,- ~"v3 ,^. /f 2 '"S. iS&y ^g-w>}-^> t %f '\ \EDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICI 1 ivnoiivn 3NOIQ3W jo Aavaan ivnoiivn 3Ni3ia3w jo Aavasn ivnou' MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDiCII 1 ivnoilvn 3NOIQ3W jo Aavaan ivnoiivn 3NOia3w jo Aavaan "IVNOIi> 2 X/Tffcft-' q V^DlT\X 2 Q_ MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDlCtr T3 1 IVNOIIVN 3NOI03W JO Aavaan IVNOILVN 3NOICI3W JO ASVaail 1VNOI11 MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY Of MEDICI o CL 1 IVNOILVN 3NIDIQ3/. ••* *":r*X / DENTAL METALLURGY: 3? Manual for the Use of Dental Students BY CIIAS. J. ESSIG, M.D., D.D.S., PROFESSOR OF MECHANICAL DENTISTRY AND METALLURGY IN THB DENTAL DEPARTMENT OF THE UNIVERSITY OF PENNSYLVANIA. SECOND EDITION, REVISED PHILADELPHIA: '^^ftEniC^^'^'S.8- "White Dental Mfg. Co. WLJ £18a 1888 Copy-right. The S. S. White Dental Manufacturing Co. 1882. Copy-right by Chas. J. Essig, 1887. all rights reserved. press of PATTERSON i. WHITE, PHILADELPHIA. preface to the second edition. rpilE practical results of the publication of the first edition of the Manual of Metallurgy, then the first text-book of the kind adapted to the use of dental students, showed that it filled a want long felt, and demonstrated the necessity for preparing a second edition. I have accordingly carefully revised the work, and while it has not been greatly enlarged, the more recent improvements in the reduction of metals and the formation of alloys and amalgams used in dentistry have been incorporated. As in the first edition, my aim has been to avoid extraneous or merely hypothetical matter, as well as that be- longing to the purely chemical study of the metals, his knowledge of which it is presumed the student should derive from other sources. The success of the first edition, in so far as it has been of service to students of dentistry, and the kindly reception accorded it by my collaborators are subjects of peculiar gratification and grateful appre- ciation on my part. Charles J. Essig. 111 preface to the first edition. n^HTC object of the author in the preparation of this Manual was to place in the hands of stu- dents of dentistry an outline of the scientific princi- ples involved in the reduction of the metals, their properties, the modifications resulting from alloying, and their application to dental uses. While the properties of many of the metals have been only incidentally or illustratively referred to, special consideration has been given to those most commonly used by dentists. In Chapter Y a resume of the author's experiments in the formation of alloys for amalgams is given. These were made for the purpose of enabling him to present the subject systematically to the students who sat under his teachings; and, while the chapter is far from being a complete treatise on the subject, it is hoped that it will prepare the dental student for a better comprehension of that branch of metallurgy, occupying, as it does, so conspicuous a place in the practice of dentistry. As the atomic weights, specific gravity, and fusing- points of the metals are somewhat differently stated by various authors, the figures given in this Manual v VI have been made to correspond to those in Fownes's Chemistry,—the text-book commonly used by dental students. The decimal system has been used to express pro- portions, in the belief that its comprehensiveness and simplicity, would commend it. In expressing temperatures the Centigrade and Fahrenheit scales have been occasionally referred to separately. The rule for translating one into the other, and comparative tables, have been omitted because they are to bo found in Fownes's and nearly all the other recent works on chemistry, with some one of which the student is supposed to be more or less familiar. C. J. E. CONTENTS. CHAPTEB I. Metallurgy . II. The Metallic Elements III. Properties of the Metals IV. Alloys .... V. Amalgams VI. Modus of Melting Mktals . VII. Combinations of Mktals "p tallic Elements riTH Non -Me- VIII. Gold .... IX. Silver X. Platinum . XI. Iridium XII. Palladium XIII. Iron . XIV. Mercury . XV. Copper XVI. Zinc . XVII. Cadmium . XVIII. Aluminum XIX. Lead XX Tin . XXI. Electro-Metallurgy..... 1* 1 rii CHAPTER I. METALLURGY. HPHE art of separating metals from their ores, or from simple combinations with non-metallic ele- ments, and their application to useful purposes, may be regarded as a separate branch of chemical science. It is essential that the student, before commencing its study, should acquire a good preliminary knowl- edge of chemistry and mechanics. The empirical reduction of the ores of metals seems to have been practiced at a very remote period, its origin being attributed to Tubal Cain, seventh only in descent from Adam.* The remains of numerous mines have been discovered on the southern and eastern borders of the Ural Mountains, in which have been found hammers and chisels of copper, and other instruments of the same metal of which the uses at present are unknown.f That these implements were those of a wandering people would seem to be evi- denced by the absence of any traces of masonry in tho neighborhood; and the fact that no iron tools were found near them would indicate their great antiquity. ._______________,________________________________» * Fourth ohapter of Genesis Tubal Cain is spoken of as an "in- structor of every artifioer in brass and iron." | Pereey's Metallurgy. 2 (9) 10 DENTAL METALLURGY. Gmelin found in the eastern part of Siberia the remains of nearly one thousand smelting-furnaces, very primitive in character, surrounded by heaps of scoria, broken pottery, and other evidences that metallurgical operations of considerable magnitude had been at some distant period carried on in that locality.* The alchemists of the Middle Ages were the metal- lurgists par excellence of that period, and there is evidence that they were acquainted with chemical processes for the reduction of metals, which were brought to a great state of perfection, thus showing that the practical part of metallurgy was far in advance of the theoretical.^ * Percey's Metallurgy. f The following experiments, from manuscripts discovered by M. Ferdinand Hoefer, will serve to convey an adequate idea of the status of metallurgy from the third and fourth centuries down to a compar- atively recent date: " Experiment No. 1.—A piece of red-hot iron is placed under a bell, which rests in a basin full of water. The water diminishes in vol- ume, and a candle, being introduced into the bell, sets fire at once to the gas inside. Conclusion—water changes into fire. " Second Experiment.—A piece of lead, or any other metal except gold or silver, is burned in contact with the air. It immediately loses its primitive properties, and is transformed into a powder or species of ashes or lime. The ashes, which are the product of the death of the metal, are again taken and heated in a crucible, together with some grains of wheat, and the metal is seen rising from its ashes and re-assuming its original form and properties. Conclusion—metals are destroyed by fire and revivified by wheat and heat. "Third Experiment.—Argentiferous lead is burned in cupels com- posed of ashes of pulverized bones; the lead disappears, and at the end of the operation there remains in the cupel a nugget of pure silver. Conclusion—lead is transformed into silver." (It is probable that upon this and analogous facts was founded the theory of the transmutation of metals.) METALLURGY. 11 To these early experimenters, however, must be awarded the credit of great industry. They knew nothing of the metals as ultimate bodies, nor of the particular force governing their union with the non- metallic elements, and finding that an earthy matter, such as an ore of iron, became converted by fire into a metal, they naturally believed the change of earth into metals to be possible, and in the search for gold, the philosopher's stone, etc., they really, by mere accident, discovered many valuable chemical agents. In this way sulphuric, nitric, and hydrochloric acids were produced, and these, made to act upon the metals, in turn yielded the metalline salts. Thus it will be seen that from the gradual aggre- gation of facts resulting from the pursuits of the alchemists ultimately sprang an exact science, and toward the latter part of the sixteenth centuiy appeared a set of investigators of a very different " Fourth Experiment.—A strong acid is poured on copper; the metal is acted upon, and in process of time disappears; or, rather, is trans- formed into a green, transparent liquid. Then a thin plate of iron is plunged into the liquid, and the copper is seen to reappear in its ordi- nary aspect, while the iron in its turn is dissolved. Conclusion—iron is transformed into copper." Practically the fourth experiment, quoted from Jules Andrieu's paper on " Alchemy," written for the Encyclopaedia Brittanica, is electro- lysis, the principle by which a compound of a metal with a non-metal is decomposed by galvanic electricity; but the transmutation theory was generally accepted as accounting for the phenomena noticed in this experiment, and it would seem that at least some of the savants of tho period were sufficiently shrewd and unscrupulous to turn the process to profitable account, since we find that "St. Thomas Aquinas, in his theological writings, forbids the sale of 'alchemist's gold,' and in a spooial treatise on the subject unmtisks an imposture of the ohar- latans of the day, who pretend to make silver by projecting a sublimate of white arsenic on copper." 12 DENTAL METALLURGY. order, who, instead of wasting their time in the pur- suit of such fanciful theories as that of transmutation, etc., devoted themselves to the unraveling of the principles that govern the composition and forma- tion of bodies already known. Thus, Paracelsus* was the first to distinguish the true character of some of the well-known salts, such as alum and cop- peras, showing that they contained metals—a matter of great importance at that day, inasmuch as it eventually led to the discovery that many of the well-known crystalline salts were compounds of dis- similar elements, as a metal with a non-metallic body, and to a knowledge of the particular force governing their union; and finally the investigations of Beecher and Stahl of Cronstadt, Klaproth, Wollaston, Ber- zelius, Wohler and Deville, and others, have dispelled many illusions and rendered accurate the present literature of the subject. During the latter half of the eighteenth century the list of metals was aug- mented by new discoveries, and the application of the voltaic current to the decomposition of the alka- lies by Sir Humphrey Davy in 1807-8 added a dozen or more. The employment of the spectroscope by Kirchhoff and Bunsen, in 1860, brought to light so many new metals that the total number now exceeds fifty. •"Paracelsus, though the author of many fanciful doctrines, seems to have been the first to offer a true chemical explanation of the action of mercury, lead, etc., upon the human system. CHAPTER II. THE METALLIC ELEMENTS. "TV/TODERN CHEMISTEY assumes that the metals ■*- are elementary bodies, yet there have been other theories presented regarding their ultimate character, and it is thought by some that " when man shall have mastered that great power of nature, elec- tricity, many of the so-called elements will be found probably to be compound bodies." Others have enter- tained the theory of but one ultimate element;* while nearly all agree that as we advance in knowl- edge new elements will be brought to light. The old philosophers applied the term element to imaginary principles of matter, such as fire, water, and air; while the elements of the alchemists were salt, sulphur, and mercury. The term is now used as synonymous with simple body, or one of the un- decomposable constituents of any kind of matter, or that which cannot be divided by chemical analysis. The elements known at present number sixty-six, * Prof. Graham's Researches with Hydrogen, in 1869. 2* (13) 14 DENTAL METALLURGY. divided into the metallic and non-metallic. Of the former there are fifty-two, as follows : COMBINING NAMES. SYMBOLS. WEIGHTS. Aluminum. Al. 27-4 Antimony. Sb. (Stibium) 122 Arsenic. As. 75 Barium. Ba. 137 Bismuth. Bi. 210 Cadmium. Cd. 112 Caesium. Cs. 133 Calcium. Ca. 40 Cerium. Ce. 92 Chromium. Cr. 52-2 Cobalt. Co. 58-8 Copper. Cu. (Cuprum) G3-4 Davyum. Da. (?) Didymium. D. 95 Erbium. E. 168-9 Gallium. Ga. 68 Glucinum. Be. (Berryllium) 9-4 Gold. Au. (Aurum) 197 Indium. In. 1134 Iridium. Ir. 198 Iron. Fe. (Ferrum) 56 Lanthanum. La.' 936 Lead. Pb. (Plumbum) 207 Lithium. Li. 7 Magnesium. Mg. 24 Manganese. Mn. 55 Mercury. Hg. (Hydrargyrum) 200 Molybdenum. Mo. 96 Nickel. Ni. 588 Niobium. Nb. 94 Osmium. Os. 199-2 Palladium. Pd. 106-6 Platinum. Pt. 1974 Potassium. K. (Kalium) 39-1 THE METALLIC ELEMENTS. 15 COMBINING NAMES. SYMBOLS. WEIGHTS. Rhodium. Rh. 104-4 Rubidium. Rb. 85-4 Ruthenium. Ru. 104-4 Silver. Ag. (Argentum) 108 Sodium. Na. (Natrium) 23 Strontium. Sr. 87-6 Tantalum. Ta. 182 Terbium. Ter. 148-5 Thallium. Tl. 204 Thorium. Th. 235 Tin. Sn. (Stannum) 118 Titanium. Ti. 50 Tungsten. W. ( Wolframium) 184 Uranium. U. 240 Vanadium. V. 51-2 Yttrium. Y. 92 Zinc. Zn. 65-2 Zirconium. Zr. 89 6 Of these, only about fourteen are employed in true metallic condition. These are: Antimony, Magnesium, Aluminum, Mercury, Bismuth, Nickel, Copper, Platinum, Gold, Silver, Iron, Tin, Lead, Zinc. Twelve are more or less extensively used in medi- cine, and in the arts as coloring pigments and for alloying purposes. These are: Arsenic, Lithium, Barium, Manganese, Cadmium, Potassium, Calcium, Sodium, Chromium, Titanium, Cobalt, Uranium. 16 DENTAL METALLURGY. The remaining twenty-six are as yet of little or no practical use in the metallic state. Seven of the metals play a more or less important part in the maintenance of animal and vegetable life. These are: Aluminum, Manganese, Calcium, Potassium, Iron, Sodium. Magnesium, The metallic elements are divided by metallurgists into two classes,—the noble and base metals. The first are those which are capable of being separated from combinations with oxygen by merely heating to red- ness. The base metals are those whose compounds with oxygen are not decomposable by heat alone. The noble metals are ten in number, as follows: Mercury. Hg- 200 Silver. Ag- 108 Gold. Au. 197 Platinum. Pt. 197-4 Palladium. Pd. 106-6 Rhodium. Rh. 104-4 Ruthenium. Ru. 104-4 Osmium. Os. 199-2 Iridium. Ir. 198 Davyum. Da. (?) The base metals are further subdivided according to their affinity for oxgen and other chemical prop- erties. CHAPTER III. PROPERTIES OF THE METALS. A METAL may be defined as an elementary sub- stance, usually solid at ordinary temperatures,* insoluble in water, fusible by heat, and possessing a peculiar luster, commonly spoken of as a "metallic luster; " an expression sometimes used in describing the appearance of substances which present a similar condition of surface. To these qualities must be added those of conducting heat and electricity, which the metals possess to the greatest extent, and the power of the metals of replacing hydrogen in chemical reactions ; as, when zinc is placed in contact with hydrochloric acid it displaces the hydrogen and unites with the chlorine to form zincichloride (chlo- ride of zinc), thus: Zn + 2HC1 = ZnCl2 + H2 liberated. Another characteristic of the metals is their basic properties when united with oxygen. Arsenic and tellurium are by some regarded as intermediate links between the metallic and non- metallic bodies. Watts, in his " Dictionary of Chem- istry," says of tellurium that " this element, though decidedly metallic, must be classed as a member of ■ Mercury is an exception, being fluid at the ordinary temperature. Tt freezes at —40° F. (17) 18 DENTAL METALLURGY. the sulphur family;" probably in consequence of its poor conducting qualities and the acid character of its oxides. Bloxam does not regard arsenic as a metal, and states that, though "some authorities class it as such on account of its metallic luster and property of con- ducting electricity, yet it is lacking in the quality of forming a base with oxygen, a property common to all the true metals; " and asserts that " the chemical character and composition of its compounds connect it in the closest manner with the phosphorus group." On the other hand, we find some of the non-metallic bodies possessing the chemical but not the physical properties of the metals. Thus, the real nature of hydrogen has long been an interesting point of dis- cussion among chemists, some supposing it to be a metal in a gaseous form. Dumas and others prophe- sied that " if ever the means of liquefying hydrogen is found, it will present the appearance of quick- silver," and their grounds for this belief are its uniformly basic properties. Others contend that it is a neutral substance, possessing both the basic prop- erties of a metal and the chlorous qualities of a gas. In 1869 is was announced that Prof. Graham, an eminent English chemist, had discovered the metallic hydrogen. " This new metal, baptized ' hydroge- nium,' was white, magnetic, of a specific gravity about 2, and appeared to have some analogy to mag- nesium." This discovery excited much speculation. Upon verification, however, the new metal was found to be a compound of palladium and hydrogen, in which the former had absorbed 700 or 800 times its bulk of the latter. PROPERTIES OP THE METALS. 19 Again, the existence of a hypothetic compound metal called ammonium, and having the constitution NH4, has been assumed as the only method of ex- plaining the perfect analogy that exists between the salts of ammonium and those of some of the metals, actual experiments having already strengthened this theory, at first founded only on analogy.* The metals are all quite opaque, with the single exception of gold, which, however, is only transpa- rent in leaves of a highly attenuated condition, when it transmits green light.f The Color of the metals ranges from the pure white of silver to the bluish hue of lead. Between these two the major part of the others may be found. About five run from light yellow to deep red. These are, barium and strontium, pale yellow; calcium, somewhat deeper in color; gold, when pure, of a rich yellow; and copper, the only red metal. It was at one time supposed that the mineral titanium, well known to dentists as a dark red (copper-colored), crystalline substance, used in a finely-divided state as a coloring pigment in the manufacture of por- celain teeth, was a metal. It was so pronounced by Wollaston. Wohler and Deville, however, demon- strated that the red mineral is an oxide, and they verified their statement by producing the metal it- self, which is of a steel-gray color. The color of the metals is modified by alloying.J * E. Miller, Treatise on Chemistry. f It is by some believed that the absence of transparency in the other metals may only depend upon our inability to obtain them in a suffi- ciently attenuated condition. J See Chapter on "Alloys." 20 DENTAL METALLURGY. Luster.—This characteristic of the metals is prob- ably the result of perfect opacity, by which the rays of light are reflected from the surface. Odor and Taste are possessed by some few of the metals. The greater number, however, are destitute of these qualities. Iron, copper, and zinc, when heated, evolve peculiar odors, and one means of de- tection of arsenic is the odor of garlic observed when that metal is exposed to an elevated temperature. Odor and taste may depend upon voltaic action. The former may be noticed in a marked degree when holding in the hand a mass of an alloy composed of gold, platinum, tin, and silver prepared for use as amalgam. The moisture of the hand, aided by its heightened temperature, seems to promote the elec- trical action. Fusibility.—All metals admit of being reduced to a liquid state by the application of heat, but the tem- perature at which they melt differs widely. Thus, mercury retains its liquid form to 39° F. below zero, and is always fluid at ordinary temperatures. Po- tassium and sodium fuse below the boiling-point of water; tin, lead, and antimony below redness. Gold, silver, and copper require bright redness. Iron, nickel, and cobalt fuse at white heat, while platinum, iridium, rhodium, titanium, etc., become fluid only when exposed to a powerful voltaic current or the flame of the oxyhydrogen blow-pipe. PROPERTIES OP THE METALS. 21 Table of Fusing-points of the Principal Metals. fe ■& -£ M .£P2 g , o E3JM ,2,2 a g >^2 >> _ a; fr Mercury FAilft. —39° Rubidium +101.3 Potassium 144.5 Sodium . 207.7 Lithium 356 Tin 442 Cadmium 442 Bismuth 497 Thorium 561 Lead 617 Tellurium, rather less fusible than lead. Arsenic, unknown. Zinc.....773 Antimony, just below redness. Silver . Copper . Gold . Cast-Iron Pure Iron Cobalt . Manganese Palladium Molybdenum Uranium Tungsten Chromium Titanium Cerium . Osmium Iridium Rhodium Platinum Tantalum FAHR. 1873 1996 2016 2786 22 DENTAL METALLURGY. Capacity for Heat.—The metals, in common with other bodies, have their specific heat. This consists in the amount of heat.required to raise equal weights of different metals from the same to another given temperature. Thus, if we express by 1 the quantity of heat necessary to raise a weight of water from 0°C. to 1°C, that which must be supplied to elevate the same weight of the following metals to that tem- perature would be as follows :* Mercury Gold . 0.03332 0.03244 Iron 0.1138 Nickel 0.1086 Cobalt 0.1070 Zinc . 0.0956 Copper Palladium 0.0952 0.0593 Silver . 0.0570 Cadmium 0.0567 Tin . 0.0562 Antimony Lead . 0.0508 0.Q314 Platinum 0.0311 Bismuth 0.0308 Now, if we should take equal bulks of these metals and expose them for the same length of time to exactly the same heat, and then place them simul- taneously upon a cake of wax, we would observe those of the above table with the highest figures, such as iron, instantly melting their way through the wax, while those of the lowest capacity for heat, such as bismuth, would remain on the surface. * Phillips's Metallurgy, p. 13. PROPERTIES OP THE METALS. 23 Expansion by Heat.—Metals expand when heated, but this property is not uniform, some possessing it to a greater or less extent than others. Within cer- tain limits of temperature this takes place propor- tionately to the amount of heat to which they are exposed. Zinc possesses a rather high degree of expansibility, and is consequently useful for the pur- pose of making dies for swaging metal plates for artificial dentures. By many dentists it was for- merly thought that a metal, to be well suited for this purpose, should be entirely destitute of this property, so that after casting the die should not, in returning to its former condition in cooling, be smaller than the plaster model, the object per se being to have the plate fit the plaster cast perfectly ; whereas, the real pur- pose should be to make the plate fit the mouth close!}', the plaster model being only a means to that end. Plaster expands in setting. From the impression to the model two expansions are gone through before the fac-simile of the mouth in plaster is obtained; hence, a plate made to fit such a model perfectly must necessarily be somewhat larger than the mouth,—a condition unfavorable to atmospheric adhesion. On the other hand, a plate made to fit the zinc will not be found too small for the mouth, but will, provided the impression is a good one and represents perfectly the conformation of the mouth, afford a very close-fitting plate. Even better results might be expected where the plate is somewhat smaller than the mouth, because such a condition would, in entire upper dentures, throw any undue pressure upon the alveolar ridge, while that portion of the plate covering the palatine arch would barely be in contact 24 DENTAL METALLURGY. with the tissues; the pressure along the ridge would quickly promote absorption of the remains of the alveoli, and a uniform adaptation of the plate to the mouth would soon follow. On the contrary, if the plate be made to fit the plaster cast, and is a trifle larger than the mouth, the pressure will be thrown upon the palatine arch at the back edge of the plate, at a region not likely to change by absorption, as is the case with the alveolar ridge, and hence the margin of the plate will imbed itself in the tissues and cause much discomfort and impair the usefulness of the denture. Much time and thought have been expended in the effort to discover some alloy which, in connection with the properties of hardness and fusibility, shall possess that of non-expansibility when heated. Har- ris's " Principles and Practice of Dentistry " gives no less than nine different formulae. The author is satis- fied that the property of expansibility in zinc as used in the dental laboratory constitutes one of its most valuable qualities, as it gives us the means of com- pensating for the yielding of the tissues and the absorption along the ridge which nearly always fol. lows the first insertion of an artificial denture. Table of Expansion of Metals for each degree from o° C. to ioo° C* Gold .... 0.00155155 Silver .... 0.00190868 Platinum . . . 0.00099180 Palladium . . 0.00100000 Copper . . . 0.00171733 Iron .... 0.00123504 0.00284836 Tin . . . . 0.00193765 Zinc (cast) 0.00294167 " (ham'r'd) 0.00310833 Bismuth . 0.00139167 Antimony 0.00108333 * Phillips's Metallurgy. PROPERTIES OP THE METALS. 25 Power of Conducting Heat.—The metals are the best conductors of heat among the solid bodies. The quality of transmitting heat is possessed by them in variable degrees. The following table shows the relative approximate ratio of conductivity of heat of each of the metals commonly used in the mechanic arts : 100 73.6 53.2 23.6 14.5 11.9 11.6 8.5 '8.4 6.3 2.8 1.8 Power of Conducting Electricity.—Metals conduct electricity nearly in the ratio of their capacity of transmitting heat. Dav}r, Becquerel, and Dr. Matthiesen have, at different times, more or less extensively experimented upon this characteristic quality of the metals. Among the results of Dr. Matthiesen's investigations are the facts that debas- ing a metal or alloying it greatly diminishes its con- ducting power, and that elevation of temperature has the same effect, and that between 32° and 212° F. (or 0° and 100° O), great diminution takes place,— not- uniformly, however, as some lose it more in pro- portion than others.f Silver Copper Gold Brass * Tin. Iron Steel Lead Platinum German Silver Rose Fusible Metal Bismuth . "■ Zinc is probably between brass and tin. f Makins's Metallurgy, p. 17. 3* 26 DENTAL METALLURGY. A rough means of determining the relative con- ducting power of metals consists in connecting the poles of a voltaic battery by a wire through which the current will pass freely. Now, if the wire be too small for the transmission of the electricity supplied to it, the obstruction will be manifested by the wire becoming red-hot.* Hence the relative capacity of metals for this pur- pose may be observed by employing equal battery- power upon wires of the same, diameter of different metals, and noting the length of the portion of each which can thus be heated. Conversely, the same means may be employed to indicate the quantity of electricity, or the capacity of the battery itself. In this case the wire is made to demonstrate the power of the battery by the length of wire which the battery is capable of rendering incandescent. A good demonstration of the relative conducting power of different metals may be made by construct- ing a chain of alternate links of platinum and silver wire. This will show, while the current of electricity is passing, red-hot platinum links alternating with cool silver ones. Platinum, being much the inferior conductor, offers such an impediment to the passage of the current that great elevation of temperature results, while the silver, being a good conductor, offers no check to the free passage of the electricity. The power of metals of conducting electricity is *This was fully shown in some of the electro-magnetic mallets made some ten or twelve years ago, in which the wire was too small for the accompanying battery-power. They worked very well for a few min- utes, when they became hot and ceased working. PROPERTIES OP THE METALS. 27 shown in the following table from Matthiesen (Phil. Trans. 1863): Silver Copper Gold Zinc Iron Tin Lead Antimony Bismuth 100 99-95 77-96 2902 16-81 12-36 8-32 4-62 1-24 at 32° P. Malleability, Ductility, and Tenacity.—The qualities of malleability, ductility, and tenacity differ widely in the metals. The term Malleability, when applied to such a metal as gold, signifies that by hammering or rolling its surface may be extended in all directions, and that it is capable of being thus reduced to very thin leaves or sheets without fracture of its continuity at the edges during the process of attenuation; when applied to other metals, the term should be understood as expressing this quality relatively. Gold is the most malleable of the metals, and is capable of being made into leaves of 80 0X0 0 0 of an inch in thickness, each grain of which will cover a surface of 54 sqr. inches. In the following list, by Kegnault,* the metals are arranged in the order of their malleability : 1. Gold. 2. Silver. 