as the investient contracts and the plate at the same time expands when heated, a change in the form of the latter must occur whenever the force exerted by the shrinking plaster exceeds the expansive force of the metal; and when the peculiar form of the upper plate is con- sidered we can readily conceive how a slight contraction of the plaster of the thickness mentioned may "warp" or " spring " the plate when its uniform linear expansion and contraction is so effectively opposed. The change in the form of the base from this cause will, according to the author's observations, be found, in an upper plate, to exist on each side of the sloping walls of the palate, embracing the posterior half or two-thirds of the plate at these points,—the change manifesting itself in an inward displacement of the lateral walls of the plate midway between the summit of the palatal arch and the most depending portion of the ridge. We would suggest in explanation of this result that, as the plaster contracts with sufficient force to carry the plate with ENTIRE DENTURES. 243 it, the sides of the latter are approximated, while the palatal portion is, at the same time lifted up. Now it seems plain that inasmuch as the portions of plate overlapping the ridge is encased in and embraced by the plaster, and as the palatal portion is arched in form with its convexity presenting to the plaster and therefore self-sustaining in respect to its own pe- culiar form, the special configuration of these parts cannot suffer any appreciable change ; but as they are forced toward the common centre of the mass, their relation to each other is alone changed, and this changed relation must necessarily eventuate in a deformity of those parts of the plate which offer the least resistance to the contractile force of the plaster. In obedience to this necessity, the sides of the plate along the sloping walls of the palate, which from their form are neither resistant or self-sustaining under pressure,- and whose inward displacement is unopposed by any counter-force, is projected in toward the centre of the palatal excavation in proportion as the borders and central portions are approxi- mated, or converge in the direction of the centre of the piece. The practical effect of this approximation of the lateral and posterior borders, and internal displacement of the plate, is to make the latter " bind" upon the outer and posterior borders of the alveolar ridge, and to throw the central portion of the plate from the roof of the mouth. To obviate, as far as practicable, any change in the form of the plate which may result from the contraction of the plaster investient, various expedients have been suggested, but the following will sufficiently counteract the influence of the plaster by permitting an unobstructed expansion and contraction of the metallic base. Take a band of tolerably thick copper plate as wide as the plate and teeth are deep; bend it to the form of the plate, but large enough to leave a space of nearly half an inch between it and the teeth, the ends being united to each other back of the plate by riveting or otherwise. Holes are then made in the band at numerous points throughout its extent, through which wire is introduced and interlaced on 244 MECHANICAL DENTISTRY. the inside-in such a way as to form loops,—the latter extend- ing in to within a short distance of the teeth. The plaster is then filled into the space between the band and teeth even with the cutting and grinding surfaces of the latter; the palatal surface of the plate is also covered with plaster and may be connected with the outer portion by a very thin layer at the edge of the plate, or the two may be entirely disconnected. The expansion of copper being very nearly that of gold, the body of the plaster, when heat is applied, will be carried in advance of the borders of the plate as the latter expands, while the thin portion of plaster at the edges of the plate will allow the central portion of the latter to ex- pand with but little or no interruption. On cooling, the entire mass will contract together- and assume its original form, unless warping is induced by other agencies acting independently of the enveloping plaster, as excess or unequal distribution'of solder, irregular heating, &c. It is not, ordinarily, necessary to provide by any special expedient against warping of the lower plate, as any slight change of form consequent on contraction will not materially affect its adaptation to the lower jaw,—its only effect being to impart to the substitute a slight lateral play upon the ridge. The plaster on the inside of the lower piece may be cut away to the edge of the plate, while that external to the teeth should not be added in greater quantities than is barely sufficient to hold the latter in place whilst lining and solder- ing them to the base. The plate being properly invested, all portions of the wax attached to the inner surface of the teeth and plate should be thoroughly removed with suitable instruments; after which stays are to be adjusted to the teeth. In reference to the method of forming and adjusting stays, little need be added to what has already been said when treating of partial dentures. One method not there specified, consists in first fitting to each tooth separately, in the usual manner, a thin stay formed of platinum, which is temporarily fastened to ENTIRE DENTURES. 