3. Tin. 4. Copper. 5. Cadmium. 6. Platinum. 7. Lead. 8. Zinc. 9. Iron. 10. Nickel. 11. Palladium. 12. Potassium. 13. Sodium. 14. Mercury (frozen). Ductility signifies that property which renders a * Phillips's Metallurgy, p. 412. 28 DENTAL METALLURGY. metal capable of being drawn into rods or wires, usually accomplished by passing an elongated piece of metal through a series of gradually diminishing holes in a steel draw-plate; the granular particles of the metal are thus extended into fibers. One grain of gold has been drawn into a wire 550 feet long. To accomplish this result a compound wire is made, of gold covered with silver, the tenacity of the latter being taken advantage of to enable the gold to be carried through the successive holes of the draw- plate, until the greatest possible attenuation is reached; after which it is immersed in nitric acid, which dissolves the silver, leaving a gold wire jrfa of an inch in diameter. In the following table the metals are arranged according to their ductility: 1. Gold. 5. Copper. 9. Nickel. 2. Silver. 6. Zinc. 10. Palladium. 3. Platinum. 7. Tin. 11. Cadmium. 4. Iron. 8. Lead. Tenacity is the power possessed by metals of sus- taining weight, or of resisting rupture, when a bar or rod is exposed to tension. As the fitness of metals for certain purposes in the industrial arts depends largely upon this property, it is of the utmost im- portance to know the relative tenacity, not only of the different metals, but of different alloys. This is usually ascertained by preparing wires of exactly equal diameters. These are suspended by one end from a fixed bar, and to the other extremity weights are gradually and carefully added until the wire breaks. The weight which causes the fracture rep. PROPERTIES OP THE METALS. 29 resents, when compared with other wires similarly treated, the relative tenacity of the metal. Elevation of temperature, even within rather cir- cumscribed limits, affects the tenacity of metals to a marked degree, generally diminishing it. There are some exceptions, such as iron and steel. On the other hand, malleability and ductility are only developed in some of the metals by an elevated temperature. thus, it was found that zinc, which had previously been of no use in an unalloyed state, was rendered perfectly malleable and capable of being rolled into very thin sheets merely by heating to between 248° and 302° F. (=120° and 150° O), and it has conse- quently come very largely into use. If carried much beyond this point, however, say to 400° F. (=205° 0.), it becomes very brittle, and may even be reduced to powder in an iron mortar. A rather unsatisfactory demonstration of this fact sometimes occurs in the fracture incident to the falling upon the hearth or floor of a hot zinc die, carelessly removed from the molding sand in the laboratory, its brittleness being so extreme at 500° F. that it might be broken into a number of pieces. Magnesium, aluminum, and some other metals, which at ordinary temperatures are nearly destitute of ductility, have that quality greatly increased by heating, and are then readily drawn into wire. In alloys these qualities are diminished by heating. Thus, the great tenacity and ductility of brass are entirely destroyed by simply heating to dull redness. Again, while it is claimed that in pure gold tenacity is increased by heating, it is quite certain that 18- carat gold is rendered brittle at red heat. 30 DENTAL METALLURGY. The following table* gives the results of experi- ments on the tensile strength of a few of the metals at temperatures between 15° and 20° C. NAME OF METAL. For wire of 1 Sq. MM. Section, Weight (in Kilos) causing Permanent Elonga-tion of 1-20,000. Breakage. Gold, drawn 13.5 27 " annealed . 3.0 10 Silver, drawn 11.3 29 " annealed . 2.6 16 Platinum, drawn 26. 37 " annealed 14. 23 Copper, drawn . " annealed 12. under 3. 40 30 Iron, drawn 32. 61 " annealed under 5. 47 Palladium, drawn 18. 37 " annealed . under 5. 27 The following table shows the order of relative capacity of the metals for sustaining weight: 1. Iron. 4. Silver. 7. Tin. 2. Copper. 5. Gold. 8. Lead. 3. Platinum. 6. Zinc. It has been observed that students and others very often fail at first to appreciate the difference between these properties, and they not infrequently fall into the mistaken idea that the three qualities of mal- leability, ductility, and tenacity are possessed to an equal extent by each metal. If, however, we take gold, for example, the most perfectly malleable and ductile of the metals, we shall find that in tenacity Annales de Chimie et de Physique (iii), vol. xii, Wertheim. PROPERTIES OP THE METALS. 31 it ranks considerably below some of the others, and the greatest care is necessary in drawing a piece of pure gold into even a moderately fine wire, and be- yond a certain limit, past which platinum or copper may be carried with safety, gold would not possess sufficient tenacity to overcome the resistance to which it would be exposed in passing through the smaller holes of the draw-plate, and fracture would result. Iron, on the other hand, which exceeds all the other metals in tenacity, is in malleability inferior to gold, silver, copper, platinum, lead, zinc, tin, and cadmium. Crystalline metals, such as bismuth, antimony, and arsenic, do not possess these properties. They are easily broken by even slight blows of a hammer, and the two latter may be reduced to powder in a mortar, It is stated that brass drawn into wire will often after a time, become crystalline in texture and brittle by slow change of molecular arrangement.* Crystallization.—It is stated that under favorable circumstances all the metals will assume a crystalline form. It is known that some of them, as gold, silver, etc., are found native as cubes or octahedra, or in slight modifications of these forms; and metals in a crystalline form may be obtained by electrolysis. For example, silver may be obtained in the form of crystals nearly pure by introducing strips of copper into a solution of argentic nitrate. A piece of zinc introduced into a solution of plumbic nitrate will precipitate the lead in the form of feathery crystals. Gold may also be deposited in this form from solution by the introduction of a stick of phosphorus.' Nearly all the metals yield crystals when deposited from * Makins's Metallurgy, p. 10. 32 DENTAL METALLURGY. their solutions by electric currents of feeble intensity. The beautiful preparation known as Watts's Crystal Gold is formed in this way. Gold so prepared is generally in a high state of purity. Elasticity and Sonorousness may be conferred upon the metals by alloying. Thus, iron does not possess these qualities until combined with the proper pro- portions of carbon, when by subsequent tempering the highest degree of elasticity is developed, and pieces of steel of different lengths, as arranged in the dulcimer, when struck by a small wooden hammer, are capable of giving off the most musical sounds. Again, a very great amount of elasticity is obtained by the admixture of copper and zinc in the form of brass, from which a spiral spring may be made, equal to that from any other alloy. It is curious to observe how this quality may be developed by the admixture of two metals, each of which, examined separately, is soft and destitute of anything like springiness. Thus, gold and platinum, both soft metals, when combined in the proportions of, say, 1 grain of the latter to 1 dwt. of the former, of 20-carat fineness, will afford a decidedly elastic alloy, suitable for clasps for artificial dentures. A tolerably elastic alloy may be formed by combin- ing platinum with a small amount of iridium. This alloy is frequently employed in the construction of artificial dentures.* Sonorousness is obtained to the greatest extent in alloys of copper and tin, known as bell-metal. Volatility.—All metals are probably more or less * See chapter on " Platinum and its Alloys." PROPERTIES OP THE METALS. 33 volatile, although only a certain number admit of being converted with any degree of facility into a Btate of vapor, even at the highest temperatures. Some of the conspicuously volatile metals are zinc, cadmium, mercury, arsenic, tellurium, potassium, and sodium; while a few others have the property of communicating characteristic colors to flame, and are probably volatile to a limited extent. Metals are sometimes characterized as " fixed," as gold, copper, nickel, etc., and " volatile" (during fusion), as cadmium, zinc, etc. Arsenic may unques- tionably be regarded as belonging to the latter group, passing as it does without fusion from the solid to the gaseous state. Gold has been known to volatilize under certain conditions. Makins states that it is doubtful whether it is at all volatile per se, but if alloyed with copper it has been shown by Napier to be considerably vol- atilized, so that quantities amounting to 4£ grains could be collected during the pouring of 30 pounds' weight from a crucible. According to Makins, gold has been known to volatilize when mixed with silver and lead and cupelled together, he having collected considerable quantities of each metal from the chim- ney of an assay furnace after only a few weeks' use. Agents which may Volatilize a Metal.—Concentration of solar rays in the focus of a lens; the Voltaic cur- rent; the oxyhydrogen blow-pipe flame. M. Despretz employed the three in conjunction, by which means he volatilized magnesium, and with a powerful Bunsen battery alone he reduced carbon by volatilization to the state of a black powder.* * Percey's Metallurgy. 4 CHAPTER IV. ALLOYS. IV/TOST of the metals are capable of uniting with one another, forming a class of compounds termed alloys, in which may be observed to a greater or less extent the properties of the several constitu- ents entering into the union. From a purely scientific point of view, the study of the alloys is an interesting one, as they are not only mixtures of metals possessing certain distinct qualities, but in reality are true chemical compounds. In the appearance which often accompanies the union of the metals, and in the properties of the resulting alloys, we may frequently observe the phenomena which characterize chemical affinity, such as heat and incandescence, resulting in the formation of sub- stances having a definite composition, distinct crys- talline form, and properties differing from those of their constituents. When a piece of clean sodium is rubbed in a mortar with dry mercury, the former dissolves, and a pecu- liar seething sound, resembling that caused by the immersion of a hot body in water, is produced, due to the evolution of heat which accompanies the com- bination, the mercury rising rapidly in temperature as the pieces of sodium are added. As the mercury (34) ALLOYS. 35 cools, the resulting alloy, which is brilliantly white, crystallizes in long, needle-like forms from the middle of the liquid, and the excess of mercury may be poured off. Alloys are generally harder and more fusible than the metals of which they are formed, and as many metals are unfit in the pure state for use in the me- chanic arts, owing to extreme softness or high fusing- point, these properties are modified to suit various requirements by the admixture of other metals. Thus, as a base for an artificial denture, pure gold would be too soft to withstand, without bending, the force to which the fixture would be exposed during mastication, but by the addition of sufficient copper and silver to reduce the gold to .750 (18 carats) the necessary rigidity may be obtained without materially affecting the other properties. Again, it is often de- sirable to unite several pieces of the same metal or of different metals. This is accomplished by means of a class of alloys called solders, generally formed of the metal upon which they are to be employed with the addition of some other metal which will considerably lower the fusirig-point without affecting the color, as it is desirable that the place of union should not be noticeable. For example, a solder suit- able for use in prosthetic dentistry should fuse at a much lower temperature than the plate upon which it is to be used. Its color should be as nearly as pos- sible the same, and what is even more important, it should withstand the action of the fluids of the mouth nearly as well. These properties may be obtained by the addition of small quantities of silver, copper, or brass. 36 DEN'EAL METALLURGY. The value of many of the metals for industrial uses is very greatly enhanced by alloying. Thus, copper, which is unfit for casting and too tough for turning, may, by the addition of zinc, be rendered not only harder and more elastic, but the fusing-point of the resulting compound will be so much lower than that of the copper alone as to render the casting of it a matter of no great difficulty, while at the same time it will be found susceptible of being turned in the lathe with facility. The tendency on the part of metals to unite in definite proportions may be studied in connection with platinum, iridium, gold, rhodium, ruthenium, and silver, when fused with tin. If the latter metal is in excess, after cooling a metallic ingot is obtained resembling closely the tin ; but by the action of strong hydrochloric acid upon this the excess of tin may be dissolved, leaving crystals of a definite alloy of the tin and the noble metal, which cannot be further dissolved by the same acid, but are soluble in nitro- hydrochloric acid, even when the precious metal con- tained, whether rhodium, ruthenium, or iridium, is in the free state absolutely insoluble by that agent. It must not, however, be assumed that all the alloys employed in the industrial arts are the result of defi- nite combination dissolved in an excess of one of the metals. Many combinations are capable of co- existing in the same alloy. This may be demonstrated in an alloy of tin, lead, and bismuth, which melts below the boiling-point of water. Heated to 25° C, and then permitted to cool, it will be observed, by the assistance of the thermometer, that the fall of temper- ature is twice distinctly arrested. The cause of ALLOYS. 37 this phenomenon has been assumed to be the produc- tion in the compound of a less fusible alloy, which in solidifying evolves heat, and thus for a time retards the gradual cooling of the mass. It may, therefore, bo assumed that true chemical combinations may occur between two metals, notwithstanding the fact that such union may be masked by excess of one of the constituents. Matthiesen, in an elaborate paper on the subject, states that " an alloy maybe, first, a solidified solution of one metal in another; second, a chemical combina- tion ; third, a mechanical mixture; or fourth, a solid- ified solution or mechanical mixture of two or all of the above." In simple mechanical mixture of two metals there is often a tendency to separate. This is noticeable in some alloys of silver and copper by an absence of perfect homogeneity in the ingot. Again, some of the metals form mixtures so decidedly mechan- ical that on being allowed to stand after fusing they will separate, the one possessing the highest specific gravity settling to the bottom. This may be observed when lead and zinc are mixed. Matthiesen, however, found that the lead retains 1-6 per cent, of the zinc, while the zinc retains 1-2 per cent, of the lead.* Density.—Theoretically, it might be supposed that the density of an alloy would be the mean of its con- stituents. Such, however, is not always the case, as the resulting number is sometimes equal to, or greater or less than, the theoretical mean. The density of alloys of gold and silver is less than the mean of the components, in consequence of expansion; while brass and alloys of lead and antimony vary in the opposite * Makins's Metallurgy, p. 62. 4* 38 DENTAL METALLURGY. direction, through a condensation of their constituents. But in the formation of some alloys there is no altera- tion of volume, and the density of such will corres- pond to that obtained by calculation as the mean of their constituents. The following table,* by Thenard, gives examples of variations of density in alloys: f Alloys Possossing a Groater Specific Gravity than the Moan of thoir Components. Gold and Zinc. " " Tin. " " Bismuth. " " Antimony. " " Cobalt. Silver and Zinc. " " Lead. " " Tin. " " Bismuth. " " Antimony. Copper and Zinc. " " Tin. " " Palladium. " " Bismuth. " " Antimony. Lead and Bismuth. " " Antimony. Platinum and Molybdenum. Palladium and Bismuth. Color is always modified by alloying. It is gener- ally such as might be expected to result from the mixture of the metals entering into the formation of * Phillips states that it is doubtful whether some of the mixtures inoluded in this table should be regarded as alloys. f Phillips's Metallurgy. Alloys Having a Specific Gravity Inferior to the Mean of thoir Compononts. Gold and Silver. " " Iron. " " Lead. " " Copper. " " Iridium. " " Nickel. Silver and Copper. Copper and Lead. Iron and Bismuth. " " Antimony. " " Lead. Tin and Lead. " " Palladium. " " Antimony. Nickel and Arsenic. Zinc and Antimony. ALLOYS. 39 the alloy. There are a few instances, however, where it is different. Thus, three parts of silver to seven of gold yields a green alloy, and nickel, added to brass, produces an alloy of silvery whiteness. Malleability, Ductility, and Tenacity.—These prop- erties are generally much changed in metals by alloy- ing ; malleabilit3T and ductility being diminished, and in some cases entirely destroyed, even in the combi- nation of two very ductile metals, as is the case with gold containing a small quantity of lead, ductility being completely lost. Again, gold and platinum, two exceedingly ductile metals, are rendered much harder and somewhat elastic by admixture. The union of a brittle and a ductile metal yields a brittle alloy. According to Mr. Makins, antimony, a metal so brittle that it may be broken up in a mortar, when added to gold to the extent of yttVu" Vai% Wl^ make the gold quite unworkable. Tenacity is generally increased by alloying. The following results were obtained by Matthiesen, by employing wires of the same gauge and noting the weights which caused their rupture before and after alloying: LBS. alloyed with 12 per ct. Tin, 80 to 90 " " " Copper, 7 " "Tin..... 7 " " Copper ... 70 " " Platinum . 75 to 80 Steel (Iron alloyed with Car- bon) ......above 200 Generally speaking, the hardness of metals is in- creased by alloying them. A familiar instance is LBS. Copper, unalloyed ,25 to 30; Tin, k under 7; Lead, u " 7; Gold, ii 20 to 25; Silver, ii 45 to 50; Platinum, ii 45 to 50; Iron, ii 80 to 90; 40 DENTAL METALLURGY. standard gold or silver. Neither of these when un- alloyed is sufficiently hard to resist attrition to the degree required for currency; but the addition of one-tenth of its weight of copper to either metal in- creases its hardness to the required point. Ninety- four parts of copper with six parts of tin form an alloy so brittle that it may be broken with a hammer. Fusibility.—The fusing-point of an alloy is always lower than that of the least fusible metal entering into the composition of the alloy. Thus, an alloy com- posed of five parts of bismuth, three of lead, and two of tin, melts at 91° O, less than the boiling-point of water, while tin alone fuses at 227-8° O, and lead at 325° O, and the addition of a small amount of cad- mium to the above alloy will further reduce the fusing- point to 140° F., or 60° C. Lead combined with a small portion of silver is more fusible than the former in a state of purity, and an alloy may be formed of potas- sium and sodium, which remains fluid at ordinary temperatures of the air. This phenomenon has been explained by Mat- thiesen. He says that " matter in the solid state ex- hibits excess of attraction over repulsion, while in the liquid state these forces are balanced, and in the gaseous state repulsion predominates over attraction, and similar particles of matter attract each other more powerfully than dissimilar particles do. The attraction subsisting between the particles of a mix- ture will be sooner overcome by repulsion than will the attraction in the case of a homogeneous body; hence mixtures should fuse more readily than their con- stituents." * * Makins's Metallurgy, p. 65. ALLOYS. 41 Composition of Alloys.—A statement of the average proportions in which metals enter into the best-known alloys, the composition of which is generally very variable, is given in the following table : Coinage of Gold Silver Vessels Silver Jewelry Aluminum-Bronze rGold . \ Copper Gold Jewelry and Plate { „ I Copper „.. „ . ( Silver Silver Coinage . . . . < _ I Copper r Silver \ Copper r Silver \ Copper r Copper 1 Aluminum Specula of Telescopes . j ?PP r Pinchbeck...../^°PPer \ Zinc . Brass.......j °°Pp6r \ Zinc . (Copper Zinc . Nickel f Lead . I Antimony (Copper Tin . Zinc . German Silver Type-metal Bronze Cannon Bronze Bells Bronze Cymbals f Copper \Tin . rCopper \Tin . f Copper I Tin . . . 90 . . 10 75 to 92 25 to 8 90 10 95 5 80 20 90 to 95 10 to 5 . . 67 . . 33 . . 90 . . 10 67 to 72 28 to 33 50 25 25 80 20 94 to 96 6 5 90 10 78 22 80 20 42 DENTAL METALLURGY. Tin . . 100 Antimony 8 Bismuth . 1 Copper . 4 Tin . . 92 Lead . . 8 Tin . . 82 Lead . 18 Tin . . 67 Lead . . 33 Alloys used as plates for artificial dentures and those constituting solders will be described with the metals forming their bases. Decomposition.—When the alloy contains a volatile metal, like zinc or mercury, heat decomposes it, but the temperature required to expel the last trace of the volatile metal must be considerably higher than that metal's normal temperature of ebullition. If the alloy is composed of a noble metal and zinc, lead, or tin, and it is desired to free it from the impurity, this may be accomplished by exposure to a high tempera- ture, and the addition, while the metal is fluid, of some substance rich in oxygen, such as potassium nitrate. By this means the base metal is converted into an oxide, and is then dissolved and held in solution by the borax, which should be used as a flux in the crucible. Metals in combination with mercury may be separated by the application of heat, the mercurj- volatilizing at 600° F. In.the case of particles of amalgam, how- ever, the temperature to which tne pieces are exposed should be at least a bright-red heat. Influence of Constituent Metals.—Mercury, bismuth, tin, and cadmium give fusibility to alloys into which they enter; tin also gives hardness and tenacity; lead English Metal . . Pewter . . . . Liquid Measures . Plumbers' Solder . ALLOYS. 43 and iron give hardness; arsenic and antimony render alloys brittle. f It has been observed that phosphorus and arsenic, when added to alloys of copper and tin, have the power of deoxidizing or eliminating metallic oxides, which are invariably present. The well-known phosphor- bronze owes its closeness of grain and superior tenacity to the addition of phosphorus, and it is claimed that " when arsenic or arsenical compounds are made to unite, under suitable conditions, with alloys of copper and tin, known as bronze or gun-metal, it imparts to them several remarkable and, for many purposes in the arts, desirable properties—among others and prin- cipal of which are homogeneity, hardness, elasticity, greatly increased tensile strength and toughness, and a peculiar smoothness, rendering it a valuable anti- friction metal for journal-bearings," etc. The arsenical compounds of alloys of copper and tin are also more fluid when molten than are other known alloys of copper and tin—a property which renders them capable of filling out sharply and with- out flaws the most intricate molds. Liquation.—The constituents of an alloy heated gradually to near its point of fusion frequently unite to form new compounds, and if the fluid portion is poured off, there remains a solid alloy less fusible than the original. Copper is separated from silver by this process. In bars of silver alloyed with copper, a curious tendency on the part of the latter to separate and aggregate at the edges, as the fused mass assumes the solid form, has been observed. Mr. Makins states that, as a result of careful examination of Mexican dollars and crown pieces, he found the variation 44 DENTAL METALLURGY. between the center and edges to range in the former from one to six, and in the latter from one to four milligrammes. He gives as the average of a number of experiments on twenty-four crown pieces a mean variation of two milligrammes, and as the quality in which the greatest tendency to separate is shown that of 900 parts of silver to 100 of copper.* Temper.—Modified conditions of hardness and elas- ticity of a metal, it has been shown, may be obtained by admixture with other metals and by sudden varia-. tions of temperature, as in the case of the alloy of 94 parts of copper and 6 of tin, which forms a bronze so brittle that it may, when heated and slowly cooled, be pulverized with a hammer; but if, on the contrary, it is cooled rapidly, by immersion in cold water, it becomes malleable. The treatment of iron mixed with carbon (steel) is just the opposite, the greatest degree of hardness being attained by suddenly cooling the heated mass. Preparation.—When the alloy is to be formed of a noble and one or more of the base metals, the former should be thoroughly fused first; the latter is then added, and the whole covered with charcoal, to prevent oxidation, and then thoroughly mixed by stirring or agitating. When it is designed to lower the fusing-point of gold or silver for use as solders, by the addition of brass, etc., the precious metal should first be thor- oughly fused with a sufficient quantity of borax; the brass, in the convenient form of wire, should then be quickly thrust into the melted gold or silver. It will almost instantly mix with the melted mass, and * Makins's Metallurgy, pp. 379, 380. ALLOYS. 45 the borax, if in sufficient quantity, will cover the liquid alloy, and thus protect from oxidation by con- tact with the atmosphere. The action of acids upon alloys is generally more energetic than upon a simple metal, but it varies according to the relative amounts of their constitu- ents. Silver alloyed with a large proportion of gold is protected from the action of nitric acid. Sometimes, however, the reverse of this is seen, and metals . which are totally insoluble in certain menstrua are made to dissolve in them by the addition of a metal on which they have the power of acting. Thus, plat- inum, although of itself insoluble in nitric acid, may be dissolved by it when sufficiently alloyed with silver. Alloys consisting of two metals, one readily oxi- dizable, the other possessing less affinity for oxygen, may be readily decomposed by the combined action of heat and air. In this case the former metal will be rapidly converted into an oxide, excepting perhaps the last portions, which may in some degree be pro- tected from further action by the oxide already formed. This increased affinity for oxygen exhibited by alloys is probably an electrical phenomenon, the study of which belongs rather to the science of chemistry than to metallurgy. For further light on this subject the student may refer to Fownes's, Bloxam's, or other standard works on chemistry. 5 CHAPTER V. AMALGAMS. A MALCAM—the name given to an alloy of mer- cury with one or more other metals. The amalgams are a very numerous class of compounds, and many of them are used largely in the arts. Some amalgamations are formed merely by contact of the metals, and are accompanied by evolution of heat; others are obtained by the action of mercury on a salt of the metal, or the action of the metal on a salt of mercury, thus developing in some cases a weak electric current. The constituents of amalgam compounds are not generally held together by strong affinities, hence many of them may be decomposed by pressure, and all by high temperatures. Tin amalgam is used for " sil- vering " mirrors; gold and silver amalgams in gilding and silvering; while amalgams containing gold, silver, tin, platinum, and, in some cases, cadmium, zinc, copper, and other of the base metals, compounded according to many different formulae, have been used very extensively in dentistry. An amalgam of zinc and tin is employed for tho rubbers of electrical machines. An alloy for dental amalgams should possess the qualities of strength and sharpness of edge, and (46) AMALGAMS. 47 freedom from admixture with any metal favorable to the formation of soluble salts of an injurious character in the mouth. It should be capable of maintaining its color, although in an alloy composed of several dif- ferent metals absolute freedom from discoloration, under the conditions to which an amalgam filling is exposed, cannot readily be obtained. It should also bo capable of retaining its shape, as the tendency on the part of many amalgams to assume a globular form after their introduction, thus leaving the edges of the cavity unprotected, is probably a frequent cause of failure in this class of fillings. Undue expansion, although not so likely to occur as some other changes, would be equally a source of failure. According to Mr. Fletcher, amalgams of silver and mercury expand, sometimes sufficiently to split a tooth; and Mr. Kirby states that "amalgams of pure silver, either the precipitate or filings, expand greatly." With a suitable instrument for measure- ment, he was able to determine the change in bulk of such an amalgam, in which he found the extent of expansion to reach one-fortieth of its diameter. Many old amalgam fillings have the appearance of projecting from the edges of the cavity as though there existed some force behind or beneath sufficient to push them out. There seems to be some diversity of opinion respecting the cause of this, and while by some it is attributed to expansion, others believe it to be due to contraction. Favoring the latter theory, Mr. Fletcher states that he has found it to occur only with those amalgams which are known to shrink, and he suggests that the plug may be raised or forced out by the " decomposition of tooth-substance and the 48 DENTAL METALLURGY. formation of gas under the loosened plug, the driving down and accumulation of food underneath, or some similar cause." It is evident that an amalgam liable to contract or expand to a marked extent is not to be relied upon as a filling-material. Discoloration of dental amalgams depends largely upon the formation of sulphides. The fluids of the mouth, in every case where the most scrupulous cleanliness is not observed, may be said to contain sulphur in combination with hydrogen, as dihydric sulphide (H2S), resulting from decomposition of par- ticles of food having a lodgment between or adhering to the teeth. The affinity of sulphur for both silver and mercury is so active that we may reasonably assume that not only the discoloration of amalgam fillings, but in many cases their failure to prevent a recurrence of decay, is due to the action of that element upon the alloy. Nor is it safe, in compound- ing alloys for dental amalgams, to depend upon the protecting influence of metals which do not possess the same affinities, such as gold and platinum; for while these metals individually may remain wholly unaffected by contact with sulphur, it does not neces- sarily follow that their presence in an alloy will secure the same immunity to such metals as silver and mercury. There are doubtless other causes for the discolora- tion of amalgams, some of them purely adventitious, depending upon the administration of certain reme- dies in diseased, abnormal conditions of the fluids of the mouth, or the presence of vegetable acids in articles of food, such as fruit. An amalgam filling may retain its integrity of AMALGAMS. 49 surface, while, at the same time, the darkening of the tooth-substance unmistakably indicates chemical action at its peripheral portion, doubtless due to im- perfect adaptation, favoring the ingress of the eroding agent. It appears that almost any amalgam filling may be kept bright by friction, whether of the brush or from mastication, and it seems equally certain that all such fillings will blacken if the position which they occupy protects them entirely from friction. Again, an amalgam filling may retain its original color and brightness of surface, and yet not protect the tooth ; and, conversely, a filling of this class may exhibit a great degree of surface-discoloration and yet fully preserve the tooth from further decay, peripheral discoloration being much the worse con- dition of the two. In a number of experiments made with some of the well-known amalgams, such as Townsend's, Ar- rington's, "Standard Alloy," Lawrence's, Walker's, etc., as well as with some of higher grades, it was found that with care in using the proper quan- tity of mercury, and in packing the mass into clean glass tubes, subsequently filled with colored fluid and closely sealed, there was after several weeks not the slightest apparent leakage; and yet, when the same tubes were thrown into a solution of sulphureted hydrogen, the edges were attacked, and marked dis- coloration occurred at the periphery of the fillings, while the surface directly exposed to the action of the sulphur, and not in contact with the glass, was but slightly clouded. The edges had the appearance of having been eroded, as by an acid. This experiment is not merely speculative, as it is simply filling a cavity 5* 50 DENTAL METALLURGY. with amalgam and then exposing it to sulphur in the form usually found in the mouth. The result is pre- cisely similar to that which is observed in the great majority of amalgam fillings in actual service. It would seem, however, that the purely theoretical test of covering the plug with colored fluids, such as indigo, blue ink, etc., known as the " color-test," is not to bo relied upon in testing peripheral adaptation, for all the plugs used in these experiments had apparently excluded the passage of a solution of indigo or ink. Yet, that they did not perfectly seal the tubes, although introduced under very favorable conditions, was plainly shown by the result, which indicated that solution of one or more of the constituents of the alloy had taken place, accompanied doubtless with the formation of new compounds, consisting of sul- phides of silver and mercury, and in some instances, probably, of copper. Influence of Different Metals in Dental Alloys.—Tin dissolves very easily in mercury, but the alio}' hardens slowly and imperfectly. Without admixture with other metals it is unfit for use in the form of an amalgam in the mouth. It is also well known to possess a tendency to draw away from the edges of any cavity into which it may be packed, and to as- sume a globular form, and it never becomes suffici- ently hard to answer the requirements of a filling- material. Mixed with other metals, tin serves to facilitate amalgamation and to afford different degrees of plasticity. Between mercury and silver the affin- ity is but slight. By the addition of tin, however, union is facilitated. Silver apparently unites very readily with mercury. AMALGAMS. 51 Yet it will be found, upon close examination, that complete solution of pure silver filings will not take place until after long contact, unless the silver is in a finely divided state and the mercury heated. Under these circumstances, amalgamation will be more readily accomplished. Amalgams of silver and mer- cury alone are said to expand. Silver with tin added forms an alloy very white in appearance, but more easily oxidized than either of the constituents; mixed with mercury, an exceed- ingly unctuous and plastic amalgam is obtained, but it is somewhat slow in hardening. There is much diversity of opinion in regard to the contraction and expansion of this alloy. Dr. Hitchcock and Mr. Tomes both claim contraction* for it, while Mr. Kirby states that an " amalgam of an alloy of 3 parts of silver and 2 of tin contracts slightly at first, but finally expands about y£V f These variable results may depend upon the quantity of mercury employed. The author is satisfied that when an alloy of silver and tin is used no excess of mercury should be present, and when this precaution has been carefully observed he has found the results to be quite as good as those obtained with the alloys of the higher grades. Thus, 500 milligrammes of Arrington's amalgam, composed | of silver 40 per cent., tin 60 per cent., mixed with 160 milligrammes of mercury, withstood the sulphureted hydrogen test quite as well as those containing gold and phitinum. It should be borne in mind that alloys composed of tin and silver require much less * Transactions New York Odontologioal Society, 1874. f Ibid. J Hitchcock, Ibid. 52 DENTAL METALLURGY. mercury to render them plastic than those contain- ing gold and platinum in addition. Probably the most unfavorable property observed in an alloy of silver and tin is slowness in hardening, which favors the ingress of fluids by capillary force. The direct influence which silver exerts upon an amalgam of tin and mercury is to lessen the tendency to assume the spheroidal form, and to facilitate set- ting. In this respect its action is similar to that of gold; but while the latter further lessens these injurious tendencies, and confers similar properties, it cannot be made to supersede silver. In my experi- ments with these curious compounds I formed an alloy consisting of Gold ..... 500 milligrammes. Platinum .... 500 " Silver .... 2000 " Tin.....2500 " An amalgam of this alloy hardened almost instantly, so that a filling of it might be inserted and finished at one sitting. For the sake of experiment, another ingot was made, from which the silver was omitted. The result was an exceedingly brittle alloy, which could only be made to unite with mercury by heat- ing, and even then with difficulty, and it did not harden sufficiently to be of any use as a filling-ma- terial. Thus it will be seen that silver fills an impor- tant place in dental alloys, since without its presence amalgamation becomes difficult, and rapidity in hard- ening is not secured. Gold combines with mercury at all temperatures, but for rapid amalgamation an elevation of tempera- ture is required, and the process is accelerated if the AMALGAMS. 53 gold is in a state of fine division. With mercury alone it does not harden well; added to tin, it to a certain extent facilitates setting, but does not harden sufficiently for use in the mouth, though it prevents the tendency to draw away from the edges. But it is when added to an alloy of tin and silver that the greatest benefit is derived from its presence. I found that an alloy consisting of Gold.....500 milligrammes, Silver .... 2000 " Tin.....2500 " when mixed with mercury in the proportion of 500 milligrammes of the alloy to 250 milligrammes of mercury, retained its sharpness of edge, hardened well in a few minutes, and apparently filled all the requirements of a dental amalgam. The author is aware of the statement which has been made that gold added to an alloy of tin and silver retards hard- ening, but this is doubtless an error, and the presence of an excess of mercury is the real cause of the tardi- ness in setting. Platinum, in the usual form of plate or wire, does not readily unite with mercury. . A very smooth and plastic amalgam may, however, be formed by rubbing some finely-divided platinum, such as is obtained by precipitation, with mercury in a heated mortar. An amalgam composed of platinum and mercury alone does not harden well. The properties of an alloy of tin and silver are also greatly impaired by the addition of platinum in any considerable quantity. The author found an alloy consisting of tin 2500 milligrammes, platinum 500 milligrames, to be ex- ceedingly brittle, and with so little affinity for mercury 54 DENTAL METALLURGY. that amalgamation was only accomplished by eleva- tion of temperature and much rubbing, while the property of setting was almost entirely lost. If platinum be added to an alloy of gold and tin, the same negative results are observed. When com- bined with tin, silver, and gold, however, the influ- ence of platinum becomes apparent. With the proper proportion of mercury, it seems to confer upon such an alloy the property of almost instantly setting, as well as much greater hardness. Thus, it will be seen that the qualities claimed for platinum per se belong in reality to the combination of tin, silver, gold, and platinum with mercury, since if either one of the others is omitted the platinum do.es not even remain passive, but actually by its presence causes marked deterioration of the qualities essential in a dental amalgam. Alloys containing platinum amalgamate less read- ily than those wherefrom it is absent; yet when union has once begun they seem to require a larger quantity of mercury to render them plastic. By care- ful experiment, I found that with an alloy of Platinum .... 500 milligrammes, Gold.....500 « Silver.....2000 '« Tin .... 2500 '< the smallest amount of mercury which could be em- ployed without impairing the strength and general working qualities of the amalgam, was 300 milli- grammes to 500 milligrammes of the alloy; while with another ingot composed of Gold.....500 milligrammes, Silver.....2000 «< Tin.....2500 " AMALGAMS. 55 160 milligrammes of mercury to 500 of the alloy afforded a perfectly good result. The proper quantity of mercury should be ascer- tained by careful experiment, as the statements of manufacturers or venders are not always reliable. In order to ascertain the proportion of mercury re- quired by different alloys, a small quantity of the latter should be taken, say one gramme, and, after weighing, the mercury may be carefully added and mixed by rubbing until the mass assumes a semi- coherent state. It should then be weighed again, to determine accurately how much mercury is present. It may then be introduced into a glass tube and con- densed by means of instruments slightly warmed. Should the proportions not be correct, another trial may be made, and the quantity of mercury increased or diminished as indicated by the results of the first experiment. The quantity of mercury required by each alloy is probably definite, so that a tolerably accurate approxi- mation of the composition of an alloy should be ascer- tained by carefully noting the required proportions of one to the other. Different methods are employed for the attainment of this object. Probably the most common is to mix the alloy with a large excess of mercury, and then to express the surplus of mercury either by compression with the fingers or through the pores of a piece of chamois-leather. The first involves the loss of some of the alloy, which is carried away with the surplus of mercury, and neither is to be relied upon as a means of excluding an excess of mercury. There are also several methods employed in mixing 56 DENTAL METALLURGY. amalgams. Probably the most common one consists in simply rubbing the alloy and the mercury together in the palm of the hand. This is certainly the most expeditious, though not a very neat way, and much has been said about manifestations of the physiologi- cal effects of mercury following a long continuance of the practice. The author has made considerable effort to ascertain the correctness of this theory, in view of the fact that certain phases of ill-health have been attributed to absorption of mercury through this method of mixing amalgams, but he has been unable to trace a single case of ptyalism or any other well- marked sign of mercurial poisoning to this cause. It would be well to remember,- however, that the active properties of mercury are developed by a state of fine division, and that there is nothing un- reasonable in the theory that mercury, highly com- minuted by rubbing in the palm of the hand, may find its way into the system and produce con- stitutional disturbance. In view of these facts, it would bo a proper precaution to prevent contact of the mercury with the skin. This may be accom- plished by covering the hand with a piece of rubber dam, forming a sort of mitten, leaving the fingers free and having an opening for the thumb to pass through. Small porcelain and glass mortars are also employed in promoting amalgamation, but they do not effect the desired purpose speedily, in consequence of the granules of the alloy becoming burnished by the attrition of the pestle. Heating the mortar will however, greatly facilitate union. Mr. Fletcher has recently called attention to a AMALGAMS. 57 simple and effective method of mixing amalgams. The required weight of filings and mercury are put into a long, narrow test-tube or bottle, and well shaken for a few seconds. The percussive force brought to bear upon the mass promotes prompt union. . Some difficulty may be encountered in introducing amalgam fillings when the mercury has been reduced to the minimum. The semi-coherent mass, almost in the form of a powder, is not easily conveyed to the cavity, especially if it is in the superior arch. Fletcher has devised a sort of mold by which amalgams mixed so dry as to be unmanageable ordinarily may be rapidly shaped into a convenient, workable form. It consists of a cylinder, into which the semi-coherent mass is poured. By means of a piston the powder is compressed into disks of the desired thickness, which may be introduced into the cavity and solidly com- pressed with slightly warmed instruments. It is claimed that this method practically does away with the chief objection to the use of very dry amal- gams. Should the amalgam harden and become un- manageable before the completion of the filling, it may be rendered plastic and cohesive without disturb- ing the process of crystallization or the property of setting, by the use of slightly-heated instruments. A further addition of mercury will be found to greatly impair, if not destroy, these properties. Amalgams may be regarded as chemical compounds having definite proportions, and capable of being dissolved in an excess of mercury, in which condition, however, they will no longer be found to fulfill the requirements of a filling-material. Hence, the opera- 6 58 DENTAL METALLURGY. tor should in no case attempt to restore the plasticity of an amalgam which has once hardened, by a further addition of mercury. Forming Alloys for Amalgams.—The putting to- gether of the constituents of an alloy composed of tin, silver, gold, and platinum, is a matter of no great difficulty, as it does not require an extraordinary. degree of heat, and it may be easily accomplished in a stove or furnace such as is usually employed for heating or cooking purposes. The small reverbera- tory furnaces devised by Mr. Fletcher will be found to answer the purpose admirably. The author has used one successfully in a large number of melting operations. Their cost is only about $3.50 each. The only difficulty likely to be met in forming an alloy of this description is oxidation of the tin, and the formation of certain definite compounds having a tendency to separate from the mass, thus causing an ingot which is not homogeneous. Oxidation of the tin may take place at the instant of union with the platinum, and it is therefore preferable to melt the platinum and silver together first, and then add the tin and gold. A quantity of borax should be fused in the crucible before the metals are melted, the objects being to prevent adhesion of the alloy to the sides of the crucible, to facilitate pouring, and to dissolve and hold in solution any oxide which may be present. Lastly, a layer of broken charcoal should be placed over the mass before the heating. This will perfectly protect it from oxidation. The formation of definite alloys, it has been shown, takes place with the gradual cooling of the mass; the fusing-point and density of these being greater AMALGAMS. 59 than of that which remains fluid, they manifest a tendency to settle to the bottom of the crucible in a solid state, and in some cases do not leave the cruci- ble in pouring. Thus the ingot may not possess the desired composition. Again, the alloy may assume a semi-solid form, floating in masses in the more fluid portion, settling at the sides or bottom of the mold at the moment of pouring, the result being an in- got which is not uniform in composition, in conse- quence of which it has been recommended that dif- ferent parts of every ingot should be tested ; but the difficulty may be entirely avoided by carrying the heat to the point of complete fusion and pouring while still very hot, before the tendency to separate is de- veloped. Oxidation of the surface from contact with the at- mosphere will retard amalgamation. It is therefore better not to reduce the entire ingot to a state of fine division. It will be found to unite more readily with the mercury, if freshly filed off as required for use. This can easily be effected with one of the coarse files sold at the depots as vulcanite files. Other metals, such as palladium, copper, cadmium, bismuth, antimony, and zinc, have been used as con- stituents of amalgams. Copper is said to control shrinkage, while it in- creases the tendency to discoloration. It is also be- lieved to exert a preservative influence on the tooth- structure. The salts of copper formed around amal- gam fillings certainly do permeate and may pro- tect the tooth-structure from further decay, but too much reliance should not be placed in the therapeutic theory in connection with this class of amalgams. 60 DENTAL METALLURGY. It is exceedingly difficult to determine such a question. Amalgam is in many cases placed in teeth of such good quality that almost any filling-material would answer a conservative purpose, and the examples which are often presented of the long duration of such fillings may prove nothing beyond the density and superior quality of the tooth-structure. Careful and intelligent observation and experiment alone can satisfactorily solve a question of this kind. Brittleness may be increased or diminished accord- ing to the quantity of platinum present, and the re- sulting amalgam will exhibit this quality equally with the alloy. An amalgam filling should always be strong enough to retain its integrity of edge under the force of mastication. A formula from which the author has obtained very good results is as follows: Silver ...... 40 grammes. Tin......60 <«• Gold......3 " Platinum.....3 " Larger percentages of gold and platinum afford no better results; * but, on the contrary, the alloy is rendered more brittle thereby; its affinity for mer- cury is lessened, while its capacity for the latter is increased, as shown under the head of "Platinum." The more recent experiments with amalgams seem to favor the substitution of zinc for platinum. The great improvement in amalgam filling-mate- rials which was expected from the use of platinum * Under the assumption that amalgams are benefited in proportion to the amount of gold contained, Dr. W. G. A. Bonwill recommends that from 10 to 20 per cent, of gold be dissolved in the mercury used with the alloy. AMALGAMS. 61 has not been realized, but on the other hand alloys containing zinc instead of platinum have exhibited better qualities than had previously been attained. Soon after the publication of the first edition of this work, the author changed the above formula, using zinc instead of platinum, and the general working qualities of the amalgam seemed to be improved by the alteration. A number of fillings made with it at the time have since been frequently examined, and in maintenance of peripheral integrity and color have been in every respect satisfactory. Dr. Louis Jack has obtained excellent results from amalgam alloys prepared according to the following formulae : AMALGAM. No 5. No. 6. No. 7. R Gold ..... £ oz. <5 oz. 12 dwts. Silver...... 12-oz 1£ oz. If oz. Tin...... 3^oz. 8} oz. 3Joz. Pure Zinc..... 3 dwts. 4 dwts. 3 dwts. From actual experience with the above, the author can testify to the excellence of their general work- ing qualities and their durability. They afford, how- ever, very quick amalgams, and should not be mixed with mercury until the cavity is quite ready for the reception of the filling; or, if found to set too quickly, that quality may be modified by using less gold in the formula. Palladium and mercury are said to make a very good filling. According to Mr. Thomas Fletcher, however, all alloys into which palladium enters as a 6* 62 DENTAL METALLURGY. constituent are utterly worthless ; combined with tin and silver, it has been found to be unsatisfactory. Mr. Fletcher gives the results of a number of experi- ments with tin, silver, and palladium* " The alloys given below were made from chemically-pure metals. They were melted first at a high temperature, under a layer of charcoal, in a clay crucible, with constant stirring. They were then poured quickly into a thick and cold iron ingot-mold; broken up and remelted three times, to insure uniformity as far as possible. Pd. 1, Ag. 5. Eesult powdery and unmanage- able. Pd. 1, Ag. 5, Sn. 1. Kesult ditto. Both readily combined with Hg. Pd. 1, Ag. 5, Sn. 2. Eesult same as above. Pd. 1, Ag. 5, Sn. 3. Eesult very dirty to mix; makes a leaky plug. Pd. 1, Ag. 5, Sn. 6. Eesult similar to last. Pd. 1, Ag. 3, Sn. 5. Eesult very dirty ; does not combine properly with mercury. Pd. 1, Ag. 6, Sn. 5, Au. 1. Eesultsimilar to last. Pd. 1, Sn. 4. Eesult very dirty; does not set at all." In consequence of the uniformly bad results of these combinations, Mr. Fletcher states that for the present he has discontinued any further experimen- tation in this direction. The claim, however, has been made for palladium,! that when used as a single metal amalgamated with mercury, like Sullivan's amalgam (copper and mer- * British Journal of Dental Science. fDr. Bogue, in proceedings of New York Odontological Society, Dental Cosmos, 1884, p. 403. AMALGAMS. 63 cury, see chapter on " Copper "), it does not change in form or bulk, or discolor the teeth, and though it turns quite black, it is a thoroughly reliable filling provided the cavity be properly prepared. It is said that the union of palladium and mercury is a true chemical one, and is accompanied with the phenomena usual in such cases of heat and incandescence. Qualitative and Quantitative Examinations of Amal- gam Alloys.—It is important for the dentist employing filling-materials of this class to be well informed as to their composition, and it is also often desirable to be able to determine the composition of old amalgam fillings, the constituents of which, besides tin and silver, are unknown. With a compound belonging to the latter class the first step, after weighing, would be to free it from mercury by heating to redness, the loss of weight at the second weighing indicating the amount of mercury which was present. Alter this it is to bo treated as an alloy, one gramme of which may be placed in a test-tube or other suitable glass vessel, and acted upon by a sufficient quantity of chemically-pure nitric acid. The silver is dissolved and converted into argentic nitrate. The tin is oxi- dized and becomes metastannic acid (5Sn02, 10H2O). The latter, in the form of a white powder, settles to the bottom of the vessel. If gold forms part of the alloy, it will be recognized even in the smallest quan- tity, by the very decided purple color which it im- parts to the precipitate, due to the formation of purple of Cassius. Platinum may be readily detected by the presence in the test-tube of the finely-divided metal, quite black in color. At this point in the experiment, if neither gold nor 64 DENTAL METALLURGY. platinum is present, the quantitative estimation of the alloy is not difficult, and may be accomplished as follows: The solution should be rendered neutral by evaporation, and diluted with a large quantity of dis- tilled water. The oxidized tin will be found at the bottom of the vessel. After pouring off the solution, this should be washed, dried, and rendered anhydrous by heating to redness, when it is ready for weighing, every 100 parts indicating 7866 of tin. Returning to the solution, the silver is next precipitated by sodium chloride or hydrochloric acid, and may be collected b}^ filtering. If cadmium is present in the remaining solution, sulphureted hydrogen will throw it down in the form of a yellow powder (sulphide of cadmium), the color distinguishing it from zinc sulphide, which is white- The alkalies, potassa, soda, and ammonia, throw down the oxide of cadmium as a white hydrate. Ammonic carbonate also produces a white precipi- tate, which is insoluble in excess of the precipitant. The latter quality also distinguishes it from zinc, which is soluble under similar conditions. Zinc may be precipitated from a solution by potas- sic carbonate, added in sufficient quantity to decom- pose any ammoniacal salts, if present, as such would prevent the precipitation of the carbonate of zinc. The whole is now to be evaporated to dryness, hot water added, the zinc salt boiled, collected by filter- ing, and lastly, after washing, ignited to drive off the carbonic acid, when oxide of zinc remains. In quan- titative estimation, zinc is always weighed in this form, the oxide being calculated as containing 80-24 per cent, of the metal. AMALGAMS. 