245 the tooth by splitting and spreading apart the ends of the rivets with a small chisel-shaped instrument. The teeth are then removed from the investient and partially imbedded side by side in plaster, the platinum strips remaining un- covered. The plaster and teeth may then be raised to a full red heat with a blowpipe or by placing them in the furnace. Small pieces of gold plate, of equal fineness with the base, are then placed upon the surfaces of the platinum stays and thoroughly fused with the blowpipe until they flow perfectly in around the rivets, and uniformly over the surface of the linings. If sufficient heat is applied, the solder will insinu- ate itself between the stay and tooth, and thus render the coaptation of the two perfect. Small pieces of gold plate should be added until sufficient thickness is imparted to the linings. The backings are then trimmed smoothly and bur- nished, when they may be placed back in the investment in their appropriate places. The linings which support the teeth may be united to each other laterally in sections or continuously. When the teeth are joined to each other throughout, a very small quantity of solder will be sufficient to support the teeth, provided it is well diffused along the joints uniting them perfectly at all points. The process of preparatory heating, soldering, pickling and finishing the plate, is the same in all respects as that described when treating of partial pieces, and need not, therefore, be recapitulated. In the final adjustment of the finished pieces to the mouth, and after any additional grinding of the masticating sur- faces of the teeth necessary to perfect the antagonism has been performed, such instructions should be given to the patient in regard to the care and management of the ap- pliances as will best promote their immediate and successful use. The wearer should be impressed with the absolute ne- cessity of early and prompt attention to any injuries inflicted upon the soft tissues of the mouth by the substitutes; as much future trouble and annoyance, if not permanent muti- 246 MECHANICAL DENTISTRY. lation of the parts, may result from neglect, but which may be readily averted, in most instances, by a timely removal of the sources of injury. To obviate, in some measure, the tendency to displacement of the base, which usually accom- panies the first use of artificial teeth, and especially the upper denture, the patient may be directed, when dividing food with the front teeth, to press the substance backward and upward against the cutting edges of the superior in- cisors at the same time that the opposing teeth are closed upon each other, thus dividing completely the substance seized. In reference to the mastication of food, it has been suggested to instruct the patient 'to distribute, by the action of the tongue, the portions of food as equally as possible on each side of the mouth, in this manner distributing the forces applied and thereby lessening the chances of lateral displacement of the substitute. CHAPTER XII. SUBSTANCES USED IN THE MANUFACTURE OF PORCELAIN AS APPLIED TO DENTAL PURPOSES. Single mineral teeth, porcelain blocks, continuous gum material, &c, are composed of two distinct portions,—the Body or Base and Enamel. The chief mineral substances which compose the body, are, Silex, Felspar and Kaolin. The enamel, both crown and gum, consists principally of Felspar. The various tints or shades of color are imparted to the porcelain by certain metals in a state of minute division or their oxyds. The more general properties of the mineral ingredients will be first described. Silex.—Silex, silica, or silicic acid, is a white powder, in- odorus and insipid. It forms the chief part of many familiar mineral formations, as quartz, rock crystal, flint, agate, cal- cedon", and most sands and sandstones, in some of which it occurs nearly pure. Silica, in its pure state, is insoluble in water or acids, and is infusible in the highest heat of the furnace; it melts however in the flame of the oxy-hydrogen blowpipe, passing into a transparent colorless glass. Its specific gravity is 2*66; and is composed of silicon, 48*04, and oxygen, 51*96. Only the purest varieties of silex are employed in the manufacture of porcelain teeth. It is pre- pared for use by subjecting it to a white heat and then plunging it into cold water, after which it is ground to a very fine powder in a mortar. Felspar.—This mineral substance occurs crystallized in oblique-rhomboidal prisms, and is a constant ingredient of granite, trachyte, porphyry, and many of the volcanic rocks. 