65 If copper be present, as it frequently is in amal- gam alloys, it will be instantly detected by the green- ish hue which it imparts to the solution, after the " breaking-up " by nitric acid. Indeed, the appear- ance of the contents of the test-tube will, after the student has acquired some experience, serve to con- vey a pretty clear idea of the composition of the alloy. For instance, take one gramme of Arring- ton's alloy, composed of pure tin and silver, and act upon it with nitric acid; the contents of the test- tube will be a colorless liquid and a perfectly white powder. In another test-tube dissolve some of Lawrence's alloy, and the nitrate, instead of being perfectly colorless as in the preceding experiment, will show a decidedly green tinge, and the presence of copper is verified by the blue color developed by the addition of ammonia. Gold and platinum will be detected in alloys containing these metals by the appearances already described. The presence of copper having been verified, that metal may be precipitated as cupric sulphide, by sul- phureted hydrogen. It may then be collected by filtering, and after washing and drying may be oxi- dized by nitric acid, and again be precipitated by potassa, and may then be weighed as an oxide. Should copper and cadmium both be present, the precipitate obtained by sulphureted hydrogen will consist of sulphides of cadmium and copper. Boiling in dilute sulphuric acid, however, dissolves the cad- mium so that the copper may be collected by filtering, after which the cadmium may be again thrown down by sulphureted hydrogen. The merest trace of copper in solution may also 66 DENTAL METALLURGY. be detected by placing a drop of the latter on a strip of clean platinum foil and touching it with a point of zinc. A spot of reduced copper will instantly appear. For the quantitative analysis of an alloy contain- ing tin, silver, gold, and platinum, the first step should be to remove the tin by deflagration. This is accom- plished by placing the alloy, after weighing, in a small crucible with some borax, and heating to bright redness. While at that high temperature small por- tions of potassium nitrate are added. This is de- composed ; its oxygen unites with the tin, converting it into stannic oxide (SnOa). After cooling, the crucible may be broken, and the remaining button, together with any small globules which may adhere to the sides of the crucible, collected and weighed, the loss indicating the amount of tin which was present. If the deflagration has been thoroughly performed, the button will be entirely freed of tin, and it then remains to separate the silver from the gold and platinum. This may be accomplished either by nitric or sulphuric acid. But, as the former will dissolve a considerable portion of the platinum along with the silver, sulphuric acid, which does not thus affect the platinum, affords more accurate results, and is the agent usually employed in parting opera- tions where the alloy consists largely of silver, with an appreciable percentage of platinum. The button, now consisting of silver, gold, and platinum, should be rolled into a thin ribbon, and then placed in a glass or platinum vessel with at least two and a half times its weight of concentrated sul- phuric acid. This is boiled, during which strong AMALGAMS. 67 action is evinced by copious disengagement of sul- phurous anhydride, and the silver is converted into a sulphate. The boiling is continued until all the silver is dissolved, when the gold and platinum will be found at the bottom of the digester. The liquid is now poured off, and the silver recovered from the sulphate solution by precipitation with plates of copper, which reduce it in a more or less crystalline state. The remaining alloy, now consisting of gold and platinum, should be thoroughly washed, dissolved in nitro-hydrochloric acid, neutralized by evapora- tion, then dissolved in a large quantity of distilled water. It is then ready for precipitation with oxalic acid, by which the gold is thrown down, and the platinum remains in solution for subsequent treat- ment. The gold, which is now easily collected, should be washed, dried, and heated to redness, when it is ready for weighing. Lastly, the platinum may be recovered from the solution by precipitation with ammonic chloride. When washed and dried it will be ready for weigh- ing, and every 100 parts may be considered as con- taining 44-28 of platinum. The composition of some of the principal dental amalgam alloys now in use, is shown in the following tables: COMPOSITION OF SOME OF THE WELL-KNOWN DENTAL ALLOYS.* OS 00 Tin. Silver. Gold. Plat-inum. Copper. Zinc. Cad-mium. Anti-mony. 57.5 56.85 61.5 56.00 58.37 50. 50. 40. 40. 40. 55.00 61.75 52.85 49.27 63.55 49.65 63.80 56. 50.35 35. 35. 37. 53.36 44. 60.25 50.40 50.00 ■59. 58. 51.5 44.57 50.56 42.5 42.00 34.5 37.00 37.55 50. 50. 50. 40. 60. 43.65 27.25 47.00 48.24 31.85 49.75 44.35 40. 43.35 60. 37. 58. 44.74 10. 37. 44.30 47.90 40. 39.5 43.5 50.12 44.67 0.50 0.15 0.5 2.00 0.10 0.50 5. 7. 10. 10. 3.5 10. 5. 4.00 20. 1.35 10.60 2.44 1.45 0.40 1.05 3. 0.15 0.15 0.05 0.65 0.20 0.25 0.15 2.35 46. 0.90 1.30 4. 3.35 1.65 5. 5. 6. 1.60 0.40 2.75 3.80 0.30 1.20 1.80 4. 0.30 1. 2. 0.5 1. 5.31 4.59 0.28 Pal- ladium. Arrington's (S. S. White's)........................... Blackwood's G. and P. Alloy......................... Best (Spencer & Crocker's) Old....................... Chicago Refining Co.'s (Old)......................... Chicago Refining Co.'s (New)........................ Chase's Coppered Amalgam.......................... Chase's Plastic Tin Amalgam........................ Chase's Alcohol Tight Amalgam................... Chase's Stannous Gold.................................. Chase's Incisor Tooth Amalgam.................... Caulk's White Alloy........■............................ Caulk's Par-Excellence................................ Crown Gold Alloy........................................ Dawson's White Alloy.................................. Dawson's Superior Amalgam........................ Dibble's White Amalgam.............................. Fry'8 Amalgam........................................... Fletcher's Gold Alloy (Old)........................... Fletcher's Platinum and Gold Alloy............... Flagg's Submarine...................................... Flagg's Facing............................................. Flagg's Contour Alloy.................................. Globe (S. S. White's)..................................... Grimes's Front Tooth (Old).......................... Hood's Amalgam (Old)................................. Hood & Reynolds's Gold and Platinum Alloy. Hood & Reynolds's Sans Tache Alloy............ Holmes's Star No. 1 (Old)............................. Holmes's Star No. 2 (Old).............................. Hays's Pure White (Old)............................... Hardman's Amalgam........................••........... Hardman's White Alloy.................„11Il:ll::l::. * American System of Dentistry. COMPOSITION OF SOME OF THE WELL-KNOWN DENTAL ALLOYS.*—Continued. High Grade Alloy (7% per cent. Gold)............ Harris's (Prol. J. H.) Amalgam..................... Johnson & Lund's Extra (Old)....................... Johnson A Lund's Extra (New)..................... Johnson & Lund's Virgin White Alloy........... Johnson & Lund's Atlas Amalgam................. Johnson & Lund's Extra Tough Alloy............ Justi's Superior Gold and Platinum Alloy....... King's Occidental......................................... Lawrence's (Old)......................................... Lawrence's (New)....................................... Motrin's (Old).............................................. (Motrin's) The Dentist's Amalgam.................. Oliver's Amalgam (Old)................................. «q Oliver's White Amalgam (New).................... Peirce's (Old).............................................. Prol. Essig's(Old)........................................ Parsons's Eureka Silver Alloy........................ Sterling Amalgam (Old)................................ Sterling Amalgam, D. & L. (New).................. Standard Amalgam (Davis & Co.).......'........... Standard Dental Alloy (Eckfeldt).................. Shattuck's Standard Gold Alloy..................... Sibley's Gold and Platinum Alloy.................. Temporary Alloy.......................................... Townsend's(Old).......................................... Townsend's (Improved)................................. Walker's (Old)............................................. Walker's Excelsior Gold and Platinum Alloy.. Welch's Gold and Platinum Alloy (Old).......... Welch's Gold and Platinum Alloy (New)......... Welch's Amalgam........................................ 41.5 49. 48.10 40.(10 mi. 38.00 hi.15 36.75 61.05 37.75 61.90 36.85 51.25 47.n0 59.10 35.20 54.75 42.75 47. 47. 50.43 44.06 62. OH. 59.50 37.90 59.50 37.90 50.8 46.1 55.25 44.74 40. 55. 55. 45. 40.00 55.00 62.00 31. «2.37 33.20 55.40 44.6U 4(1.60 52.(10 51.74 46.98 54.66 4 (.16 K8. 10. 58. 42.' 54.50 44.60 69. 30.5 51.60 42.00 64. 44. 51.90 46.00 61.52 48.48 Gold. 7.5 1.5 0.15 0.30 0.32 1. ....... .„„.. (1/(0 1.3 1.70 Plat- inum. 0.5 0.50 0.20 0.7 0.40 Copper, i Zinc. 0.35 ............ 0.2(1 ! 0.25 0.08 ! 3.50 5. 5.51 4. 2.5 2.5 3.00 1. 6. 0.14 4.33 4.40 1.20 0.20 3. 0.(18 2.00 1. 0.5 2. 7.00 1.80 2.50 1. 2.00 Cad- mium. 1.45 0.90 Anti- mony. Pal- ladium. ♦American System of Dentistry. OS CO 70 DENTAL METALLURGY. The following formula, with directions for forming the alloy, kindly furnished by Dr. Ambler Tees, is highly recommended by that gentleman as affording excellent results: Tin.......40 dwts. Silver.......24 " Gold.......1 dwt. Platinum......1 " "The gold, silver, and platinum to be melted first with borax and kept in a state of fusion for five minutes. The tin to be melted in a separate crucible, and the molten silver, gold, and platinum to be poured into the fused tin, and the whole quickly poured into a suitable ingot-mold and reduced to powder with a, large machinist's file." CHAPTER VI. MODES OE MELTING METALS. T7"AEI0US forms of heating apparatuses, furnaces, etc., are of great importance to those who con- duct metallurgical operations on a large scale. We shall, however, in these pages confine ourselves exclu- sively to the needs of the dental laboratory,* includ- ing a consideration of the appliances for soldering. These operations are performed by the use of the oil- or alcohol-lamp, or the gas-jet, or by means of a suitable stove or furnace. When kerosene oil or alcohol is employed, it is of the first importance to select a lamp designed not only to meet the practical requirements, but also with a view to safety. The first essential is to have the wick large enough to afford a flame of sufficient magnitude to enable the operator to solder an entire artificial denture, or to fuse from one to two ounces of gold. This would require a wick one and a quarter inches in diam- eter, and about three inches long. Its connections with the reservoir or body of the lamp in which the combustible fluid is contained should not be direct nor in such close proximity that explosive gas would * Full and detailed descriptions of the different heating apparatuses, together with the most approved processes of reducing ores and of melting large quantities of metals, will be found in Percey's, Phillips's, and Makins's works on metallurgy. (71) 72 DENTAL METALLURGY. be likely to form. The Franklin Safety Lamp, a cut of which is annexed (Fig. 1), will be found to answer every requirement. It consists of a reservoir five inches in diameter by two and a half inches deep. The wick-holder, three inches long by two and a half inches in diameter, is connected with the reservoir by a curved tube five inches long by three-sixteenths of an inch in diameter. Thus a sufficient quantity of the burning-fluid is supplied to the wick to afford Fig. 1. a constant flame, while there is no danger of the beat from the wick-holder being conducted to the reser- voir to cause an explosion. In cities and large towns where gas is available, that agent is to be preferred, on account of its greater safety and convenience. A gas-burner which will be found to answer every requirement of the laboratory may be constructed by attaching to the base of an ordinary Bunsen burner, such as is sold at the dental depots, a piece of brass tubing six inches in length 999999999942 MODES OP MELTING METALS. 73 by one and a quarter inches in diameter. Over the top of this, in order to properly spread the flame, a piece of fine brass wire-gauze is fastened by means of a ring of sheet-brass, one-quarter of an inch in width. Connection may be obtained with the gas- bracket in almost any part of the room by means of flexible rubber tubing. This will be found to answer all purposes of soldering as well as of melting small quantities of gold and silver. The blast and the flame are produced by the blow- pipe,* an instrument which has long been used by workers in metals for the purpose of soldering to- gether small pieces of metal, and for melting and reducing purposes generally. The ordinary form consists of a conical brass tube, from two hundred to two hundred and thirty or forty millimetres long, curved at the narrower end to nearly a right angle, so that the flame may be conveniently directed upon the piece of metal to be soldered or melted, as the case may be, which is held upon some suitable sup- port, such as a piece of charcoal, coke, or pumice-stone. When the blow-pipe is used in its simplest form, by the mouth, the largo end of the instrument is held between the lips, and the small end toward the flame. The blast should not be sustained by the respiratory organs, but, in order that an unbroken current may be kept up, the mouth should be filled with air, to * The use of the blow-pipe for analytical purposes is credited to Anton ■ von Swab, a Swedish Councillor of Mines, who made use of the instru- ment in the discharge of his official duties as early as 1738. Since that date its use has been widely extended, and the importance of reactions in the dry way produced under the flame of the blow-pipe is fully recognized and established. For full information on the subject the > student is referred to Plattner's " Manual of Blow-pipe Analysis." 7* 74 DENTAL METALLURGY. be forced through the blow-pipe by the muscles of the cheeks. While these are forcing the air through the blow-pipe, the connection between the chest and the cavity of the mouth should be closed by the pal- ate, which thus performs the part of a valve. The beginner is liable to fall into the error of not closing the connection between the chest and the mouth at the proper instant, and of obtaining the force neces- sary to propel the air through the blow-pipe from the lungs. That this manner of using the instru- ment may injure the organs of respiration cannot for a moment be doubted, and the operator should early acquire the proper method, above described. To avoid tiring the muscles of the lip by continual blow- Fig. 2. ing, the trumpet mouth-piece has been recommended and is shown in the annexed cut, Fig. 2. This is merely pressed against the open mouth, and an unin- terrupted blast may be kept up for a long time with- out causing the least fatigue of the orbicularis oris, since that muscle takes but a passive part in the operation. This trumpet-piece, however, should be so curved as to correspond with the shape of the mouth; otherwise it will require to be pressed very forcibly against the lips in order to prevent the escape of air. The blow-pipe should be constructed of either brass or German silver, as these alloys are but poor MODES OF MELTING METALS. 75 conductors of heat. Silver is not well suited for the purpose, because it transmits temperatures so readily that it soon becomes too hot for the fingers. A long-continued and steady flame maintained by the mouth blow-pipe is apt to cause disturbances in the flame from the collection of moisture in the tube, which is liable to be expelled by the pressure of the air. To avoid this a hollow chamber is constructed about midway in the instrument. The length of the instrument should be adapted to the eye of the opera- tor, so that the object upon which the flame is direc- ted may be distinctly seen. An improvement in these instruments has been made by Mr. Thomas Fletcher, F.C.S., of Warring- ton, England (see Fig. 3), by which temperatures be- Fig. 3. 9 yond those which can be produced by the ordinary gas and air blow-pipes are attainable. It not only gives temperatures never approached with the old blow-pipe, but it is also in every respect more con- venient, easier to use, and better adapted for every class of work. With the same amount of blowing as with the common form, this blow-pipe will do nearly double the work: if high temperatures are not required, the labor of blowing is reduced in pro- portion. The improvement consists in coiling the air-tube into a light spiral over the point of the jet. This coil takes up the heat which would otherwise be wasted, and utilizes it by heating the air in its pass- age. The author has found this form of mouth blow- 76 DENTAL METALLURGY. pipe to be well adapted for fine analytical operations by cupellation, as well as for all uses of the dental laboratory. Fletcher's hot-blast blow-pipe is so constructed that the air-pipe is coiled around the gas-pipe in a spiral form, and both are heated by three small Bun- sen burners underneath, which are controlled by a separate stop-cock, as shown in Fig. 4. It is claimed that the power of this apparatus is about double that of an ordinary blow-pipe; that when the jet is turned down to a small point it will readily fuse Fig. 4. a moderately thick platinum wire, and that its power is nearly equal to the oxyhydrogen jet. This form of blow-pipe is well adapted to continuous-gum work where the teeth are soldered to the plate with pure gold. The blast is obtained by means of a foot- blower (Fig. 5), connected with the blow-pipe (Fig. 4) by a flexible rubber tube. The reservoir of the upper portion, which holds the air, is, when the bel- lows is not in operation, merely a disk of thick coffer-dam rubber, which expands under the pressure of the air while the bellows is in motion, and thus affords a very compact, powerful, and effective ar- MODES OF MELTING METALS. 77 rangement. The step for the foot is very low and the blower may be used with ease, whether the Fig. 5. operator is standing or seated. The pressure is per- fectly steady and equal. If the rubber-disk is dis- Fig. 6. tended until forced against the net, the pressure can be increased to almost any extent desired. It will 78 dental Metallurgy. give, if required, a heavy and continuous blast through a pipe of a quarter inch clear bore. The mechanical blow-pipe is not a new idea. It has been in use by dentists for nearly fifty years, and in extensive soldering operations is one of the most val- Fig. 7. uable appliances of the dental laboratory. The instru- ment as formerly made by the late Mr. Bishop, of Philadelphia, is probably superior to the more recent forms (see Fig. 6). yfi �3014301 MODES OF MELTING METALS. 79 The Burgess blow-pipe, illustrated in Fig. 7, is con- structed on exactly the same principle as the above, and is convenient and effective. The new form of oxyhydrogen blow-pipe invented by Dr. J. E. Knapp, is perhaps the most complete and effective apparatus for soldering and melting Fig. 8. operations in the dental laboratory that has yet been devised. It may be used with equal facility in sold- ering the largest piece of plate-work, or the most delicate crown-work, and is of particular value to dentists who give attention to continuous-gum work, enabling them to readily remelt their platinum scraps. 80 DENTAL METALLURGY. It is provided with an iron stand in which is secured by a thumb-screw a 100-gallon cylinder of nitrous oxide gas. By means of a yoke and set-screw, the valve of the cylinder is connected with the tubes and valves of the blow-pipe in such manner that the pro- portions of a mixture of nitrous oxide and illuminat- ing gases are under perfect regulation and control. A cylinder of nitrous oxide gas is placed in the base or stand, and fastened with the thumb-screw A. The yoke carrying the stop-cocks and valves is attached to the valve of the cylinder, and tightened with the screw B. The pipe C is connected by a rubber tube to an illuminating-gas bracket. When the apparatus is in use the illuminating-gas is turned on and its flow regulated by the handle D. The handle G, over the outlet II, is then turned, the cylin- der valve is opened by means of the hand-wheel I sufficient to permit the escape of enough nitrous oxide gas to be detected by touching the opening II with the finger. When the desired quantity of nitrous oxide gas is obtained, the flow is directed to the mixing chamber and controlled by the handle G, which, when in posi- tion, as shown in the cut, allows the gas to pass freely into the chamber K, where it mixes with the illuminating-gas. Either or both of the burners may be used, and the desired flame obtained by regulating the pressure of the gases by the handles controlling them It is an instrument of much greater delicacy than the blow-pipes commonly used by dentists. The flame which it affords is very small, but the intensity of its heat is such that great care must be exercised in MODES OF MELTING METALS. 81 soldering small objects to prevent burning or even entire fusion of the parts adjacent to the solder. It is economical of time and materials, and its perfect cleanliness will commend it to all who work in the higher branches of mechanical dentistry. When a small quantity of gold or silver is to be melted by means of the blow-pipe, it is usually performed upon a support formed of charcoal. A good solid cylindrical piece of thoroughly charred pine coal should be selected, and divided into two equal halves by a vertical cut with a saw. Upon the end of one half a depression should be cut for the reception of the metal to be melted. On the flat side of the other half, extending to the end, the ingot- mold should be carved, of size and shape governed by the requirements of the case. The two halves should then be brought together and secured by a piece of iron or copper wire, when they will be found to prac- tically combine the requirements of crucible and in- got-mold. The depression in which the metal is to be melted and the mold or receptacle should be con- nected by means of a gutter or groove. The flame is now directed upon the metal, and when thoroughly fluid the charcoal is inverted so that the fused metal will run into the mold prepared for it in the opposite half of the charcoal. This is probably the simplest form of apparatus by which small quantities of metal can be melted, and is often employed in the dental laboratory and by jewelers. Fletcher has devised an apparatus embodying the same general principles as the one just described, for quickly obtaining ingots of gold and silver without the use of a furnace. It is shown in the accompany- 8 82 DENTAL METALLURGY. ing diagram (Fig. 9); A, representing a crucible of molded carbon, supported in position by an iron side- plate; C, the ingot-mold; D, clamp, holding ingot- mold and crucible in position; B, cast-iron stand upon which the latter swivels. The metal to be melted is placed in the crucible, A, and the flame of the blow-pipe directed upon it until it is perfectly fused. The waste heat serves to make the ingot- mold hot. The whole is tilted over by means of the upright handle at the back of the mold. A sound ingot may be obtained by the use of this simple little apparatus in a very few minutes. Simple contrivances of this kind are, however, not applicable to melting operations involving quantities ex- Fig. 9. ceeding one ounce. In such cases it I? is better to employ a crucible and A^(wkS\ any stove or furnace in which the r-a^ff"1 temperature can be raised suffi- ^mKff BA ciently. This may be accomplished ^W'^ifelpi iQ an ordinary cooking-stove, a blacksmith's forge, or a small fire- clay furnace, by the use of anthracite coal, coke, or charcoal. By far the most convenient, compact, and effective furnace for melting from one to ten ounces of gold which has ever been used is the crucible-furnace (Fig. 10), invented by Mr. Fletcher, which can be obtained at the dental depots. The furnace is per- fectly adapted to the wants of the mechanical dentist. It is composed of a substance resembling fire-clay, but much lighter in weight, and said to possess only one-tenth its conducting power for heat. The furnace consists of a simple pot for holding MODES OF MELTING METALS. 83 the crucible, with a lid and a blow-pipe, all mounted on a suitable cast-iron base. As compared with the ordinary gas-furnace it appears almost a toy, owing to its great simplicity. The casing holds the heat so perfectly that the most refractory substances can be fused with ease, using a common foot-blower. Half a pound of cast-iron requires from seven to twelve minutes for perfect fusion, the time depend- ing on the gas-supply and the pressure of air from the blower. The power which can be obtained is far beyond what is required for most purposes, and is Fig. 10. limited only by the fusibility of the crucible and cas- ing. The crucible will hold about ten ounces of gold. An ordinary gas supply-pipe of -f$ or f-inch diameter will work it efficiently. It requires a much smaller supply of gas than any other furnace known ; about ten cubic feet per hour is sufficient for most purposes. Crucibles must not exceed 2\ by 2 inches. Any common blow-pipe bellows will work the furnace sat- isfactorily except for very high temperatures (fusion of steel, etc.), for which a very heavy pressure of air is necessary. In size it is but four inches in diameter 84 DENTAL METALLURGY. by three in height. The author has used one in his laboratory for the purpose of melting gold and Bilver and for general metallurgical experiments for several years, with the greatest satisfaction ; and he has also found it to be most admirably adapted to class dem- onstration, for which purpose, as a means of illustrat- ing his lectures on metallurgy, he has had frequent opportunities to use it. A modification of the apparatus has been made, adapting it to the use of refined petroleum instead of gas as a fuel, and thus rendering it of more general Fig. 11. utility (see Fig. 11). Thus improved, it is said to be in no way inferior in efficiency to the gas-furnace. The burner of this furnace is constructed upon the principle of an atomizer, which, of course, dispenses with a wick; it is supplied with a device for regu- lating the supply of oil, which is operated by the milled nut (marked A) shown on the top of the reser- voir in the cut, and for the supply of an annular jet of air, which is regulated by turning the sleeve (marked B). This burner is so arranged that in case any ob- struction should occur it can be taken apart and cleaned by separating the burner from the reservoir, MODES OF MELTING METALS. 85 which is accomplished by loosening the small screws, drawing out the oil-tube, taking off the sleeve, B, and removing the inside tube. These furnaces are so constructed that they may be used for either gas or petroleum, the lamp being fitted for adjustment in place of the gas-burner, so that the same apparatus may be used for either. The blast is obtained by means of the foot-blower shown on page 77, which is connected with the furnace by means of India-rubber tubing, as seen in Fig. 11. Fig. 12. An injector gas-furnace has also been perfected by Mr. Fletcher, which seems to be well adapted to the wants of the dentist, chemist, or metallurgist (see Fig. 12). The construction of this apparatus is upon the principle of the injector-furnace, and it is claimed that its power and speed of working are practically without limit, depending only upon the gas- and air- supply. With a half-inch gas-pipe and the small foot- blower (see page 77) this furnace will melt a crucible 8* 86 DENTAL METALLURGY. full of cast-iron scraps in ten minutes. The supply of gas required is exceedingly small. Allowing five cubic feet of gas for heating up, it consumes about four feet of gas for every pound of cast-iron melted. For laboratory purposes it is the cheapest and most convenient furnace in use. It is very simple in construction, and consists of two parts,—an upper portion, which forms the cover, and a lower part, which holds the crucible while in operation. Mr. Fletcher has devised a gas-lamp which has given satisfactory results in melting zinc and lead for Fig. 13. dies and counter-dies, and for the fusion of all alloys which may be accomplished in an iron ladle at or below a red heat (see Fig. 13). When gas is not available the gasoline furnaces used by plumbers for melting solder have no superior in point of convenience and rapidity. These furnaces are made by C. Gefrorer, of Philadelphia, and are shown in Fig. 14. Crucibles.—The term " crucible " is applied to a chemist's melting-pot, made of earthenware or other MODES OF MELTING METALS. 87 material, and so called from the superstitious habit of the alchemists of marking such vessels with the sign of the cross. The term is now generally under- stood as designating vessels in which metals are melted in furnaces at high temperatures. A crucible should possess the power of resisting high tempera- tures without fusing or softening. It should also be capable of retaining sufficient strength, when hot, to prevent its crumbling or breaking when grasped Fig. 14. by the tongs. Lastly it should not crack either in heating or cooling. For the purpose of melting metals crucibles are made of clay with admixture of silica, burnt clay, graphite, or other infusible material. For the fusing of platinum, which requires the intense heat of the oxyhydrogen flame, they are formed of lime. For use in the dental laboratory, graphite crucibles, 88 DENTAL METALLURGY. which can be obtained at the dental depots, will be found to answer every purpose, and they are thor- oughly reliable in strength and durability. They range in size from two to four inches high, and are specially adapted for use in the Fletcher gas-furn- aces. When the quantity of metal to be melted is very small, say a half-ounce of gold, the smallest-sized Hessian crucible may be used in the small Fletcher apparatus. Crucibles suitable for melting platinum or iridium are formed of two blocks of lime, each block having a concavity or excavation, so that when the two pieces are placed together the center is hollow; it is thus designed to hold the scraps of platinum to be melted. The lower block is also arranged with a groove and lip, so that when the metal becomes fluid it may be poured into a suitable ingot-mold by in- verting the crucible. The compound flame is intro- duced by tubes passing through the center of the upper block of lime forming the cover. Before melting any considerable quantity of gold the crucible should be tested, particularly if the melting operation is to be performed in an ordinary coal-stove, where a defective crucible might be the means of a considerable loss. A small amount of borax should be placed in the vessel, which should then be exposed to a high temperature. Should it not be perfect, the borax glass will run through and glaze the surface on the outside. If the crucible is found to be impervious, it should be so inverted while yet hot that the borax glass may cover the surface of the lip or groove out of which the melted metal is to be poured. This facilitates the pouring and pre- MODES OF MELTING METALS. 89 vents any portion of the metal from adhering to the side of the crucible. Ingot-molds are constructed of various substances. For the reception of platinum melted with the oxyhydrogen blow-pipe they are formed of lime or coke; for gold and silver, they are commonly made of cast-iron, about two inches square and from an eighth to three-sixteenths of an inch thick (see Fig. 15), with slightly concave inner surfaces, as Fig. 15. the shrinkage of the ingot is greatest at the center. Ingot-molds formed of soap-stone are also employed. The ingot-mold should be heated before pouring. Rolling or laminating is accomplished by repeat- edly passing the metallic ingot between cylindrical steel rollers from three to four inches in width. These are so arranged that, by means of screws, they are capable of being brought closer together every time the gold is passed through (see Fig. 16). 90 DENTAL METALLURGY. Fig. 16. The proper degree of attenuation is determined by the gauge-plate (Fig. 17). Gold or silver is made into wire by means of the draw- plate—an oblong piece of steel provided with a num- ber of gradually diminish- ing holes enlarged on the side where the gold enters. The gold to be drawn through may be prepared in a cylindrical shape by melting and pouring into an ingot-mold provided with a chamber for the pur- pose (some ingot-molds are so constructed). The end Fig. 17. of the rod should be filed so as to readily enter the draw-plate, which must be firmly screwed in a vise. MODES OF MELTING METALS. 91 The gold is then, by means of a pair of strong pliers, drawn through the different holes of the draw-plate consecutively until the desired size is reached. At the beginning of the operation it will require frequent annealing. Soldering must also, to a certain extent, be re- garded as coming under the general head of melting operations, since it refers to the union of two or more pieces of metal by means of a more fusible alloy. The conditions of successful soldering are: 1. Contact of the two pieces to be united. 2. A clean metallic surface over which the solder is to flow. 3. A freely-flowing solder. 4. Proper amount and dis- tribution of heat. Contact of the pieces to be united is of the greatest importance. If, for example, the object to be soldered be an artificial denture, it is an indispensable require- ment that the backings be quite or very nearly in contact with the plate, and, if gum teeth be used, that each backing touch its neighbor. This is not difficult to accomplish, if the teeth have been care- fully and accurately fitted to the plate and to each other. If, however, any defects of this character are found to exist after the teeth have been invested, they should be remedied by filling such spaces or crevices with small pieces of gold or silver, as the case may be, thus rendering the continuity of the parts com- plete. By the observance of this precaution much of the vexation in soldering experienced by beginners may be avoided, and when the other conditions named have been observed the operation becomes exceedingly simple. Solder runs freely by the force of capillary attraction between two closely-fitting 92 DENTAL METALLURGY. surfaces, just as water will be drawn against grav- ity between two panes of glass in close contact. In soldering artificial dentures which have been care- fully arranged with reference to contact of all the parts to be united, the author has on several occasions completed the operation of soldering perfectly with- out using the blow-pipe at all, by merely heating the whole case to the fusing-point of the solder, in a charcoal furnace with a good draft. Thus it will be seen that the difficulties of soldering are mainly due to a violation of one or more of the rules herein given. Cleanliness should always be strictly observed in soldering operations. The parts to be united should present bright and clean surfaces. Darkening or oxidation will always occur when gold or silver, the purity of which has been reduced by alloying, is heated to redness. A weak solution of sulphuric acid and water, slightly heated, will quickly remove discoloration resulting from this cause; or, the borax employed as a flux in soldering operations, will effect the same result, by dissolving* the oxide which forms on the surface, while it also protects from further oxidation by excluding the oxygen of the atmosphere. The surfaces to be soldered should be carefully pro- tected from any contact with plaster of Paris, as there is no substance used in the dental laboratory more likely to retard the union of the parts and im- pair the final result than this. To this end, all parts over which the solder is to flow should, previous to * Borax, when fused to a glass, has the quality of dissolving metal- lic oxides, and the different colors imparted to the '" bead" is one means of discrimination in blow-pipe analysis. MODES OF MELTING METALS. 93 their investment in sand and plaster, be thoroughly covered with the cement of resin and beeswax com- monly used in the dentist's laboratory as a temporary fastening for teeth, clasps, etc., which are to be united by soldering. This will effectually exclude plaster, and it is easily removed after the investment has sufficiently hardened. A solder to be employed in dental mechanism should possess the quality of flowing freely, and be as high in grade as the attainment of that property will permit, so that it will sufficiently resist the action of the fluids of the mouth. It should also approxi- mate as nearly as possible the color of the plate upon which it is used. If the first condition, refer- ring to the contact of the plates to be united, be observed, the quantity of solder required to effect con- tinuity may be reduced to the minimum; and thus we shall have the smallest possible portion of the alloy exposed to the action of the fluids of the mouth, while we at the same time avoid the danger of frac- ture of the teeth by the contraction in cooling of an inordinate quantity of solder. The latter, for all purposes of the dental laboratory, should be in the form of plate of, sa}- No. 27 in thickness, and this should be cut into portions of sizes corresponding to the extent of the parts to be united. Thus, upon each pin a small particle of solder should be placed, just large enough to cover it. A piece of the same size Bhould also be placed near the top of each joint „ where the backings come together, while a larger piece should be placed at the points of union between backings and plate. These pieces of solder are made to adhere to their proper positions through the 9 94 DENTAL METALLURGY. agency of the borax, which is used by taking a lump, rubbing it on a piece of ground glass with clean water to a creamy consistence, and then applying to the surface by means of a camel's-hair pencil. The application and management of the heat in the operation of soldering are matters requiring both care and judgment. The temperature should at first be raised very gradually, in order that pieces of solder may not be thrown off or displaced by the puffing-up incident to the calcination of the borax, or, in the case of an artificial denture, that the porcelain teeth may not be fractured by a too sudden elevation of temperature. Both parts to be united should be equally heated; therefore the heat should be so applied in the case of an artificial denture as to raise the teeth and plate to an equal temperature; other- wise, should the plate become sufficiently hot while the teeth remain comparatively cool (a condition likely to occur unless the fuel has been built up around the outside of the investment covering the teeth), the solder, when the flame of the blow-pipe is directed upon it, will flow upon and adhere to the plate. In other words, it will manifest a preference for the hottest portion. The failure to effect an equal distribution of heat preparatory to soldering is often the cause of much vexation and delay. For example, in the process of uniting a rim to a plate by soldering, the rim, being so much smaller than the plate, will be more quickly heated, in which event the solder will fuse and flow upon the rim, and the attempt to unite it to the plate will not be suc- cessful. But to avoid such a result the flame of the blow-pipe should, as a preliminary step, be directed MODES OF MELTING METALS. 95 exclusively upon the plate until it has been heated to nearly the fusing-point of the solder, when the pointed blue flame may be directed upon the latter, and union of the rim and plate can hardly fail to take place. Supports.—In melting small quantities of gold or silver, or in soldering with the blow-pipe flame, it is necessary to perform these operations upon a support made of some suitable body, such as charcoal, coke, pumice-stone, or asbestos and plaster, charcoal and plaster, etc. Well-burned charcoal is especially suited for both purposes, as it helps to increase the heat, and, in the putting together of small quantities of gold or silver solders, prevents oxidation of the base metals which are added to reduce the fusing-point of the alloy and cause it to flow freely. Charcoal made from the light woods, such as pine, is best, because it is not so likely to throw sparks when the flame is directed upon it as are the harder coals, such as that made from oak, and, being softer, it is much better adapted to solder- ing operations in which it is necessary to hold together the pieces to be united by means of small nails or tacks thrust into the support; as, for instance, where a rim is to be soldered to a plate, the former must be brought in contact with the latter upon the char- coal, and so held during the preliminary soldering, which consists of uniting the rim to the plate with a small piece of solder at some one point; after which the accurate adjustment of the rim to the plate for the final soldering is rendered much easier. A good solid piece of charcoal, sufficiently large, should be selected, and bound with iron or copper 96 DENTAL METALLURGY. wire, to prevent its breaking into pieces. It should then receive a coating of plaster, half an inch in thickness, on all sides except the one upon which the object to be soldered is to rest. This adds to its strength and protects the fingers from being soiled in handling it. Good charcoal, suitable for use in the dental laboratory, cannot, however, always be found when wanted, and it is therefore often necessary to use some other substance which may be more easily obtained. Thus, those living in large cities may be compelled to employ pieces of coke as supports in soldering. Next to charcoal, coke is most suitable for that purpose. It is more durable than charcoal, and when such a support, composed of one large piece, or even several smaller pieces, is bound together with wire and coated with plaster, it will last a long time. Large pieces of pumice-stone also answer well for the purpose of holding small objects while the flame of the blow-pipe is directed upon them. Neither of these, however, is so well adapted as charcoal for holders when small quantities of metals are to be melted, in consequence of their greater porosity and hardness, which prevents the cutting of suitable pits for the reception of the metal to be fused. A very good support fors oldering purposes alone may be formed by filling a cup made of sheet-iron or copper, five inches in diameter by five inches in depth with a mixture of asbestos and plaster, or plaster and finely-broken charcoal. The vessel should be supplied with a wooden handle, fastened in the bottom for convenience in handling. Plattner's "Manual of Qualitative and Quantita- MODES OF MELTING METALS. 97 tive Analysis with the Blow-pipe," page 15, gives a method of artificially preparing good solid supports of charcoal which might be found of value in the dental laboratory. It consists of mixing charcoal dust (which must not be too finely ground) with starch paste. The latter is prepared by combining one part of starch with six parts of boiling water. These are stirred in an earthen pot until all the meal is converted into paste. This paste is rubbed in a porcelain mortar, with frequent additions of charcoal dust, until the mass becomes too tough for further admixture, when enough of the coal-dust is kneaded in with the hands to render the whole mass stiff and plastic. From this the desired forms of blow-pipe coals can be made, allowed to dry gradually and thoroughly, and then heated to redness in a covered vessel so as to char the starch paste. The charring may be regarded as complete when the evolution of gases from the mass ceases, or when it has been heated to dull redness. Coals thus formed are of the proper firmness, and ring like ordinary good charcoal when thrown on the table. Blocks formed of graphite and fire-clay have re- cently been furnished as supports for holding objects to be soldered, but they do not answer the purpose well, in consequence of not being sufficiently non- conducting, and they soon become so hot in the operation of soldering that it is impossible to hold one in the hand for any length of time. When the object to be soldered is an artificial denture containing a number of teeth, a support that will be found to answer all requirements is the hand- furnace, such as is now furnished by the dental depots 9* 98 DENTAL METALLURGY. (see Fig. 18). It consists of a funnel-shaped recep- tacle of sheet-iron, with a grate or perforated plate near the bottom, and a small door on one side under- neath the grate for the admission of air. The upper part of the holder is surmounted by a cone-shaped top; to the bottom is attached an iron rod, five or six inches long, terminating in a wooden handle. This apparatus is designed to serve both the purpose Fig. 18. of heating the case and as a support or holder during the soldering. For the first it is not well suited, being too small to contain fuel enough to admit of a thorough heating of the case; but when the object to be soldered has been brought to the proper tem- perature, it makes a capital holder for a set of teeth while the flame of the blow-pipe is being directed MODES OF MELTING METALS. 99 upon it. The best method of heating up a case is to place it on a gas-oven, such as is employed in the dental laboratory for general use and for heating flasks in packing rubber work, etc. A ring of cast- or sheet-iron, six inches in diameter by two inches high, should then be placed around it for the purpose of holding the charcoal, which, in pieces the size of a hen's egg, should be built around the outside of the case, so that it may be uniformly heated. The cone or top of the apparatus just described may now be placed over it. The gas is then lighted, but the full head should not be turned on until the moisture of the investment has been driven off, when it may be gradually increased until the case is heated to red- ness. About thirty minutes will be required to reach the proper temperature for soldering, when the case may be lifted from the gas-oven with suitable tongs and placed in the hand-furnace. The live coals used in heating up should also be placed around the out- side of the investment to prevent the too rapid cool- ing of the piece, should any delay in the soldering occur. When the latter operation has been satis- factorily completed, the top may be placed tightly on, and all access of air excluded, in order that the case may cool slowly and thus avoid the danger of cracking the teeth. CHAPTER VII. COMBINATIONS OF METALS WITH NON-METALLIC ELEMENTS. rPHE metals combine with the non-metallic ele- ments, to form a new class of bodies wherein none of the distinctive characteristics of the con- stituents are discernible. These are the Chlorides, Bromides, Iodides, Fluorides, Cyanides, Oxides, Sulphides. Metals also form definite compounds with nitro- gen, phosphorus, silicon, boron, and carbon. Chlorides.—All metals combine with chlorine, and some of them in different proportions, as illustrated by the stannous and stannic chlorides, the first hav- ing a formula of SnCl2, while the composition of the latter is SnCl4. The capacity of the metals for com- bination with chlorine is not uniform. The different proportions are designated by the following terms : Monochlorides, such as KC1. Dichlorides, " BaCl2. Trichlorides, " AuCl3. Tetrachlorides, " SnCb. (100) * COMBINATIONS OF METALS, ETC. 101 The chlorides may be prepared by acting upon the metals with nascent chlorine developed by hydro- chloric and nitric acids.* Some chlorides, on the other hand, are formed by bringing a current of chlorine gas in contact with the metal. In this way titanic chloride is formed, the chlorine being passed over a heated mixture of charcoal and titanic oxide. Aluminum and chromium chlorides may be similarly obtained. The rationale of the action of chlorine upon me- tallic oxides is that it drives out the oxygen and unites with the respective metals to form chlorides. The interchange may take place at ordinary tem- peratures, as in the case of silver oxide, but in others an elevation of temperature (sometimes to red heat) is required. Many metallic chlorides are prepared by acting upon the metals with hydrochloric acid. Zinc, cad- mium, iron, nickel, cobalt, and tin dissolve readily in hydrochloric acid, with liberation of hydrogen. Sometimes a chloride is obtained by substituting one metal for another. In this way stannous chloride is frequently prepared by distilling metallic tin with mercuric chloride, thus: HgCl2 + Sn = SnCl2-f.Hg. Lastly, a chloride may be prepared by dissolving a metallic oxide, hydroxide, or carbonate in hydro- chloric acid. Bromides.—Bromine unites directly with most metals, and forms compounds analogous in composi- * Aqua regia—i. c, two volumes of hydrochloric with one volume of nitric acid. 102 dental Metallurgy. tion and general properties to the chlorides. Sea- water and many of the saline springs contain native bromides, and silver bromide occurs as a natural mineral. The affinity of bromine for the metals is inferior to that of chlorine, and the latter, with the aid of heat, drives out the bromine and converts the substances into chlorides. Iodides are compounds possessing properties analo- gous to those of the chlorides and bromides, and are obtained by processes similar to those which yield the latter. With the exception of those of gold, sil- ver, platinum, and palladium, they are not decompos- able by heat alone. Fluorides are compounds formed by heating hydro- fluoric acid with certain metals, or by the action of that acid on metallic oxides. They may also be formed by heating electro-negative metals—anti- mony, for example—with fluoride of lead or fluoride of mercury. The fluorides are destitute of metallic luster, and most of them are easily fusible, and bear a close resemblance to the chlorides. Metallic oxides may be variously formed. Some metals, by mere exposure to air while heated, lose their metallic character, and, by combination with oxygen, assume a totally different appearance. There are several methods of forming oxides arti- ficially, and some oxides are capable of being con- verted into others of a higher degree. Eed lead, for instance, is thus formed, the metal being first heated without allowing it to fuse, when a protoxide of a yellow color is formed; but on further exposure to a temperature of 315-5° C, with free access of air, ad- ditional oxygen is taken up, and the mass assumes a brilliant red color. COMBINATIONS OF METALS, ETC. 103 Oxides of metals are also formed by heating a nitrate or carbonate to redness, by which means the acid will be evolved while the oxide remains. Thus, a protoxide of lead may be formed by heating the white carbonate of the metal, its color soon changing to a lemon-yellow as the acid present is driven off. Oxides of some of the metals—copper being an example—are formed by first acting upon the metal with nitric acid, and in that way obtaining a nitrate, which is dried and heated to dispel the acid, when the oxide will remain. Again, if we deflagrate some of the metals with a body containing a large proportion of oxygen, we obtain their oxides. Tin, lead, zinc, etc., are in this way removed from alloys in which they enter as prominent constituents. For example, take, say, one gramme of an amalgam alloy, consisting of tin, sil- ver, gold, and platinum, place it in a crucible and melt with borax. If crystals of potassium nitrate are then dropped into the fluid mass, the tin is converted into an oxide, and is dissolved and held by the borax glass. If this part of the process is thoroughly per- formed, the remaining button will be found to contain only the noble metals, silver, gold, and platinum, which may be easily separated and weighed, thus affording a very simple method of quantitative anal- ysis for ascertaining the proportions of amalgam alloys. Metallic oxides in the form of hydrates are ob- tained by treating an aqueous solution of a metallic salt with an alkali. Thus, the hydrated sesquioxide of iron, commonly employed as an antidote in ar- senical poisoning, is produced by adding ammonia 104 DENTAL METALLURGY. to ferrous sulphate. Zinc sulphate or cupric sul- phate, .by the addition of caustic potassa, yields bulky hydrated oxides. These in turn may be con- verted into simple oxides by heat. Superficial oxidation may occur gradually by mere exposure to air at ordinary temperatures, and the action will be accelerated by the presence of mois- ture. It frequently occurs, however, that metallic objects thus superficially oxidized are so protected by the newly-formed oxide from further access of air that oxidation can no longer go on ; but should the rusted or tarnished surface of an iron or leaden object be removed, oxidation will again occur. Many metallic oxides are formed during fusion of the metals. Lead and zinc are examples of this. The former, by continued exposure to a sufficient degree of heat may be entirely changed into an oxide, and the latter, when carried to a temperature much above its fusing-point, burns with a brilliant light, during which the oxide is evolved in the form of white fumes, the incandescence accompanying the combination being an evidence of the intense affinity which the metal at an elevated temperature has for oxygen. Other of the metals are thus combustible. The familiar experiment of converting iron into an oxide by throwing a jet of oxygen gas upon a red- hot bar of the metal is an illustration of the fact, and many metallic oxides may be thus formed by deflagration. There are, however, a few noble metals possessing so feeble an affinity for oxygen that they cannot be made to combine directly with the latter ; even when the oxides of these are obtained by chemical means COMBINATIONS OF METALS, ETC. 105 the metals separate from the oxygen upon being heated to redness.* Gold and platinum are illustra- tions of this class of metals. The latter, which is employed as a base-plate in the continuous-gum process " and for pins in artificial teeth, is subjected to the most intense furnace-heat without the slight- est oxidation of surface. Many of the metallic oxides occur in nature, a number of the metals being reduced from natural ores, which are oxides of their respective metals, such as iron, tin, manganese, chromium, etc. Sulphides.—The metals unite with sulphur and form a class of compounds which, in a chemical and economical point of view, are almost as important as the oxides. These were formerly termed sulphurets. Many of them are found as natural ores, and are generally brittle solids possessing a high metallic luster, the latter quality being so marked in some that they have been mistaken for gold. Sulphur combines with the metals in varying proportions, and it may be observed that combination takes place in proportions similar to the oxides, the only excep- tions to this analogy being the alkalies and alkaline earths,—there being but two oxides of potassium, sodium, and barium, white there are no less than five sulphides of these metals. All the metallic sulphides are solid at ordinary temperatures, most of them fuse at red heat, and some sublime unchanged. The admission of air to the heated sulphides is followed by their decompo- sition and conversion into sulphates, or, if they are exposed to higher and continued heat, into oxides. * See " Noble Metals." 10 106 DENTAL METALLURGY. The sulphides are all insoluble in water, with the exception of those of iodine, potassium, strontium, barium, and calcium. The metallic sulphides may be artificially formed by the following processes: by heating the metals or their oxides with sulphur;* and from the sulphates by heating them with char- coal, or in a current of hydrogen by passing a stream of sulphureted hydrogen through their solutions, or by adding to them a solution of an alkaline sulphide. Seduction of Metallic Compounds. The term " reduction," as used in metallurgy, refers to the different methods of separating a metal from its natural ores or from combination with any non- metallic element. In some cases this is effected by heat alone. For example, the noble metals are sepa- rated from oxygen by merely heating to 600° F. (=315-5° C.) Generally, however, the joint action of heat and reagents for which the non-metallic constitu- ents of the compound have greater affinity is required. The inventions of Eugene H. and Alfred H. Cowles, of Cleveland, Ohio, and of Graetzel, near Bremen, in Germany, will doubtless prove a most important ad- vance in metallurgy. The essential feature in the improvements of these gentlemen is the application of the intense heat of a current of electricity from a dynamo machine through a conductor of great resistance in the presence of carbon. Many of the most refractory ores, which have hitherto resisted all similar attempts, may be readily decomposed in these * Silver is an example of this. So great is its affinity for sulphur that rubber, the indurating agent of which is sulphur, cannot be vul- canized in contact with that metal. COMBINATIONS OF METALS, ETC. 107 electrical furnaces. By this means aluminum is now reduced from corundum.* The metallic compounds, whether natural or arti- ficial, are a class of bodies formed of dissimilar elements held together by the force of chemical affinity, and which are totally unlike either of their constituents. This affinity varies much in different metals. Thus, gold possesses very feeble affinities, and when combined with chlorine it may be partially precipitated by mere exposure to light or the atmos- phere. The facility with which it often passes from one element to another may be observed in the interesting process of manufacturing " shredded gold," -j- in which an acid solution of the trichloride is formed and slightly heated in a glass matrass; gum arabic or sugar dissolved in water is then added, when beautiful web-like masses of pure gold are seen to form in the liquid, but unless these are quickly removed by means of a glass spoon or dipper, they will almost instantly dissolve and the gold again unite with the chlorine. Lead, tin, zinc, iron, and many other metals evince stronger affinities; hence, they are not so readily reduced, and require, in addition to heat, the presence of other substances, such as coal, coke, charcoal, etc. In other words, it is necessary to expose them in contact with some reagent between which and the non-metallic constituents of the com- pound superior affinity exists, so that by union of these the metal may be released. Indeed, it may be truly said that all analytical operations for the reduc- tion of ores and the discrimination and estimation of *See Chapter on "Aluminum." f " Lamm's shredded gold." See "Preparations of Gold." 108 DENTAL METALLURGY. unknown bodies are performed by taking advantage of the different degrees of chemical affinity. Thus, lead which has been overheated or subjected to fre- quent or long-continued meltings becomes partially oxidized and covered with an earthy-looking mass consisting of semi-oxidized metal, formerly called the " calx." Further exposure to heat would simply have the effect of converting this into an oxide of a higher degree, but if covered with finely-broken charcoal, or other carbonaceous substance,* the latter will extract the oxygen, carbonic acid will be formed and evolved, while the metal will be restored to a free state. Chlorides.