248 . MECHANICAL DENTISTRY. The felspathic mineral formations present either a pearly or vitreous lustre, and vary in color, being red, green, gray, yellow, brown, flesh-colored, pure white, milky, transparent, or translucent. Felspar yields no water when calcined; melts at the blowpipe into a white enamel, and is unaffected by acids. It is composed, according to R-ose, of—silica, 66*75; alumina, 17*50; potash, 12; lime, 1*25 ; oxyde of iron, 0*75. It is found in various localities throughout the United States, the purest and whitest kinds being employed in the manufacture of mineral teeth. It is prepared for use in the same manner as silex. Felspar, from its ready fusibility, serves to agglutinate the particles of the more refractory ingredients, silex and kaolin; and when diffused throughout the mass imparts to the porcelain a semi-translucent appearance. Kaolin.—Kaolin, or decomposed felspar, is a fine white variety of clay, and is composed chiefly of silica and alumina, the latter being the characteristic ingredient of common clay. It is found in various localities throughout the Eastern States, and in parts of Asia and Europe. Kaolin is refrac- tory or fire-proof, but is rendered more or less fusible by the contaminations of iron and lime with which it is usually combined. The opaque and lifeless appearance character- istic of the earlier manufacture of mineral teeth was due to the introduction of a relatively large proportion of this clay into the body of the porcelain. The peculiar translucent and life-like expression which distinguishes the beautiful imita- tions of the present day, is due, in great part, to the compa- ratively small proportion of kaolin clay, and an increased amount of the more fusible and vitreous component, felspar. Kaolin is prepared for use by washing it in clean water; the coarser particles having settled to the bottom, the water holding the finer ones in solution is poured off, and when the suspended clay is deposited at the bottom of the vessel, the water is again poured off, and the remaining kaolin dried in the sun. MANUFACTURE OF PORCELAIN. 249 Coloring Materials.—The following metals and oxyds are employed in coloring mineral teeth; titanium, platina sponge and oxyd of gold being those chiefly used in producing the more positive tints, and by combining which in varying pro- portions, any desired shade of color may be obtained. METALS AND OXYDS. Gold in a state of minute division Oxyd of Gold, . Platina sponge and filings Oxyd of titanium, Purple of Cassius, Oxyd of uranium, Oxyd of manganese, Oxyd of cobalt, Oxyd of silver, Oxyd of zinc, COLORS PRODUCED. Rose red. Bright rose red. Grayish blue. Bright yellow. Rose purple. Greenish yellow. Purple. Bright blue Lemon yellow. Lemon yellow. As the preparation of most of the above colors requires great care, and a somewhat intimate knowledge of chemistry, and as the most delicate manipulations are necessary to secure accurate and satisfactory results, it is better for the mechanical operator to procure the coloring ingredients already prepared from some competent chemist, rather than attempt their production himself. For a particular descrip- tion of the various modes of preparing them, the reader is referred to Piggot's "Dental Chemistry and Metallurgy," and other works treating fully of the subject. 17 CHAPTER XIII. PORCELAIN BLOCK TEETH. The fabrication of porcelain block teeth constitutes a somewhat distinctive branch of practical dentistry, and from the delicate nature of the manipulations and long experience necessary to attain to any considerable degree of excellence in the various processes connected with their manufacture, their construction is seldom attempted by those engaged in general practice. A practical knowledge of the process, however, becomes, in some measure, indispensable on the part of those who cannot conveniently command the services of an experienced block-workman, inasmuch as cases are con- stantly occurring in practice, especially those connected with the replacement of partial sets of teeth, in which it is impos- sible to fulfill efficiently the requirements of the case with single gum teeth, but which may be accomplished in the most perfect manner by means of porcelain blocks con- structed with special reference to the condition of the parts to be supplied. The process is applicable as well also, to full arches in the form of sectional blocks. In any case, the im- practicability of readily or successfully repairing them when broken or otherwise injured, must always limit their use, in a great degree, to those cases that do not admit of other equally satisfactory means of substitution. A description of the general properties of the several in- gredients, earthy and metallic, used in the formation of block teeth, has already been given in the preceding chapter. The method of compounding and preparing the materials will next be given, with various approved recipes for body and enamel. PORCELAIN BLOCK TEETH. 251 Composition and Preparation of the Body.—The porcelain paste for the body of block teeth may be compounded from either of the following formulas. There are a great variety of recipes, differing more or less in the proportion of the com- ponent ingredients, but the following will be found to answer every practical purpose, and are such as are generally em- ployed at this time by experienced block-workmen. NO. i. NC '. in. Delaware spar, 12 oz. Spar, . 12 oz. Silex, . 2 oz. 8 dwts. Silex, . 2 oz. 8 dwts. Kaolin, . 7i dwts. Kaolin, . . 12 dwts. Titanium, 18 to 36 grs. Titanium, . 24 grs. NO. n. NO. IV. Delaware spar, 16 oz. Spar, . 8oz. Silex, . 