—With the exception of the chlorides of the metals of the alkalies and earths, all metallic chlorides are decomposed when heated in a current of hydrogen, hydrochloric acid and the pure metal being the result. The chlorides of gold and platinum are decomposed by simple ignition. Argentic chloride, when heated on charcoal, under the flame of the blow-pipe, yields pure silver and emits an odor of hydrochloric acid. Placed in water acidulated with sulphuric or hydrochloric acid, ar- gentic chlorides may be reduced by the addition of pieces of some easily-oxidiz'ed metal, such as zinc or iron, the rationale of the reaction being as follows: the zinc displaces the hydrogen of the H2So4, zincic sulphate is formed, the liberated hydrogen unites with the chlorine to form hydrochloric acid, and pure silver remains. Sulphuric acid decomposes the chlorides and con- verts them into oxides, the oxygen being supplied * In the dental laboratory beeswax is usually employed to deoxidize lead or zinc which has become thick and earthy by frequent meltings. COMBINATIONS OF METALS, ETC. 109 from the water present. Some chlorides may be decomposed by heating them with a metal which has more powerful basic properties. Thus, sodium, when heated with aluminum or magnesium chloride, will become sodic chloride, with liberation of the magne- sium or aluminum. Some chlorides are reduced by heating with a mixture of sodic carbonate and char- coal ; other carbonaceous compounds, such as sodic or calcic carbonate, are frequently used. Sulphides.—Eeduction of the sulphides, in some few instances, such as those of gold, silver, and platinum is affected by heat alone. The oxygen of the atmos- phere unites with the sulphur, which is evolved as sulphurous acid. In many cases, however, a portion of the oxygen combines with the metal, and an oxide instead of the free metal is obtained. The reduction of many of this class of ores consists simply in such interchanges. The application of heat and air in some instances converts the sulphide into a sulphate, which in turn may be decomposed at high tempera- tures and separated into sulphurous acid and a me- tallic oxide.' On the other hand, some of the sul- phides may, when heated with access of air, be converted into permanent sulphates capable of resist- ing high degrees of heat. The sulphides of the noble metals, when heated, part directly with the whole of their sulphur, leaving the metal in a pure state. Silver sulphide thus reduced is also partially oxidized, so that a small portion of argentic sulphate is formed, which requires for its reduction a still greater elevation of temperature. Eeducing agents, such as metallic iron, hydrogen, chlorine, etc., are frequently employed to combine 10* 110 DENTAL METALLURGY. with sulphur. If sulphides of lead be heated with iron, sulphide of iron and metallic lead result. This method is frequently practiced in the assay of galena, clean iron nails being heated with the ore. The sul- phides of antimony, bismuth, copper, tin, and silver are readily reduced by passing dr}^ hydrogen over them at red heat, the result of the reaction being the free metal and sulphureted hydrogen, the pro- duct of the union of the hydrogen and sulphur. Dry chlorine will also decompose them, and combine with both the metal and the sulphur. Nitro-hydro- chloric acid converts the sulphides into chlorides, and hydrochloric acid in a few instances acts simi- larly ; its hydrogen, combining with the sulphur, is evolved as sulphureted hydrogen. Strong nitric acid also decomposes them, and is often employed in analyses of ores. The sulphur being thus oxidized, the liberated metal combines with the acid to form a nitrate, mercuric sulphide or native cinnabar being the only ore which cannot be thus reduced. Oxides.—The reduction of lead, zinc, or tin, the working qualities of which have been impaired by frequent meltings with exposure to air, may be effected in the laboratory by placing the metal to be treated either in a large clay crucible or in the ordi- nary iron melting-pot employed by dentists. The semi-oxidized metal is then covered with powdered charcoal, when the reaction described above takes place, and the original properties of the metal are restored. There are some oxides to which the foregoing treatment is not applicable, but these may be reduced by passing a current of dry hydrogen over them COMBINATIONS OF METALS, ETC. Ill when heated to redness. Makins gives the following very clear description of this method of reducing oxides: " A large two-necked bottle is fitted up in the usual way for the evolution of hydrogen. This has its delivery-tube passed into a tube filled with frag- ments of calcic chloride, for the purpose of absorbing the moisture which may be carried over with the gas; to the other end of this drying-tube is connected the tube which is to hold the metallic oxide (gener- ally in a bulb blown upon this). The gas bottle should contain about a couple of quarts, so as to afford a steady supply, and the calcic chloride tube should be long and well filled. In operating, after the gas has completely driven out the air in the ap. paratus, heat is applied to the bulb containing the oxide, and its reduction will be brought about. The gas must be kept up in a good stream, so as to drive out the watery vapor formed by the decomposition. Here the hydrogen takes the oxygen of the oxide, and water is formed, while the metal is set free." There are metals whose affinity for oxygen is so strong that their union with that element cannot be broken up by such means as we have described. Deoxidation of these metals must be performed through the agency of some other metal possessing greater affinity for oxygen. For example, if oxide of iron be heated with potassium, the iron will be deoxidized, while the potassium will be converted into potash (K20). Some metallic oxides may be reduced by heating with sulphur, part of the latter abstracting the oxy- gen, with which it unites to form sulphurous acid. 112 DENTAL METALLURGY. A portion of the sulphur, however, unites with the metal, which is converted into a sulphide, or a sul- phate, or a mixture of both. These must then be treated according to the directions already given for the reduction of metals when combined with sulphur. There are also a few metallic oxides which chlorine gas will reduce. Thus, platinum is liberated from combination with oxygen when exposed to a current of dry chlorine. Probably the most powerful means of reducing metals from combination with non-metallic elements is that known as electrolysis. It consists in exposing a solution of a metallic salt to the decomposing in- fluence of the galvanic current. A demonstration of this force may be made by taking a solution of nitrate of lead (plumbic nitrate) and immersing in it a piece of zinc. The latter soon becomes covered with needle-like crystals of pure lead; the zinc re- places the lead, which is set free and deposited at the point of galvanic action. Or, the same phenomenon may be witnessed by immersing a piece of clean iron in a solution of copper, or a piece of copper in a solution of a salt of mercury,* the action only ceas- ing when all the metal in the solution is reduced. * Reinsch's test for the detection of the mineral poisons is based upon this principle. CHAPTER VIII. GOLD. Atomic Weight, 197. Symbol, Au (Aurum). r^\ OLD is one of the few metals which is found in the metallic state, and it was probably one of the first known to man. Allusions to it are frequent in the Old Testament, and jewelry and vessels found in Egyptian tombs afford evidence of the perfection attained in working it at a period earlier than the government of Joseph. There are many evidences that processes of alloying, refining, and separating gold were practiced at a very early period of the world's history. According to Pliny, the metallurgy of gold was known in his day. Vitruvius also gives a detailed account of the method of recovering gold by amalgamation from cloth into which it had been woven. It was employed in Eome for the purpose of fixing artificial teeth more than three hundred years before the Christian era, and a law of the " Twelve Tables " makes exception with regard to such gold, permitting it to be buried with the dead.* The great beauty of color and luster, and the power of resisting oxidation which gold possesses, have caused it to be valued from the earliest ages for the purpose of adornment, and as a circulating medium. * Phillips's Metallurgy. (113) 114 DENTAL METALLURGY. Occurrence, Distribution, and Properties.—Gold is of nearly universal distribution, and is found in nature chiefly in the metallic state as native gold. It occa- sionally occurs in combination with tellurium, lead, and silver, forming a peculiar group of minerals, con- fined to a few localities in Europe and America, these being the only certain examples of natural combina- tions of the metal. The most important minerals containing gold are sylvanite or graphic tellurium (AgAu)Te2, containing about twenty-four per cent. of gold; calaverite, AuTe2, containing about forty per cent, of gold, and nagyagite or foliate tellurium, the composition of which is not definitely known. It contains from five to nine per cent, of gold. The metallic sulphides, such as galena and iron pyrites, usually contain sensible quantities of gold, the lead ore being almost invariably gold-bearing. Native arsenic and antimony also occasionally contain gold, and a native gold amalgam has been found in Cali- fornia. Gold occurs in nature very nearly though never quite pure, being generally associated with silver. Other metals are occasionally found combined with it, but in very small quantities; and these foreign metals are peculiar to localities. Thus, California gold, in addition to silver, which is always present, may contain iridium; Eussian gold often contains platinum, and specimens of the native metal from Brazil will not infrequently be found to contain palladium. GOLD. 115 Analyses of Native Gold from Various Localities. Gold. Silver. Iron. Copper. United States :* California .... Europe : Vigra & Clogau . . Wicklow (river) . . Transylvania. . . . Asia: Kussian Empire— Brezovsk .... Ekaterinburg . . . Africa ; Ashantee .... South America : Brazil..... Central America . . Titiribi..... Australia : South Australia . . 9012 9016 92-32 60-49 91-81 98-96 9005 94 0 88-5 76-41 84-25 87-78 99-25 901 9-26 617 38-74 8 03 016 9-94 5-85 11-96 2312 14-90 6-07 0-65 6-15 trace. 0-78 trace. 005 6-15 trace. 077 009 0-35 0-87 003 Pure gold is of a rich yellow color, and is nearly as soft as lead. It is, with one exception (platinum), the heaviest substance in nature, being about nineteen and a half times as heavy as water. These proper- ties are all sensibly modified by admixture of other metals. Thus, the tint is lowered by small quantities *The yield of gold in the United States in 1876 was greatly in ex- cess of that of any other country on the globe, Russia being next ill quantity produced. 116 DENTAL METALLURGY. of silver, and heightened by copper. Owing to its exceeding softness, gold is commonly used alloyed, in order to render it capable of resisting the attrition to which coins and articles of jewelry are exposed. It is the most malleable of all the metals. One grain may be beaten into leaves which would cover a sur- face of fifty-six square inches, and only 3 0 ^ 0 0 of an inch thick.* Very thin gold leaf appears yellow by reflected and green by transmitted light. Highly attenuated films of gold, when heated, transmit rays of light of a ruby-red color. The pressure of a hard sub- stance on the film will, however, so change its state of aggregation that the green color will again appear. Gold is exceedingly ductile, but does not possess a very considerable degree of tenacity. A grain of gold, however, if covered by a more tenacious metal, such as silver, may be drawn into a wire five hun- dred feet in length. It also possesses the remarkable property of welding cold. Thus, the metal, in the state in which it is obtained by precipitation by oxalic acid, may be formed into disks or medals by compression between dies. The specific gravity of gold varies according to condition. In the finely-divided state in which it is obtained by precipitation by oxalic acid it is 19-36. The specific gravity of cast-gold is somewhat less, but when compressed between dies, or by the rolling- mill, it may be raised from 19-37 to 19-41. Annealing, however, will restore its previous density to nearly that of the cast metal. *The late Dr. S. S. White presented the author with a specimen, securely mounted between plates of glass, which is but -vw^-q-q of an inch in thiokness. It is transparent and transmits green rays of light. GOLD. 117 The atomic weight of gold has been variously stated. Berzelius gave it as 19667; Levol, 196-3; Wurtz, 196-5; Watts, 196-0; Bloxam, 196-6; Fownes, 197. There seems also to be a similar diversity of opinion regarding the temperature at which it fuses. Thus, Daniell fixed the melting-point at 1425° C.; Pouillet, 1200° C; Guyton de Morveau, 1380° C. The figures, 1102° C, given in Fownes's " Elementary Chemistry," are probably as nearly correct as any, and for all practical purposes will answer very well. The electric conductivity of gold is given by Matthiesen as 73-96 at 15-1° C, pure silver being 100. The conducting power, however, depends much upon the degree of purity, as the smallest addition of another metal will very considerably lower its conductivity.* The conductivity of gold for heat is stated as 53.2, as compared with pure silver, 100. Its specific heat , is 00324. Volatility.—The absence of uniformity in results of experiments with regard to this property given by different investigators would seem to leave the mat- ter still in doubt. Thus, Gasto Claveus and Kunkel describe similar experiments, wherein an ounce of pure gold was placed "in an earthen vessel in that part of a glass-house where the glass is kept con- stantly melted, and retained in a state of fusion for two months, without the loss of the smallest portion of its weight." On the other hand, Homburg, La- voisier, and Maquer state that when a small portion of gold is kept at a violent heat part of it is volatil- ized, and that a piece of silver held in the rising *See " Power of Conducting Electricity." 11 118 DENTAL METALLURGY. fumes will have its surface gilded. It is quite prob- able that, when a small portion of gold is mixed with a large quantity of zinc and heated in the air, the whole of the gold will be dissipated with the fumes of oxide of zinc. Mr. Makins has demonstrated that gold, silver, and lead, when cupelled together, volatilize. Gold may also be volatilized, when in the form of leaf or highly-attenuated wire, by passing a powerful charge of electricity through it. Forms of Native Gold.—The native metal is some- times found in the form of cubic crystals, in octahe- dra, and in irregular and more complex shapes called nuggets and dust. Crystals of gold may also be obtained artificially from an amalgam of gold one part, mercury twenty parts. The mixture is main- tained at a temperature of 80° C. for eight days. The mercury is then removed by strong nitric acid, leaving crystals of gold, which require to be heated to redness to develop brilliancy of surface. Gold is found in quartz veins or reefs traversing slaty or crystalline rocks, alone or associated with iron, copper, magnetic and arsenical pyrites, galena, specular iron ore, and silver ores, and more rarely with sulphide of molybdenum, tungstate of calcium, bismuth, and tellurium minerals. It is also found among the detritus of disintegrated rock,* associ- ated with the metals of the platinum group. In the superficial alluvial or "placer" deposits it has been remarked that the minerals with which it is found intermixed are of great density and hardness, and are the most durable constituents of disintegrated rock. *In the form of small lumps called " dust." GOLD. 119 The yield of gold in easily-worked alluvial deposits is often exceedingly small. It is stated that in the Siberian gold-washings the proportion of gold ranges from 12 grains to 1 dwt. to the ton of sand, while in the lodes which require more labor to work the pro- portion is but 8 dwts. per ton, and in the "placer" washings of California it is but 12 grains to the ton of gravel. In Australia the alluvial washings of Victoria yielded 25 grains to the ton. Vein mining being more difficult and costly, necessitates a larger yield of the precious metal, and 5 dwts., or about five dollars' worth of the precious metal, is in most gold-bearing localities regarded as a paying quantity. The method of obtaining gold from alluvial de- posits is exceedingly simple, and consists in washing away the lighter portions, leaving the heavy metallic particles. In the early daj'S of gold-mining in Cali- fornia this was accomplished by means of a pan of sheet-iron, thirteen or fourteen inches in diameter, held in the hand and its contents exposed to a stream of water; and on the large scale consists of washing the alluvial deposits into sluices or troughs by means of continuous streams of water,— mercury or amal- gamated copper plates being sometimes employed to collect the finer particles of gold. In vein mining the separation of the gold from the rock with which it is mechanically mixed consists in reducing the latter to a fine powder in grinding- or stamping-mills, and the gold is recovered by amalga- mation, or by washing the pulverulent mass through troughs lined with coarse woollen cloths, by which means the lighter deposits are carried away with the current, while the heavier metallic particles become 120 DENTAL METALLURGY. entangled in the fibers of the blanket until the sur- face of the latter is completely covered, when it is removed and its contents are washed off in a suitable vessel and reserved for amalgamation. In the treatment of gold by amalgamation the process is frequently retarded by a difficulty known as the "sickening" or "flouring" of the mercury. The latter, losing its bright metallic surface, is no longer capable of coalescing with other metals. The discovery was made by Wurtz, in 1864, that by the addition of a small quantity of sodium to the mer- cury the operation is greatly facilitated, the addition of the sodium preventing both the conditions above referred to which are produced by certain associated minerals. Some metallurgists recommend the addi- tion of 20 per cent, of zinc and 10 per cent, of tin. It has been estimated that mercury will dissolve from 0-05 to 008 per cent, of native gold of standard 650 to 850 without loss of fluidity. The solubility of the gold increases with its fineness. When the point of saturation has been reached, lumps of the solid amal- gam are introduced into an iron vessel lined with a mixture of fire-clay and wood-ashes, and provided with an iron tube, by which the fumes of mercury are passed through water and condensed, the distilla- tion being effected at a temperature below redness. The gold left in the retort is then melted in a suitable crucible. Gold is sometimes reduced from the mineral by exposing the ore, which has been previously roasted, to a current of chlorine gas. By this means the gold is converted into a soluble chloride, which is removed by washing with water. The precious metal is then GOLD. 121 recovered in the metallic form by precipitation with ferrous sulphate. This process is, when carefully performed, a very accurate one, and yields 97 per cent, of the gold present in the ore. Refining Gold. — Methods of refining gold were known and practiced in very ancient times, and many of them, though empirically employed, did not ma- terially differ in principle from those in use at the present day. Thus, in Strabo's time the gold was placed on the fire with three times its weight of salt and a quantity of argillaceous rock, which in the presence of moisture effected the decomposition of the salt. Hydrochloric acid is thus formed, which, at the high temperature employed, furnishes chlorine to the silver associated with the gold, which is con- verted into a chloride. A similar process is still practiced in South America. Among other methods for the separation of gold from silver or other contaminating metals, which have been in use from a remote period, may be men- tioned prolonged oxidation by exposure to air, and melting with sulphur, sulphide of antimony, and cor- rosive sublimate. The old "quartation" process of refining, so-called from the fact that an alloy is formed, four parts of which contains three parts of silver and one of gold, consists in first forming an alloy of the gold with silver in the proportions given. These are melted to insure homogeneity, and granulated* by pouring into water contained in a wooden vessel. The pieces" are then collected and placed in a glass or platinum vessel, and acted upon by either nitric or sulphuric ♦Granulation is usually repeated twice or thrice. 11* 122 DENTAL METALLURGY. acid. When the presence of lead or tin is suspected, these should be got rid of before subjecting the alloy to the acid; otherwise the platinum digester would be injured. The removal of lead is accomplished by cupelling the alloy, while tin may be effectually removed by fusing the alloy with potassium nitrate. On account of greater economy and the closeness with which it will act upon silver containing very small quantities of gold, sulphuric acid is at the present day the agent most commonly employed, particularly where large quantities of the alloy are to be treated. The nitric acid plan does not, as a rule, yield gold of as high a degree of fineness as the sulphuric acid treatment, but the oxidizing property of the nitric acid is of great advantage in refining gold contam- inated with antimony and other equally injurious metals. When nitric acid is employed, each ounce of the granulated alloy is treated with an ounce and a quarter of nitric acid of specific gravity 1-32. The sulphuric acid process is based upon the facts that the concentrated hot acid converts silver and copper into soluble sulphates without attacking the gold, the metallic silver being recovered from the sulphate in the form of needle-like crystals by thrust- ing copper plates* into it. The sulphate of copper resulting from the reaction is crystallized and becomes an article of commerce. The sulphuric acid should be of specific gravity 1-84, and the alloy is boiled for three or four hours in a platinum vessel with 2-5 times its weight of acid. The sulphurous acid fumes which *Iron plates are sometimes used. GOLD. 123 arise are partially condensed before being allowed to pass into the air. When the acid has ceased to act upon the metal, a small quantity of sulphuric acid of specific gravity 1-53 is added, and after a second boil- ing the contents of the vessel are allowed to settle ; the liquid is withdrawn from the gold, which rests at the bottom of the vessel, and is diluted until its density is 1-21 to 1-26. The gold is then carefully washed and melted into ingots, which generally con- tain from 997 to 998 parts of gold in the 1000. In the nitric acid process the supernatant liquid consists mainly of argentic nitrate. The silver may be recovered by precipitation with chloride of sodium, argentic chloride resulting, which in turn is exposed to a current of hydrogen, liberating metallic silver.* The dry method of refining gold before alluded to consists in placing the granulated alloy and a mixture of one part of chloride of sodium and two parts of brick-dust in alternate layers in a crucible until the latter is full, when it is covered and placed in a wood fire and kept at a dull redness for twenty-four hours. By the united action of the moisture furnished by the wood and the silica of the brick-dust the sodic chlo- ride is decomposed; its sodium combines with oxygen from the decomposition of the water, forming soda, which in turn unites with silica to form sodic sili- cate. The hydrogen of the water and the liberated chlorine form hydrochloric acid; these, at the tem- perature at which the operation must be carried on, furnish chlorine to the silver, converting it into argentic chloride. The latter, being fusible, is ab- sorbed by the brick-dust, permitting the alloy to *See chapter on "Silver." 124 DENTAL METALLURGY. be further acted upon, until nearly all the silver is converted into chloride, the gold remaining compara- tively free. There is also another cementation process given* for the purpose of acting upon the surface of gold containing a large percentage of silver, by which means it is made to resemble fine gold. It consists in subjecting the alloy, previously rolled thin and covered by the cement-powder, to a temperature slightly below its melting-point. In this operation the mixture is composed of one part of sodic chlo- ride, one part of alum, one part of ferrous sulphate, and three of brick-dust. At the high temperature necessary the sulphates are decomposed, with libera- tion of free sulphuric acid, while chlorine is evolved from the sodic chloride. These act upon the silver, which is subsequently found in the cement-powder in the form of argentic chloride. Eefining by chlorine gas, devised by F. B. Miller, of Sydney, N. S. W., in 1867, is a valuable and accurate dry method for separating silver from gold. The process is the one now practiced in the Australian mints, where it has been quite extensively employed, 1,100,000 ounces of gold having been refined by it *in S}rdney during the years 1871 and 1872, the percentage of loss during the operation being only 14 parts in the 100,000. It consists in converting the silver into chloride by the passage of a stream of chlorine gas through the molten alloy. By means of a clay pipe passing through the cover to the bottom of the crucible, and connected with the *This process is attributed to Kerl, and is described in Makins's Metallurgy, p. 244. GOLD. 125 chlorine-generator by means of a flexible tube, the gas is passed rapidly through the melted metal, and is apparently absorbed by it. The refining is considered as complete when orange-colored fumes begin to arise. As soon as this evolution of gas is noticed the crucible should be removed from the fire, to prevent the gold itself from combining with the chlorine. The chlo- ride of silver, which is fusible, should be poured off from the surface of the molten metal, and, if it retains a small portion of the gold, this may be recovered by fusing with a little carbonate of soda, which causes the gold to separate and settle to the bottom of the crucible. This method is capable of producing gold of from 944 to 1000 fine. The gold is next melted into bars, and the argen- tic chloride is reduced to metallic silver by placing it between two wrought-iron plates, and then im- mersing the whole in a vessel of water acidulated with sulphuric acid, when, after a few hours, the silver will all be reduced. It generally, however, contains a small percentage of gold, which may be recovered either by again dissolving the silver with nitric acid, when the gold will be found at the bottom of the vessel; or it may bo separated by fusing the argentic chloride, to which is added a small quantity of potassic carbonate for the purpose of reducing a little metallic silver. The latter, in subsiding through the argentic chloride, reduces the gold, which was probably combined with the chlorine. While yet hot and in a fluid state, the argentic chloride is poured off and reduced as before, when it will be found free of gold. The little button which subsides to the bottom of the vessel will be found to consist of gold, 126 DENTAL METALLURGY. the reduced silver, and some adherent argentic chlo- ride. The latter must be again decomposed by fusing with potassic carbonate. Theoretically, one cubic foot of chlorine will convert eight and a quarter ounces of silver into argentic chloride, but in prac- tice about twice that quantity is required. Thus far the use of chlorine for refining upon a large scale has proved to be much more economical and expe- ditious than the humid process, the time required to part three hundred ounces in one furnace being about two hours,' and the average cost four cents per ounce. Treatment of Brittle Gold.