3£oz. Silex, . 1£ oz. Kaolin, . i oz. Kaolin, . . 4 dwts. Titanium, 20 to 60 grs. NO. Spar, Silex, Kaolin Titanium, Titanium, . V. 2 oz. 8 dwts. 2 dwts. 4 grs. . 22 grs. The titanium is first ground in a mortar until reduced to an impalpable powder; the silex is then added and ground from one to three hours, or until there is no perceptible grit; after which the kaolin is added and thoroughly ground; and lastly the spar, adding small portions at a time, and grinding the whole until perfect comminution and intermixture of the several ingredients are effected, say from half an hour to an hour. The ingredients may be ground dry or in water,—in the latter case a sufficient quantity of clean rain water should be added, from time to time, to form a mixture of about the consistence of thick cream. After sufficient comminution is effected, the surplus water may be abstracted by pouring the mixture upon a clean, dry slab of plaster of Paris. When it acquires about the consistence of thick dough, it should be beaten with a wooden mallet, or thrown repeatedly and 252 MECHANICAL DENTISTRY. forcibly upon a marble slab, and, if prepared in quantities for future use, it should be preserved in its plastic state by con- fining it in a closely-stopped earthen jar. When ground dry, the materials are prepared for immediate use by adding to the powder clean rain water in sufficient quantity to form a thick paste; it is then well beaten on a porcelain or marble slab, and pressed, just before using, between folds of cloth to expel perfectly all particles of air that may be confined in the body of the paste. Composition and Preparation of Crown Enamels.—The enamel, which forms the external covering to the crowns of porcelain teeth, is composed wholly of felspar with such color- ing matters as serve to communicate to it the various tints or shades of complexion characteristic of the natural organs. The more positive tints, grayish-blue and yellow, are pro- duced by titanium, platinum sponge, and oxyd of gold; in- termediate colors being produced by varying the special com- binations of these ingredients. The following recipes will furnish various tinted enamels, the varieties of grayish-blue being applied to the points or coronal extremities of the teeth—the yellow to the necks; the two colors being so blended when applied as to run imper- ceptibly into each other. GEAYISH-BLUE ENAMEL. NO. I. NO. Ill Spar* . 2oz. Spar, 2 oz. Platina sponge igr. Platina sponge, igr. Oxyd of gold, . igr. Oxyd of gold, . igr. NO. II. NO. IV Spar, 2oz. Spar, 2oz. Platina sponge, igr- Flux.f . 24 grs. Oxyd of gold, . igr. Platina sponge, igr. * The Boston spar is preferred on account of its greater fusibility. f Flux is composed of silex 4 oz.; borax 1 oz.; sal tartar, 1 oz.; these are ground to an impalpable powder and packed in the bottom of a clean, light-colored crucible. A piece of fire-clay slab is then fitted into the top of the crucible and luted with kaolin clay. It is then exposed to the heat of a furnace until completely fused, when it is removed, and when cold the crucible is broken, all foreign particles or discolored portions thoroughly removed, and the remainder well pulverized PORCELAIN BLOCK TEETH. 253 - YELLOW ENAMEL. NO. I. NO. III. Spar, 2 oz. Spar, 2 oz. Titanium, . 10 grs. Titanium, 16 grs. Platina sponge, igr- Platina sponge, igr. Oxyd of gold, . igr. Oxyd of gold, . igr. NO. II. NO. IV Spar, 2 oz. Spar, 2 oz. Titanium, 14 grs. Flux, 20 grs. Platina sponge, igr- Titanium, 10 grs. Oxyd of gold, . igr. In compounding enamels from the foregoing recipes, the coloring ingredients should first be ground to a very fine powder, with five or six dwts. of the spar; the remaining portions of the latter should then be added, a little at a time, and ground for half an hour or more. The shades of color may be varied almost indefinitely by changing the propor- tions of the coloring matter. GRAYISH-BLUE ENAMEL. NO. I. NO. II. Spar. . . . 1 oz. Spar, . . . 1 oz. Blue frit * . . 5 grs. Yellow frit.j- . . 4 grs. Gold mixture,J . 20 grs. Composition and Preparation of Gum Enamels.—Either of the following recipes will furnish a good gum enamel, and may be used in connection with any of the compositions for body heretofore enumerated. NO. I. NO. II. Gum frit, No. 1, 3 dwts. Gum frit, No. 2, 3 dwts. Spar, . . 9 to 12 dwts. Spar, . . 3 to 18 dwts. * Blue frit is composed of spar, £ oz.; platina sponge, 4 dwts.; powder very finely, make up into a ball with water, and fuse very slightly upon a slide in a furnace. It is then plunged into water while hot, and when dry, finely pulverized. f Yellow frit is made by mixing intimately i oz. of spar with two dwts. of titanium, and heating as above. { Gold mixture is prepared by dissolving 8 grs. of pure gold in aqua regia, and then stirring in 12£ dwts of very finely pulverized spar. When nearly dry, it is formed into a ball and fused upon a slide, and then coarsely pulverized. 254 MECHANICAL DENTISTRY. It is recommended, in order to impart a granular appear- ance to the gum, to grind the spar somewhat coarsely; any required shade or depth of gum color being obtained by vary- ing the proportions of the frit,—the latter containing the coloring ingredients. Gum Frit, No. 1, is composed of felspar, 700 grs.; flux, 175 grs. ; oxyd of gold, or metallic gold in a state of minute division, 16 grs. The above are ground in a mortar for five or eight hours, or until they are reduced to an impalpable powder; they are then packed in the bottom of a clean Hessian crucible coated on the inside with a thin mixture of pulverized silex, and on the outside with kaolin. A piece of tile or slab is then luted with kaolin to the top of the. crucible, when it is placed in the furnace for from one to two hours, or until complete vitrifi- cation is effected. It is then removed, and when cold, the crucible is broken and all traces of adhering silex ground off; it is then broken in pieces and ground until it will pass through a sieve, No. 9, bolting cloth. Gum Frit, No. 2, is composed of spar, 700 grs.; flux, 175 grs.; purple cassius, 8 grs. The purple cassius is first thoroughly ground in a mortar, after which the flux is added in small quantities at a time, then the spar in the same manner, grinding until perfect comminution and intermixture of the several ingredients are effected. It is then packed tightly in the bottom of a clean white crucible, the inside lined with silex and a slab luted to the top, as before, and the whole exposed to a heat sufficient to fuse perfectly. It is then removed from the fire, and when cold, all foreign substances are ground off and the remaining portions pulverized until it will pass through a sieve of No. 9, bolting cloth. Having given the composition and mode of preparation of the various compounds which enter into the formation of the body and crown and gum enamels, it only remains to describe the different processes concerned in the construction of PORCELAIN BLOCK TEETH. 255 porcelain blocks from the several compositions given, and first of the method of procuring an antagonizing model. Antagonizing Model for an Entire Upper and Lower Denture constructed of Block Teeth.—The first step in the process of constructing block teeth, for either a full upper set with the natural teeth of the opposite jaw remaining, or for entire dentures for both jaws, is to secure an antagonizing model. For the latter or complete dentures, above and below, the method does not differ from that employed when single gum teeth are used. A rim of wax is adjusted to each plate in the manner heretofore described, and the plates placed in their proper position in the mouth; the wax drafts are then trimmed until the exact fullness and contour of the lips and cheeks are secured and proper relative width is given to the wax rims. Great exactness should be observed in these latter manipulations, inasmuch as the wax drafts are the only guides in the formation of the blocks, both as respects the form and fullness of the arch and the length of the teeth. The proper relation of the two pieces in the mouth is now secured, the wax rims attached to each other, and the median line of the mouth indicated on the wax, and being removed from the mouth, an antagonizing model procured in the same manner as described in a former chapter. Antagonizing Model for an Entire Upper Denture with the Natural Teeth of the opposing Jaw remaining.—A rim of wax, half an inch or more in width, is attached to the ridge of the plate and the latter placed in the mouth. The patient is then directed to close the jaws until the cutting edges and grinding surfaces of the teeth of the opposing jaw are fairly imbedded in the wax. The piece is then removed from the mouth and the wax rim detached from the plate by holding the latter for a moment over a spirit-flame. The wax is then placed upon a strip of paper with the side indented by the teeth looking upward, the surface of the wax oiled and a batter of plaster poured upon it, filling the imprints of the 256 MECHANICAL DENTISTRY. teeth and running back an inch and a half or more behind the wax, raising the plaster to a level of half an inch above the wax. When the plaster is sufficiently condensed, it is turned over, the wax removed without fracturing the plaster teeth, and a crucial groove made in the surface of the model posterior to the teeth. This constitutes the lower section of the antagonizing model, and is a representation of the teeth of the lower jaw. The upper section is next obtained in the following manner. A second rim of wax, in width equal to the required length of the teeth, is adjusted to the plate as before and placed in the mouth. The exact contour and full- ness of the arch required is then given to the external or labial surface of the wax draft, and the lower edge cut away until the required approximation of the jaws is secured and the points of all the teeth remaining below touch the wax at the same instant. The patient is now required to close the jaws gently upon each other until a slight indentation is made in the wax by the opposing teeth; the median line of the mouth is then marked upon the wax and the plate. removed. The plate and wax are now adjusted to the lower section of the model, the points of the plaster teeth being received into the indentations in the wax made by the natural teeth. The upper and posterior surface of the lower section of the model having been varnished and oiled, and the exposed surface of the plate also oiled, a mixture of plaster is poured in upon the latter and back upon the model, raising the whole to a level of half an inch above the plate. The two sections, when the latter portion of plaster has consolidated, are then separated, reserving the lower part of the antagonizing model for future use. Forming a Matrix for Molding the Body preparatory to Carving the Teeth.