—The slightest admix- ture of such metals as arsenic, antimony, tin, lead, etc., is sufficient to seriously impair the ductility of gold. In 1856 the coining operations of the mint of England were much embarrassed by the importation of brittle gold, in which the contaminating metal did not exceed the j^-q part of the mass. To purify the gold it was exposed while in a state of fusion to a stream of chlorine gas, which removed the dele- terious substances by converting them into volatile chlorides. The toughness of gold may also be re- stored by throwing a small quantity of corrosive sublimate on the surface of the molten metal, the vapor of which converts the metallic impurities into chlorides, which are volatilized. In the dental laboratory gold is liable to become contaminated with small particles of lead or zinc. These may be effectually removed by melting with a mixture of potassium nitrate and borax, when the foreign metals will be oxidized and dissolved in the slag. Another process consists in adding to the melted mass about 10 per cent, of black oxide of copper. But this GOLD. 127 plan is objectionable, because the crucible is liable to become much corroded, and even perforated, and the standard fineness of the gold lowered, by a portion of the copper being reduced to the metallic state. The gold of this country^is often found to contain iridium, the presence of which greatly impairs the metal for coinage and other purposes. The little hard grains occasionally met with in gold, upon which the file makes no impression, consist of iridium, or a native alloy of osmium and iridium, and are not com- bined with the gold, but merely disseminated through it. The only dry method of separating iridium from gold consists in alloying the latter with three times its weight of silver, by which means the specific gravity of the metal is so much lowered that the iridium, which is very infusible and of a specific gravity of 21-1, will subside to the bottom of the crucible, when the gold and silver alloy may be poured or ladled off. As some gold will remain with the residue, more silver must be melted with it, the operation being repeated several times, until nearly all the gold is removed. What is left is then acted upon by sulphuric acid to dissolve the silver, when the iridium and some finely-divided gold will be left. These may be separated by washing. Iridium may also be separated from gold by the wet process. The gold is melted with three times its weight of silver, and granulated to insure admix- ture. The alloy is then treated with nitric acid, which dissolves the silver, leaving the gold and iridium at the bottom of the vessel. The gold may now be acted upon by nitro-hydrochloric acid. The iridium may then be collected and washed to free it 128 DENTAL METALLURGY. from any portion of the gold. The latter may be recovered from its solution by precipitation, oxalic acid or sulphurous acid being usually employed. Preparation of Chemically-Pure Gold.—None of the methods which have been described can always be relied upon to afford absolutely pure gold. When nitric acid is employed in the quartation process, gold may be obtained from 993 to 997 parts in the 1000, while sulphuric acid will frequently yield gold up to 998 thousandths. Eecent assays, made by Messrs. DuBois and Eck- feldt, assayers at the United States Mint, of some of the most prominent foils give the following results*: No. 1. Abbey's Non-cohesive . 998-8 9987 " 2. Wolrab's .... 999-2 999-3 " 3. Quarter Century, S. S. W. Den. Mfg. Co. . . 9991 9991 " 4. Rowan's Decimal foil . 999-9 9998 There are several methods by which chemically- pure gold may be obtained. Usually, ordinary refined gold, obtained by one of the methods above described, is dissolved in nitro-hydrochloric acid. The excess of acid is driven off, and alcohol and chloride of po- tassium are added for the purpose of precipitating platinum, if any is present. The chloride of gold is then dissolved in pure distilled water, until each gallon does not contain more than half an ounce of the chloride. Any silver present will be converted into argentic chloride, which will settle to the bottom of the vessel, after which the supernatant liquid should be carefully removed by means of a syphon. The gold may be precipitated by a stream of care- * American System of Dentistry. GOLD. 129 fully-washed sulphurous anhydride, or by the addi- tion of oxalic acid. The precipitated metal is washed with dilute hydrochloric acid, distilled water, ammo- nia water, and again with distilled water, and is then ready for melting. This is done in a clay crucible, with a small portion of bisulphate of potash and borax. The melted metal should be poured into a stone ingot-mold. By this method gold, of which the purity was 999-96, has been prepared, the precipi- tant being oxalic acid; but gold precipitated by that agent from an acid solution containing copper is always contaminated with cupric oxalate, to avoid which the solution should be heated, with the addi- tion of potash, when a soluble double oxalate of cop- per and potash is formed, leaving the gold in the pure state. The aqua regia used in the preparation of chemi- cally-pure gold should consist of two parts of hydro- chloric and one part of nitric acid. The specific gravity of the former should be about 1-16, and of the latter 1-45. Each ounce of gold will require for its solution about three and one-half ounces of the mixed acids. The action of this upon the metal will in the beginning be quite energetic, but as the solution ap- proaches saturation the application of moderate heat is required to dissolve the last portion of the gold. The greatest care must be exercised in the separation of the gold solution from the argentic chloride, which subsides to the bottom of the vessel, and also to rid the liquid of the small portion of silver held in solu- tion by the acid. The solution is cautiously trans- ferred to an evaporating dish by means of a syphon, and heat is applied, and as the bulk is gradually re- 12 130 DENTAL METALLURGY. duced by evaporation more argentic chloride will be separated and deposited at the bottom. The super- natant liquid should again be carefully poured or syphoned off, and this should be repeated as often as the residue appears in the dish. When the solution has become viscid and of a deep-ruby color, the heat is discontinued, and the auric chloride soon crystallizes in a mass of prismatic forms. It should then be dis- solved and largely diluted with distilled water, acidu- lated by a few drops of hydrochloric acid, and, after standing for a few days to permit a further subsidence of argentic chloride, it should be filtered, when it is ready for precipitation. This may be accomplished by quite a number of different reagents, but the form of the precipitated metal depends much upon the nature of the precipitant, and it may be thrown down in a spongy condition, in sheets resembling foil, as a pow- der, in a more or less crystalline state, and in scales. The affinity of gold for other bodies is so weak that care must be observed lest partial reduction be effected by merely adventitious conditions. The highly di- luted neutral solution of the trichloride just described is quite liable to such accidents; indeed, it may occur from exposure to air, atmospheric nitrogen probably being the active agent. The addition of pure water to such a solution may also cause slight precipitation, but the dilute solution may be protected from pre- mature precipitation by acidulation with a small quantity of hydrochloric acid. The best agents for the precipitation of gold are oxalic acid, sulphurous acid, and ferrous sulphate. Oxalic acid will precipitate several forms of gold, from sponge-like masses to the different crystalline GOLD. 131 or powdery forms. Its action is, however, slower than the others, and it requires to be slightly heated. The reaction is shown in the following equation: 2AuC]3+3H2C204=6HCl-f6C02+2Au. The chlorine of the auric chloride unites with the hydrogen of the oxalic acid to form hydrochloric acid, the copious evolution of gas noticed during the precipitation being the escape of the carbonic acid formed by the remaining elements of the oxalic acid- The gold is thus set free. The so-called " shredded gold," somewhat exten- sively used by dentists in filling teeth a few years since, was produced by the addition of sugar or gum arabic to an acid solution of gold. The exact modus operandi is as follows : The pure gold is dissolved in nitro-hydrochloric acid, and, without evaporating the solution, it is diluted by the addition of about two- thirds its bulk of pure water. Clean gum arabic, dissolved in boiling water to the amount of one-third the bulk of the gold solution, is added to the latter, and the whole poured into a glass matrass or evapo- rating-dish and placed over a steam bath. When the proper temperature is attained, gold in the form of leaves, shreds, or fibers will be observed floating in the liquid. When these become sufficiently coherent, to admit of removal they are lifted out by a vulcan- ized rubber spoon attached to a glass rod, and placed in a filter. This operation is continued until the gum aiabic or sugar, assisted by heat, has caused the precipitation of all the gold held in solution. The web-like masses are then thoroughly washed, dried and heated to dull redness. As the elements entering into the composition of sugar and gum 132 DENTAL METALLURGY. arabic are identical with those of oxalic acid, the reaction is probably the same as that which occurs when the latter is employed as the precipitant. With care in the application of the proper amount of heat) the action of precipitants of this class is capable of regulation, thus affording uniform results. When the precipitated gold is intended for plate or bars, it should be well washed, and fused in a perfectly new crucible. Sulphurous acid precipitates gold generally in the form of a scaly metallic powder; hence it does not afford masses sufficiently coherent or sponge-like for use as a filling-material for the dentist. The reaction which takes place is thus explained: 2AuCl3+3H20+3H2S03=6HCl+3H2SOi+2Au. The water present is decomposed, its hydrogen uniting with the chlorine of the auric chloride to form hydrochloric acid. The oxygen of the water, attracted to the sulphurous acid, converts it into sulphuric acid, and the gold is thus liberated. Ferrous sulphate precipitates gold in the form of a light-brown powder. Of the sulphate crystals about four times the weight of the gold is dissolved in water. This is added to the auric solution. After the finely-divided gold has entirely subsided, it should be boiled several times in dilute hydrochloric acid, in order to free it from all traces of the iron with which it is liable to be contaminated. The interchange, which in this reaction results in the liberation of the gold, is expressed by the following equation : 2AuCl3+6FeS04=Fe2Cl6+2(Fe23SOJ+2Au. The ferrous salt parts with a portion of the iron to the chlorine of the gold salt, thus forming fer- GOLD. 133 ric chloride and ferric sulphate, while the gold is liberated. As stated before, the reduction of the metal from the trichloride may be effected (in, however, a less satisfactory manner) by many different reagents, some of which are purely elementary. Thus, sulphur, sele- nium, carbon (charcoal), and phosphorus, each, when introduced into a heated solution, becomes coated with a film of metallic gold. Eeduction may also be accomplished by some of the gaseous bodies contain- ing hydrogen. Thus, gold may be precipitated by arscniureted and antimoniureted hydrogen. Many of the base metals, such as bismuth, zinc, etc., also reduce gold from solution in the form of a brown powder. It is also reduced on a platinum pole by the electrical current. In this way the beautiful form of gold made by A. J. Watts, of New York, is produced. In a solution of auric chloride plates of pure gold are suspended. These are connected with a battery, so that as the solution loses its gold by deposition of the metal it is re-supplied from the sus- pended plates. By this means large masses of per- fectly pure crystal gold may be obtained. Gold may also be precipitated by some of the metallic salts, of which nitrate of mercury and chlo- ride of antimony may be named as examples. Quite a number of organic substances will also precipitate it, a prominent example of which is gallic acid. The tartrate, citrate, and acetate of potassium will also reduce it. Some of the members of this class, how- ever, require the addition of heat, and to obtain prompt action with these agents the solution should be quite neutral. 12* 134 DENTAL METALLURGY. Alloys of Gold.—The most important alloys are those with silver and copper. The coinage of the present day, from which the dentist usually obtains his plate, is mainly an alloy of gold and copper, in the proportion of 900 parts of gold in 1000. Gold coins were first introduced in England by Henry III, in 1257. They were of pure gold. Edward III, in 1345, established a standard of 994-8, and in 1526 Henry VIII issued crowns of the double-rose, of the standard 916-6. In 1544 the standard of all gold coins was reduced to 916-6, and again in 1548 to 833-4. Mary restored the old standard, 994-8. In Elizabeth's reign coins of both standards (916-6 and 994-8) were issued. In America the standard of gold coin is 900. Alloys of gold for bases for artificial dentures should be of such fineness as will enable them to resist chemical action of the fluids of the mouth, while at the same time they should possess the requisite hardness, strength, and elasticity. These properties are usually conferred by the addition of copper and silver, or either of these metals singly; or by copper, silver, and platinum. The quality of gold which is to be introduced into the mouth should, as a rule, be of a standard of fineness not less than eighteen carats. Care, however, should be observed in remelting the scraps and filings of the drawer, that the grade of the gold be not lowered by the admix- ture of old plates, backings, etc., containing portions of solder. Indeed, much the safer rule is to remelt only new scraps, on which no solder has been used. The scraps.and filings of doubtful quality may be sent to the mint for coinage, the charge for which, GOLD. 135 on the one hundred dollars (the minimum amount received by the United States mint), is less than 1 per cent. Gold exceeding nineteen carats in fineness will generally be found too soft and yielding for use in the mouth. The amount of force which the plate must sustain in mastication is much greater than might be supposed; hence, if the degree of purity of the alloy be too high, the requisite amount of rigidity and strength will be wanting, and the plate will soon bend to such an extent that it will no longer fit the mouth. This difficulty may, however, be avoided in the higher grades of gold plate intended for dental purposes by a slight admixture of plat- inum, by which much greater tenacity is obtained; otherwise the plate will require strengthening by doubling at such points as are most liable to bend. Plates for partial cases necessarily require a great deal of strengthening. This adds considerably to the expenditure of time and labor in the construction of the plate. It increases its weight and does not always render the plate sufficiently rigid to with- stand the force of mastication. Gold plate suitable for dental purposes may be prepared according to the following formulae, from Eichardson's "Mechanical Dentistry: " Gold Plate 18 Carats Fine. No. 1. No. 2. Pure Gold . 18 dwts. Gold Coin . 20 dwts. Pure Copper . Pure Silver . 4 " 2 " Pure Copper . Pure Silver . 2 " 2 " No. 3. Gold Plate 1< ) Carats Fine. No; 4. Pure Gold . 19 dwts. Gold Coin . 20 dwts. Pure Copper. Pure Silver . 3 " 2- " Pure Copper Pure Silver . 25 grs. 40-f- grs. 136 DENTAL METALLURGY. Gold Plate 20 Carats Fine. No. 5. No. 6. Pure Gold . . 20 dwts. Gold Coin . 20 dwts. Pure Copper . . 2 " Pure Copper 18 grs. Pure Silver . . 2 " Pure Silver . 20+ grs. Gold Plate 21 Carats Fine. No. 7. No. 8. Pure Gold . . 21 dwts. Gold Coin . 20 dwts. Pure Copper. . 2 " Pure Silver . 13-f- grs. Pure Silver . . 1 dwt. No. 9. Gold Coin ... 20 dwts. Pure Copper ... 6 grs. Pure Platinum . . 7f- grs. Gold Plate 22 Carats Fine. No. 10. Pure Gold ... 22 dwts. Fine Copper ... 1 dwt. Pure Silver . . .18 grs. Pure Platinum . 6 " Gold Plate 18 Carats Fine. No. 11. United States Gold Coin ($60) . . 64$ dwts. Pure Silver......13 " On account of its greater strength and power of resisting chemical action of the fluids of the mouth, many dentists prefer to use gold plate twenty or twenty-one carats fine, in which the reducing con- stituents are copper and platinum, the following formula being an example: Gold Coin . . . .20 dwts. Pure Platinum . . .10 grs. The union of platinum with gold yields an alloy possessing great strength and considerable elasticity. Such an admixture, however, has its disadvantages. GOLD. 137 Owing to its increased strength and stiffness, a much thinner and lighter plate may be employed without the additional labor and cost of doubling the plate at what, in partial cases composed of ordinary 18-, 19-j or 20-carat gold, would be weak points. It may also be justly claimed for gold alloyed with platinum that it will perfectly resist the action of the fluids of the mouth. On the other hand, the richness of color of the gold is always more or less impaired by the admixture of platinum. But perhaps the greatest objection to be urged against the employment of platinum-gold is the increased difficulty of swaging a plate composed of it so that it shall perfectly conform to all the de- pressions and irregularities of the model. Having invariably found, when the alloy contained any con- siderable percentage of platinum, that the ordinary method of swaging between zinc and lead was not effective, the author has for more than fifteen years employed zinc for counter-dies as well as for dies,—a procedure which entirely overcomes any difficulty in swaging.* Gold for use in the formation of clasps should always contain sufficient platinum to render it much more elastic than the alloys usually employed in the plate or base, so that on the application of force upon the denture in the act of mastication the clasp, though it may yield slightly, will always spring together again and accurately embrace the tooth which it surrounds. In the perfect adjustment of clasps to remaining teeth the following points are of impor- tance : First, the model must be as accurate as a plaster impression will afford; second, the clasp *See chapter on " Zinc." 138 DENTAL METALLURGY. should be thrown around the thickest or most promi- nent part of the tooth; third, the clasp should be so arranged as to fit accurately the convexity of the tooth. To successfully accomplish this the gold, of about No. 26 of the standard gauge, should be cut by pattern, and before any attempt is made to fit it to the tooth it should be bent with the clasp-benders to correspond with the rounded surfaces of the natural tooth. Lastly, the contact of the clasps with the tooth should be uniform. The ends of the clasp should be free, and it should be attached to the plate at one point, so that but little of its circumference shall be included in the union; otherwise, if a large proportion of the clasp be soldered fast to the plate, much of the quality of elasticity will be lost. The following formulae will afford alloys of twenty carats' fineness, suitable for clasps, backings, etc., wherever elasticity and additional strength are re- quired : Formula No. 1. Formula No. 2. Pure Gold . . 20 dwts. Coin Gold . 20 dwts Pure Copper . . 2 " Pure Copper . 8 grs. Pure Silver . . 1 dwt. Pure Silver . 10 " Pure Platinum . 1 " Pure Platinum 20 " Alloys of Gold employed in Dentistry as Solders.— These are a class of alloys formed of the metal to be united, the fusing-point of which is reduced by the addition of silver, copper, and brass.* No. 2. 14 Carats Fine. No. 1. 14 Carats Fine. American Gold Coin, f 10.00 Pure Silver . . 4 dwts. Pure Copper . . 2 " American Gold Coin, 16 dwts. Pure Copper, 3 dwts. 18 grs. Pure Silver . . 5 dwts. * See page 35. GOLD. 139 2$ dwts. 20 grs. 35 " 20 dwts. No. 3. 14 Carats Fine. Pure Silver . Pure Copper . Pure Zinc 18-carat Gold Plate (formula No. 11) No. 5. 16 Carats Fine. Pure Gold . 11 dwts. Pure Silver . 3 " 6 grs. Pure Copper 2 " 6 " No. 7. 18 Carats Fine. No. 4. 15 Carats Fine. Gold Coin . Pure Silver . Pure Copper . Brass 6 dwts. 30 grs. 20 " 10 " Gold Coin Pure Silver Pure Copper Brass 30 parts. 4 « 1 part. 1 " No. 6. 16 Carats Fine. Pure Gold . 11 dwts. 12 grs. Pure Silver 3 " Pure Copper 1 dwt. 12 " Pure Zinc . . . 12 " No. 8. 20 Carats Fine, for Crown and Bridge-work. American Gold Coin (21.6 carats fine) $10 piece, 258 grs. Spelter Solder . 20.64 " No. 9. 20 Carats Fine, same use as No. 8. Pure Gold . . 5 dwts. Pure Copper . . 6 grs. Pure Silver . . 12 " Spelter Solder . . 6 " Spelter solder, composed of equal parts copper and zinc, is sometimes employed as a constituent in the preparation of gold solders for the purpose of reducing the fusing-point. Thus, some dentists use an alloy composed of 18-carat Gold.....6 dwts. Granulated Spelter Solder ... 6 grs. An alloy of this composition is exceedingly brittle, and hence difficult to roll into plate without break- ing into many pieces. Its color is good, but the author has noticed that the surface of such solders, after flow- ing, is apt to be pitted with small holes, and has not the solid and uniform appearance that is desirable. 140 DENTAL METALLURGY. This may be due to the oxidation and escape of some of the zinc. Methods of Reducing Gold to a Lower or Higher Standard of Fineness, and of Determining the Carat of any given Alloy.—The gold alloys used in the labo- ratory are generally made from pure gold or gold coin, the standards of which are definitely fixed. A few simple rules are here given,* by which the operator may readily determine the quantity of alloy necessary to reduce either coin or pure gold to any desired standard. To ascertain the carat of any given alloy, 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. For example, take the following: Pure gold......18 parts. Copper ....... 4 " Silver.......2 " 24 " The result may be thus expressed: 24 X 18 -f- 24 = 18 carats. To reduce gold to a required carat, multiply the weight of gold used by 24 and divide the product by the required carat. 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 the alloy to be added. To raise gold from a lower to a higher carat, multiply the weight of the alloyed gold used by the number representing the proportion of alloy in the * Richardson's Mechanical Dentistry. GOLD. 141 given carat; divide the product by the figures repre- senting the quantity of alloy in the required carat. The quotient is the weight of the mass when reduced to the required carat by adding fine gold. Thus, to raise one pennyweight of 16-carat gold to 18 carats, the numbers representing the proportions of alloy are obtained by subtracting 18 and 16 from 24. The statement is 6:8::l:li; from which it will be seen that, to raise one penny- weight of 16-carat gold to 18 carats, one-third of a pennyweight of pure gold must be added to it. Again, if instead of using pure gold we desire to raise the fineness of one pennyweight of 16-carat gold to that of 18, by the addition of, say 22-carat gold, the numbers representing the proportions of the alloy would be found by subtracting, in the example given, 16 and 18 from 22, the result being 4:6:: 1:1}. Hence each pennyweight of 16-carat gold would re- quire a half-pennyweight of 22-carat gold to raise it to 18 carats. The fineness of gold may be expressed in decimals or in parts called carats. The former is the system employed at the United States Mint and by metal- lurgists and chemists, while the latter is the usual method of expressing the grade of alloys of gold among dentists and jewelers. The following table will show the relation of one to the other : 13 142 DENTAL METALLURGY. Carats. Decimals- Pure gold ...... 24 1000 English coin 22 916-6 American coin . 21-6 900 Dentists' gold 20 833 3 n a 19-2 800 Jewelers' gold, best . 18 750 " " good . 15 625 " " low grade 12 500 Common jewelers' solder 8 3333 There are many different alloys used in the arts. The greenish alloy used by jewelers contains 70 per cent, of silver and 30 per cent, of gold. " Blue gold " is stated to contain 75 per cent, of iron. The Jap- anese employ a compound of gold and silver, the standard of which varies from'350 to 500. This alloy is exposed to the action of a mixture of plum-juice, vinegar, and sulphate of copper. They also possess a number of bronzes, in which tin and zinc are re- placed by gold and silver. The alloy known as shi- ya-ku-do, extensively used by the Japanese for sword ornaments, contains 70 per cent, of copper and 30 per cent, of gold. Alloys of Gold and Silver.—The density of those natural alloys, the composition of which varies from AuAg6 to Au6Ag, is greater than that calculated from the densities of the constituent metals. Gold and silver unite in all proportions, affording alloys of several tints, ranging from the color of silver to that of gold. By the addition of silver the hardness of gold is increased, and it is rendered more fusible, while its malleability is not materially diminished. GOLD. 143 Gold as found in nature always contains silver, and all specimens of native silver will likewise be found to contain gold. Gold and Platinum.—Equal weights of the two metals yield an alloy of good malleability, with, how- ever, some dullness of color. An excess of platinum renders the alloy infusible in an ordinary blast-fur- nace. One part of platinum to 9-5 of gold will afford an alloy of the same density as platinum. Gold and Tin.—Alloys of tin and gold are hard and brittle, and the combination is attended with contraction. Thus, the alloy SnAu has a density 14-243, instead of 14-828, as indicated by calculation. Gold and Mercury.—-These combine at all tempera- tures, but the union may be greatly facilitated by heating, and a state of fine division still further assists the process. It is stated that an amalgam composed of six parts of mercury to one of gold crystallizes in four-sided prisms, and that, if the mercury is then dis- tilled off, the gold is left in an arborescent state. The operation of coating the surface of brass or copper objects with gold, extensively practiced some years ago, and known as "fire-gilding," was based upon the amalgamation of gold and mercury. The process is as follows: The article to be gilded is given a uni- form coating of an amalgam made by heating six parts of mercury with one part of gold, with the surplus mercury removed by squeezing. The operation is greatly facilitated by first rubbing the surface with mercurous nitrate. It is thus given a superficial layer of mercury. It is now gently heated over some burn- ing charcoal, the amalgam in the meantime being kept uniformly distributed over the surface by means 144 DENTAL METALLURGY. of a soft brush. As the heat is continued and the mercury is gradually driven off, the surface assumes a dull yellow color. It is then ready for polishing, which is accomplished by means of a wheel-brush moistened with vinegar. Verdigris mixed with bees- wax is applied, for the purpose of removing any re- maining mercury by means of the affinity which the latter has for the acetate of copper. This operation, sometimes called " water-gilding," is so dangerous to health, in consequence of the liability of the operator to inhale the volatized mercury, that it has been almost entirely superseded by electro-gilding. Gold and Copper yield a class of alloys of a reddish color (between gold and copper), which are much harder than either of their constituents. The mal- leability of the gold is not, however, much affected by admixture of copper, provided the latter is pure. It is stated that seven parts of gold with one of cop- per exhibits the greatest degree of hardness which it is possible to obtain by union of these two metals. In the American coinage the alloy is chiefly copper; hence the coins are red in color and very hard. Gold and Palladium.—These metals are stated to alloy in all proportions. Chenevix states that an alloy composed of equal parts of the two metals is gray in color, less ductile than its constituents, and has the specific gravity of 1108. W. Chandler Eob- erts, assayer of the Eoyal Mint, London, states that an alloy of four parts of gold and one of palladium is white, hard, and ductile. According to Makins, the merest trace of palladium with gold will render the latter very brittle. Graham has shown that a wire of palladium alloyed with from twenty-four to GOLD. 145 twenty-five parts of gold does not exhibit the re- markable retraction which, in pure palladium, attends its loss of occluded hydrogen. Gold and Zinc.