—As the process of forming a matrix in which to mold the porcelain paste, giving the general form and outlines to the blocks before carving the teeth, is the same for an upper and lower denture, it will be sufficient to describe the method as it relates to the superior arch. A PORCELAIN BLOCK TEETH. 257 Fig. 86. matrix for an entire denture above or below, whether con- sisting of three, four, or six blocks, is ordinarily made to consist of three distinct pieces independently of the plate and model, and is constructed in the following manner. Three conical-shaped holes are made in the sides of the model, one in front and one on each side, to furnish a fixed articulation for the three sections forming the external walls of tlie matrix. The appearance of the model when thus prepared, with the plate and wax rim in place, is exhibited in Fig. 86. The sides of the model and external face of the wax are now oiled, and both surfaces covered with a batter of plaster to the depth of a fourth or a half of an inch, extending from the base of the model to the lower edge of the wax, and posteriorly about half way on each side of the model to form a matrix for the front block, or the two anterior blocks, if the arch is made to consist of more than three sections. Supposing the wax removed from the plate and this front piece in place, the several parts will present the appearance shown in Fig. 87. The plaster rim forming the external wall of the front block being removed, plaster is again added, as before, to the outer surfaces of the model and wax, extending it from the heel of the plate on each side forward an eighth or a fourth of an inch in advance of the posterior extremi- ties of the plaster rim first formed. When hard, the plaster is trimmed even with the edge of the wax draft, and the two pieces removed from the model. The matrices formed by these lateral sections when readjusted to the model with the wax removed are shown in Fig. 88. Having thus provided 258 MECHANICAL DENTISTRY. Fig. 87. a matrix determining the general outline and length of the teeth for the entire arch, the wax draft is removed and the Fig. PORCELAIN BLOCK TEETH. 259 plate thoroughly cleaned preparatory to molding the paste,— before doing which, however, the line upon the wax indicating the median point of the mouth should be extended across the model. Molding the Porcelain Paste preparatory to Carving the Teeth.—In the process of constructing an entire denture, it is impracticable, owing to the shrinkage of the body, to form a single continuous block or full arch without materially changing its relation and adaptation to the metallic base, and also to the natural organs in cases where the latter are remaining in the opposite jaw; hence it is customary, as before intimated, to divide the arch into sections,—usually three; a central front block embracing the incisors and cus- pidati, and two lateral blocks including the bicuspids and molars on each side; or the denture may consist of four blocks, dividing the arch between the central incisors, and also between the first and second bicuspids on each side; making the two anterior blocks to consist each of a central and lateral incisor, a cuspidatus and anterior bicuspid, and the posterior blocks of the second bicuspid and the two molars. Again, the arch is sometimes divided into six blocks,—an anterior embracing the central and lateral inci- sor and cuspidatus, a central comprising the bicuspids, and a posterior including the molars. If constructed in three sec- tions, as is ordinarily the case, the front block should be molded and carved first. The material for the body, if in a dry state, is mixed with a sufficient quantity of clean rain water to form a thick batter, and mixed thoroughly in a mortar. It should then be poured upon a dry slab of plaster of Paris, and when the excess of water is absorbed, removed and well beaten with a spatula on a marble or porcelain slab until it assumes a somewhat pasty form; it may then be well pressed between folds of cloth to force out any remain- ing portions of confined air. The plaster rim forming the matrix for the front block is now adjusted in its proper po- sition to the model, and its inner surface, as well as that of 260 MECHANICAL DENTISTRY. the plate, oiled; the porcelain paste is then packed into the matrix as compactly as possible, filling it even with the upper edge of the plaster rim. When the paste has been worked in as solidly as possible, patting it with the fingers or suitably formed instruments as successive portions are added, it should be trimmed even with the edge of the plaster rim and the palatal surface cut away to near the thickness required for the teeth included in the block, leaving it some- what thicker, however, to compensate for the shrinkage of the body, and to allow for small portions which will be cut away in carving the teeth. The plaster rim forming the ex- ternal border of the matrix is now loosened by tapping gently upon the model and then carefully removed. The mark upon the model, showing the mesial line of the mouth and indicating the proper position of the central incisors, is then extended across the block, after which the width of each ad- joining tooth is lined off, making each one as much broader than will be required in the finished piece as the porcelain composition will shrink in baking,—this, in a block em- bracing the six anterior teeth, will be equivalent to about one-third or one-half the width of a bicuspid on each side. If the case is one requiring a full denture above and below, the operator should next proceed to mold the front block for the lower arch in the same manner as described for the upper. The two sections of the antagonizing model are then placed together, and the proper relative width for the lower teeth indicated upon the inferior block,—the drawn lines upon the upper block serving as a guide. The points to which the posterior extremities of the front block extends on each side of the plate should be marked upon correspond- ing points of the model above and below to enable the mani- pulator to determine how far the side blocks should be ex- tended anteriorly when molding the paste for the latter,— the marks upon the model being subsequently transferred to the lower edges of the lateral sections of plaster concerned in the formation of the side matrices. Before removing the PORCELAIN BLOCK TEETH. 261 front blocks from the plates preparatory to carving the teeth, the surface of the paste may be dried somewhat by throwing upon it, with a blowpipe, a broad spreading flame from a spirit lamp. The blocks are then carefully detached by rapping lightly upon the model, assisted by gentle trac- tion with the fingers. The front blocks being removed from the upper and lower plates, the side sections of plaster con- cerned in the formation of the posterior matrices are ad- justed to the model, and, being oiled, the paste filled in as before described, extending each block forward beyond the point occupied by the cuspidatus of the front block a dis- tance equal to about one-third or one-half of the width of the latter. These are then cut away even with the edges of the plaster rims and trimmed on the palatal sides, leaving them somewhat thicker than will be required for the bicuspids and molars. The plaster walls of the matrices are then removed; the two parts of the articulating model placed together, and the relative width and position assigned to the upper and lower teeth by drawing lines across the external surface of the blocks. They are then separately removed from the plates in the manner before described, and the necessary ad- ditional portions of paste added to the grinding surfaces to compensate for the contraction of the body in baking. In constructing a full upper denture with all or a portion of the natural organs remaining below, the proper width to be given to the upper teeth, as well, also, as the required rela- tion or antagonism of the artificial with the opposing natu- ral teeth, may be readily determined by applying the lower portion of the antagonizing model representing the teeth of the under jaw, and marking upon each block, as it is being molded, the necessary width and position of each tooth above,—being careful to make allowance for shrinkage by adding to the length, width and thickness of each block as much as will compensate for the contraction of the body. In every other particular, the process is conducted in the same manner as heretofore described. 262 MECHANICAL DENTISTRY. Carving the Teeth.—The teeth are first separated by draw- ing between them a thread attached to a small bow, and it may be observed in this connection that the most careful and delicate manipulation is required in handling the blocks while carving to prevent portions of the paste from crumb- ling away, a tendency that may be counteracted, in some measure, by moistening the paste occasionally with a little water taken up on the point of the carving knife. The gene- ral outline of each tooth having been traced upon the exte- rior surface of the block with the point of the instrument, the operator proceeds next to give the distinct and charac- teristic form to the crowns, and the harmonious and agreea- ble effects produced will depend upon the fidelity with which the manipulator copies nature in the form and arrangement of the teeth. The requirements of individual cases are too varied in their nature to admit of specific directions in respect to their formation,—a careful study of the modified forms of the natural organs, combined with some degree of manipula- tive tact, will enable any one, after sufficient experience, to attain to satisfactory results in this particular. After the teeth are formed, and the body of the block is reduced to the required thickness, superfluous portions extending from the ends of the block should be cut away, leaving enough, however, projecting to allow for grinding when jointing and