—It appears that these two metals possess a strong affinity for each other, but all the alloys of zinc and gold are more or less brittle, accord- ing to the quantity of zinc present. Care should, therefore, be observed in the dental laboratory, where so much zinc is employed, that small particles of it do not find their way into the gold filings. There are certain other metals which, when mixed with gold in quantities as small as the y-gVg- part of the mass, render it quite brittle and unworkable. These are bismuth, lead, antimony, and arsenic. Compounds of Gold.—Two compounds of gold with oxygen have been obtained,—Au20 and Au203,—but neither of them is of any great practical importance. The chlorides of gold correspond in composition to the oxides. Auric chloride, or trichloride, as it is more com- monly called, is prepared by dissolving gold in nitro- hydrochlorie acid. The excess of acid is driven off by evaporation at a temperature not greater than 280° F. (=138° C.); otherwise a part of it at least will be converted into aurous chloride. The crystals obtained by this process are ruby-red in color, and very deliquescent. The composition of auric chlo- ride is AuCl8; atomic weight, 303-1. Aurous chloride is obtained by heating the crys- tallized auric chloride in a porcelain evaporating- dish to about 347° F. (=175° C). If the tempera- ture is carried much beyond this point, say, to 392° F. (=r200° C), the compound will be decomposed 13* 146 DENTAL METALLURGY. into metallic gold and chlorine gas. Aurous chloride is yellowish in color and nearly insoluble in cold water. Boiling water, however, converts it into auric chloride and metallic gold. Its composition is AuCl; atomic weight, 232-1. Auric chloride is the most important of the com- pounds of gold, and is the source from which most of the preparations of gold used in the arts are ob- tained. There are iodides of gold resembling the chlo- rides in many respects. Berzelius also described an aurous sulphide. These are, however, not important. Purple of Cassius, named for the discoverer, M. Cassius, is employed by manufacturers of porcelain teeth in obtaining the gum color, and in the indus- trial arts for imparting a red color to glass and por- celain. It is a compound of gold, tin, and oxygen, which are believed to be grouped according to the formula Au2O.Sn02, SnO, Sn02+4H20* It may be prepared in the humid way by adding stannous chloride (SnCl2) to a mixture of stannic chloride (SnCl4) and trichloride of gold. Seven parts of gold are dissolved in aqua regia and mixed with two parts of tin, also dissolved in aqua regia. This solution is largely diluted with water, and a weak solution of one part of tin in hydrochloric acid is added, drop by drop, until a fine purple color is produced. The purple of Cassius, in a state of fine division, remains for a time suspended in the water, but finally sub- sides as a purple powder. The fresh precipitate dis- solves in ammonia, and exposure to light decomposes Bloxam's Chemistry, Organic and Inorganic. GOLD. 147 the purple solution, during which process its hue changes to blue, and it finally becomes colorless, and metallic gold is precipitated, the binoxide of tin being left in solution. The dry method* is the one now employed by manufacturers of porcelain teeth in the preparation of gum-enamel. Two hundred and forty grains of pure silver, twenty-four grains of pure gold, and seventeen and a half grains of pure tin are placed in a crucible, with sufficient borax to cover the mass, and melted. In order to insure a thorough mixture of the different metals, the melted mass should be poured from a height into a vessel of cold water, and this process of granulation should be repeated at least three times, but at every melting the alloy should be well covered with borax to prevent loss of the tin by oxidation. The vessel into which the melted mass is poured should not be a metallic one. The component parts of the alloy having now been thoroughly incorporated, the next step is to collect the granulated mass and separate from it any adherent particles of glass of borax. The metal is then put into a glass or porcelain evaporating-dish (the Berlin porcelain is the best), and sufficient chemically-pure nitric acid is added to cover the metal. The dish is now placed over a sand-bath, and gentle heat applied and continued until chemical action ceases. If at this point it is found that all the metallic particles are * The dry method of preparing purple of Cassius and the process of manufacturing the gum-enamel were imparted to the author by the jate Professor Wildman, to whom is due the credit of having brought the preparation of bodies and enamels to their present high state of excellence. 148 DENTAL METALLURGY. dissolved, the dish may be removed from the batb. Should any solid particles be found in the solution, a little more nitric acid must be added, and the opera- tion continued until all are dissolved. The silver having been entirely dissolved by the nitric acid, the solution should be poured off, and the remaining oxide carefully washed until the last trace of silver is removed. After several washings with a large quantity of pure warm water, the latter should finally be tested with a clear solution of common salt, and if it remains clear, without show of milki- ness, the silver is all removed. When the oxide is sufficiently washed, the purple of Cassius should be dried by gently heating, after which it is ready to be incorporated with the silicious materials. The process of making gum-enamel is divided into three stages: first, the preparation of the oxide ; sec- ond, fritting, or by the aid of heat uniting the me- tallic oxide with the silicious base ; and, third, diluting the frit so as to form the desired shade. The first we have already described. The frit is formed by mixing eight grains of the metallic oxide (purple of Cassius) with seven hundred grains of feldspar, and one hun- dred and seventy-five grains of a flux composed of Pure quartz.....4 ounces ; Glass of borax.....1 ounce ; Sal tartar .....1 ounce ; fused into a glass and ground fine. The oxide is placed in a smooth Wedgwood mortar and ground separately as fine as it is possible to get it. The flux is then added in small quantities, and the levigation continued, after which the feldspar may be added and treated similarly. It is of the highest importance GOLD. 149 that the mass be reduced to the utmost degree of fineness, and an expert workman will spend six or eight hours at least in levigating the quantity given in the formula. While the mass is being ground in the mortar foreign substances, such as small particles of wood, etc., must be carefully excluded ; otherwise, during the vitrifying process, these will be converted into carbon, which will be sure to reduce a portion of the gold in fine metallic globules distributed through- out the mass. The vitrifying or fritting process consists in pack- ing the mass, after the most thorough levigation, in the whitest sand crucible that can be obtained. (Dark- colored crucibles are liable to injure the frit by con- tamination with iron.) This must be provided with an accurately-fitting cover made of the same material, or a suitable top may be formed of a piece of slide such as is used in burning continuous-gum work. Before placing the frit in the crucible, the interior sur- face of ihe latter should receive a thin coating of very fine quartz, made into a paste with water, to prevent the frit from adhering to it during fusion. The frit in a dry state is then packed in, and the cover tightly luted to its place with kaolin. The crucible is then to be buried in a strong anthracite-coal fire, and to remain there until the contents are fused. The time required to do this will depend upon the size of the crucible and the intensity of the heat. Any ordinary coal-stove provided with a good draught will answer; but the fuel must be packed around and over the crucible, and the heat carried to the highest attainable point. Usually about two hours will be required to thoroughly fuse the mass, after which it is removed from the fire and permitted to cool. 150 DENTAL METALLURGY. The vitrified mass is removed from the crucible by breaking the latter. Every particle of adhering quartz or portions of the crucible should be cleared from the surface. It is then pulverized to a fineness which will allow it to pass through a No. 10 bolting- cloth sieve, and is ready for the third stage in the preparation of gum-enamel, which consists of dilut- ing the frit with the proper amount of feldspar. As the strength of the coloring-pigment varies accord- ing to the degree of fineness attained during the levigation, it is usually necessary to make several tests in order to arrive at the desired shade. This is accomplished by mixing separately several differ- ent lots in the fo Gum frit Feldspar Gum frit Feldspar Gum frit Feldspar [lowing p rope >rtions: 1 part; 2 parts ; 1 part; 3 parts ; 1 part; 4 parts. These are applied to marked pieces of porcelain body and fused in the usual way, the result deter- mining the proportions necessary to produce the de- sired shade. There is a compound known as " silicate of gold," used in ceramic dentistry to impart a life-like yellow tint to porcelain teeth. This is prepared by grind- ing together in a Wedgwood mortar one hundred and twenty grains of coarse feldspar, ten grains of gold foil, and eight grains of flux.* These are *White bottle-glass, which does not contain lead or iron, may be used to reduce the fusing-point of enamels, but, owing to the uncer- tainty of the composition of glass, most of the manufacturers of porcelain teeth make a fine glass, for this purpose, of the following GOLD. 151 ground until the gold is entirely cut up, when the mass is made into a ball and placed on a slide and fused, after which it is again ground fine and is then ready for use. Hyposulphite of gold and soda, the sel d'or of the photographers, is a double salt formed by adding a solution of one part of trichloride of gold to a solu- tion of three parts of hyposulphite of soda. Alcohol, in which the double salt is soluble, is then added. The formation of this compound may be explained by the equation, 8(N02S203)-f-2AuCl3=Au2S203, 3 (Na2S203)+6NaCl+2(Na2S406). Fulminating Gold.—When ammonia is added to trichloride of gold, a buff-colored precipitate results; which explodes violently when gently heated. Its exact composition is not well established. Discrimination of Gold.—Protochloride of tin is a characteristic test for gold, affording a purple-brown precipitate. The smallest portion of gold dissolved in a large quantity of water may be detected by the addition of a few drops of this reagent. Thus, a pale brown precipitate may be obtained in a pint of water containing but one-fiftieth of a grain of gold. Ferrous sulphate is also a delicate test for the presence of gold, and will detect the merest trace of it. By this reagent the gold is thrown down in the form of a brown powder, which, after washing, proportions : Pure quartz, four ounces ; glass of borax, one ounce ; sal tartar, one ounce. These are first ground separately, then thor- oughly mixed, and placed in a white crucible provided with a cover, which must be tightly luted, and then thoroughly fused in the fire. If perfectly pure materials are used the result will be an exceedingly brilliant, colorless, and transparent glass. 152 DENTAL METALLURGY. drying, and heating to redness, yields the metal in a finely-divided state. Sulphureted hydrogen (H2S) added to a solution of trichloride of gold affords a brown precipitate of auric sulphide. Nitrate of mercury also precipitates from solution a brown powder, which after heating yields finely-divided gold. Finely-divided gold, sus- pended in water, imparts a violet or red color to it. Colored fluids containing minute particles of gold in a state of suspension may be obtained by the action of phosphorus dissolved in ether upon a very weak solution of gold- in aqua regia. After standing for a long time the fine particles of gold are deposited, having the same tint as that which they previously exhibited when suspended in the liquid. The blue particles, being less minute, are soonest deposited, but the red particles require many months to settle down. The one-hundredth of a grain of gold is capable of imparting a deep rose-color to a cubic inch of fluid, and the different colors thus produced are taken advantage of in painting upon porcelain, a beautiful ruby-red color being the result of the pigment thus obtained. Assays of Gold Ores, Quartz, etc.—The specimens of ores should first be heated to redness, and then thrown into cold water to facilitate powdering, which may be accomplished in an ordinary Wedgwood mortar. Several lots of three hundred grains each should be weighed and examined separately, and the assays made from these averaged for the result. To the separate portions of powdered ore equal weights of litharge, half their weights of sodic carbonate, and about half of powdered charcoal, are added and thor- GOLD. 153 oughly mixed with the ore. Each portion is then placed in a crucible, a little borax sprinkled over the top, and it is ready for heating in a suitable blast- or wind-furnace. The heat at first should be gradual, so that the active effervescence caused by the escape of carbonic acid from the soda salt may not force por- tions of the mixture from the crucible. After a short time, however, the danger from this cause having passed, the heat may be carried to bright redness, or until the whole has fused. An ingot-mold with two apertures has been recommended for the reception of the fused mixture, which at this point is ready for pouring, the slag being turned into one concavity and the reduced metal into the other. The button will be found to consist of lead and gold, the former re- duced from the litharge and the latter from the aurif- erous ore. These are to be separated by cupellation. If the ore contains much iron pyrites, or is of the nature of " sweep " (the name given to residues which accumulate in the dental laboratory and other places where gold is worked), it will be necessary to roast it in a shallow fire-clay dish placed in a muffle; and, in the case of pyrites containing about seven penny- weights to the ton, the operation should be conducted with one thousand grains. The roasted ore is then fused with a mixture consisting of red lead, one thou- sand grains; sodic carbonate, six hundred grains; powdered charcoal, forty grains, and borax, five hun- dred grains. The mixture is introduced into a clay crucible, which it should half fill, and is fused in an air-furnace. The button of reduced lead may be removed either by pouring the contents of the cru- cible into a mold, or by breaking the crucible when cold. 14 154 DENTAL METALLURGY. Assay by Scorification. — Scorification resembles cupellation,* but the oxide of lead produced in the operation, instead of sinking into a porous cup, is held in a flat saucer of fire-clay, and dissolves the earthy constituents of the ore, leaving the precious metal to pass into another portion of lead, which re- mains in the metallic state. About two hundred grains of the roasted ore are placed in the scorifier, and intimately mixed with five hundred grains of granulated and fifty grains of borax lead. The con- tents of the scorifier are fused in a muffle. Air is admitted to oxidize the greater portion of the lead. At the conclusion of the operation the litharge should be perfectly fluid and cover the molten lead. The slag may be freed from particles of precious metal by the addition, at the conclusion of the operation, of a small quantity of powdered anthracite, which reduces a portion of the litharge to metallic globules, which fall through the slag and unite with the lead button. The gold is then separated by cupellation, and the silver, with which it is almost always asso- ciated, by parting with nitric acid. Assaying.—This term refers to the quantitative estimation of one constituent of an alloy or mineral, and is accomplished by cupellation when the alloying metal is copper, and "parting" when the debasing metal consists of silver. Usually both operations are necessary. From five to sixteen grains of the gold are wrapped in sheet-lead, with pure silver equal to two and a half times the quantity of gold supposed to be present. The weight of lead employed where * See page 155. GOLD. 155 the assay is standard gold* is 8 to 1, and the ratio of the weight of lead to the weight of copper assumed to be present is 100-1. The assay is now to be treated by cupellation, a process which is thus briefly and clearly described by Mr. W. Crookes: " The gold alloy is fused with a quantity of lead and a little silver, if silver is already present. The resulting alloy, which is called the ' lead button,' is then submitted to fusion on a very porous support, made of bone-ash and called a ' cupel.' The fusion is effected in a current of air, which oxidizes the lead. The heat is sufficient to keep the oxide of lead fused. The porous cupel has the property of absorbing melted oxide of lead without taking up any of the metallic globules, exactly in the same way that blotting-paper will absorb water while it will not touch a globule of mercury. The heat being continued, and the current of airf always passing over the surface of the melted lead button, and the oxide of lead or litharge being sucked up by the cupel as fast as it is formed, the metallic globule rap- idly diminishes in size until at last all the lead has been got rid of. Now, if this were the only action, little good would have been gained, for we should have put lead into the gold alloy and taken it out again. But another action goes on while the lead is oxidizing in the current of air. Other metals, except * 22-carat, or coin. t The process of cupellation is generally performed in a furnace provided with a muffle for the reception of the cupels, and arranged so as to admit of a current of air over the fused button. The lead used in cupellation should be of absolute purity; otherwise, as lead is always liable to contain silver, the latter would necessarily combine with the assay and vitiate the accuracy of the result. 156 DENTAL METALLURGY. the silver and gold, also oxidize, and are carried by the melted litharge into the cupel. If the lead is, therefore, rightly proportioned to the standard of alloy, the resulting button will consist of only gold and silver, and these are separated by the operation of parting, which consists in boiling the alloy (after rolling it into a thin plate) in strong nitric acid, which dissolves the silver and leaves the gold as a coherent sponge."* As the accuracy of the result of an assay is liable to be influenced, either by retention of silver or copper, or by loss of gold by volatilization in the muffle, solution in the acid, or retention in the cupel, it is necessary to employ " check assays," made on pure gold, with which the alloy assay is weighed in com- parison; and, as will be seen, the weight of gold indicated by the balance is liable to be either greater or less than the quantity originally present in the alloy. The following formulaf will serve to show the correction to be applied: Let 1000 be the weight of alloy originally taken. p, the weight of the piece of gold finally obtained. x, the actual amount of gold in the alloy expressed in thou- sandths. a, the weight of gold (supposed to be absolutely pure) taken as a check, which approximately equals x. b, the loss or gain of weight experienced by a during the process of assay, expressed in thousandths. k, the variation of " check gold" from absolute purity expressed in thousandths. Then the actual amount of fine gold in the check-piece =a(l—j^s), and x, the corrected weight of the assay, will =p— lea,d destroyed the coining qualities of gold." Gold reduced to standard fineness by lead is light-yellow in color, and quite brittle. The contents of the dentist's gold-drawer are always liable to contamination by small pieces of lead, the latter being much used in the form of thin sheets in the making of patterns by which the gold or silver plate is cut. As the working qualities of the precious metals are seriously impaired by its presence, means should be instituted to insure its com- plete removal. This may be accomplished by cupel- lation, or by melting the gold or silver in a crucible, and adding nitrate of potassium when the point of complete fusion has been reached. Lead and platinum, like tin and platinum, appear to possess considerable affinity for each other, and an alloy of the two can be formed at a comparatively low temperature. An alloy of lead and platinum is very hard and brittle. With palladium also lead forms a very hard and brittle alloy. * See chapter on " Silver." 248 DENTAL METALLURGY. The most valuable alloys of lead are those which it forms with tin, antimony, and bismuth, constituting solders, pewter, type-metal, etc. For the discrimination of lead, the student is referred to Fownes's or other standard works on chemistry. CHAPTER XX. TIN. Atomic Weight, 118. Symbol, Sn (Stannum). rpHE metal tin has been known for probably three thousand years. It is found in all parts of the world, chiefly as oxide. In reducing the ore it is first powdered and roasted to free it of sulphur and arsenic. It is then exposed to a high temperature with charcoal, and the metal is thus liberated. Pure tin is white in color, and is perfectly soft and malleable. It has a density of 7-3, and its fusing- point is 458-6° F. (237° C). It is but slightly acted upon by air, but when heated much above its melt- ing-point it oxidizes freely, and is converted into a yellowish-white powder,—the well-known polishing- putty. The action of nitric acid upon tin is to con- vert it into a white hydrated dioxide. It is dissolved by hydrochloric acid, assisted by heat, and forms stannous chloride. Nitro-hydrochloric acid acts upon tin with much energy, converting it into stannic chloride. Alloys.—Tin is readily dissolved in mercury (see page 50). With silver it forms a malleable alloy, which is considerably harder than tin. The late.Dr. Bean used tin alloyed with a small percentage of sil- ver for lower sets, which he cast directly upon the teeth after the ordinary cheoplastic method. 22 (249) 250 DENTAL METALLURGY. Alloys of tin and silver, in which the former is slightly in excess, are much used as amalgam alloys. Tin 10, silver 8, gold 1, is also frequently employed in filling teeth; and tin 10, silver 8, gold 1, copper 1, has, according to Fletcher, been largely used as " gold and platinum " amalgam. It is stated that from 5 to 7 per cent, of copper has the property of replacing platinum in amalgams, conferring the quick-setting quality claimed for platinum.* Dr. G. F. Reese has formed an alloy for a base for artificial dentures, composed of 20 parts of tin, 1 of gold, and 2 of silver, f This is cast directly upon the teeth, the process being similar to the cheoplastic method. The alloys which have been used in the cheoplastic process are chiefly composed of tin, silver, bismuth, and, in some instances, cadmium and antimony. According to Makins, gold and tin form a malleable alloy,J and gold reduced to standard by pure tin re- tains its malleability. Tin and platinum in equal proportions afford a hard and quite brittle alloy, fusible at a comparatively low temperature. When it is remembered that the fusing- points of these metals almost represent extremes of temperature, it would seem that their union must be attended with difficulty, but, as has already been stated,|| it is probable that some affinity exists between * T. Fletcher. f "Alloys and Amalgams Chemically Considered," J. Morgan Howe, M. D. J A precipitated alloy of gold and tin, having the form of a black powder, may be formed by acting upon a concentrated solution of trichloride of gold with stannous chloride. || See chapter on " Alloys." TIN. 251 the two, as platinum is readily dissolved by and alloys with the fused tin. With palladium tin is said to form a brittle alloy. With lead tin forms the chief part in the alloys used for soft-soldering, and in the compounds known as pewter and Britannia-metal. Tin solders are com- posed of two parts of tin to one of lead. Pewter consists of four parts of tin to one of lead, while Britannia-metal is formed by the addition of small quantities of antimony and copper. Alloys of tin and lead are harder and tougher than either metal singly, and they are more fusible than the mean of their constituents. The addition of bismuth to such an alloy lowers the melting-point to a remarkable degree, and the fusing-point is still further reduced by the addition of cadmium. Thus, an alloy composed of 15 parts of bismuth, 8 of lead, 4 of tin, and 3 of cadmium, fuses at 145° F. == 63° C. The alloy known as " Wood's metal," occasionally employed by dentists in replacing teeth on vulcanite plates, is composed of 7 parts of bismuth, 6 of lead, and 1 of cadmium, and fuses at 180° F. = 82° C, a point much below the boiling-point of water. In replacing a broken tooth by means of Wood's metal the usual dove-tail is cut in the rubber plate with a fine saw, the tooth is fitted to its place, and the fusible alloy is packed in with a spatula heated in a spirit-lamp. Lead 75, tin 5, and antimony 20 parts, is the com- position of the best form of type-metal. With copper tin affords a number of very useful alloys. Bell-metal is formed of 78 parts of copper to 2 of tin. Gun-metal is formed of 90 per cent, of 252 DENTAL METALLURGY. copper to 10 per cent, of tin. Speculum-metal is formed of 6 parts of copper, 3 of tin, and 1 of arsenic. Babbitt-metal—tin 12 parts, antimony 3, copper 2 —is sometimes used in the dental laboratory for dies, and is thought by many to be superior to zinc for this purpose. Dr. L. P. Haskell recommends the formula, tin 72.72, copper 9.09, antimony 18.18. Bronze is an alloy of copper and tin, and some- times zinc. It is affected by changes of temperature in a manner precisely the reverse of that in which steel is affected, becoming soft and malleable when quickly cooled, and hard and brittle when allowed to cool slowly. The art of making bronze was prac- ticed before any knowledge of the working of iron existed, and it was used at a very early period in the manufacture of weapons. Commercial tin is liable to contain minute quan- tities of lead, iron, copper, arsenic, antimony, bis- muth, etc. Pure tin may be precipitated in crystals by the feeble galvanic current excited by immersing a plate of tin in a strong solution of stannous chlo- ride. Water is carefully poured on so as not to dis- turb the layer of tin solution. The pure metal will be deposited on the bar of tin at the point of junc- tion of the water and the metallic solution. Perfectly pure tin may also be obtained by dis- solving commercial tin in hydrochloric acid, by which it is converted into stannous chloride. After filtering, this solution is evaporated to a small bulk, and treated with nitric acid, which instantly converts the stan- nous chloride into stannic oxide. This is thoroughly washed and dried, and exposed to red heat in a cruci- tin. 253 ble with charcoal. A button of pure tin will be found at the bottom of the crucible. Pure tin in the form of foil is frequently used in filling teeth, for which purpose it doubtless ranks next to gold. Tin foil is also employed in connection with non-cohesive gold in filling approximal surfaces of cavities in bicuspids and molars. Two sheets of foil, one of gold and the other of tin, are placed together and made into mats or cylinders. These are carefully packed against the cervical margins of the cavity. The frequent failure of ordinary gold fillings at this point has led some practitioners to entertain the theory that between the tooth-substance and the gold there is galvanic action, to which the lime-salts of the tooth yield, and that by the com- bination of two metals, whether tin and gold or amalgam and gold, the galvanic action is confined to the metals, the tooth-substance being thus protected. The appearance of a filling formed of tin and gold would seernto confirm this theory, as it soon becomes dark in color, and presents a surface resembling amalgam, but it effectually protects the margins from decay.* Solvents.—Tin is readily dissolved by either of the three mineral acids. Sulphuric acid converts it into stannic sulphate. Tin dissolved in hydrochloric acid forms stannous chloride. By the action of dilute nitric acid tin is not dissolved, but is converted into stannic oxide, which settles to the bottom of the vessel as a white powder. This, when rendered anhydrous by heating to redness, affords the well- known polishing-powder called " polishing-putty." * Prof. James Truman, Report of Proceedings of Odontological Society of Pennsylvania, November, 1881. 22* 254 DENTAL METALLURGY. Chlorides.—There are two chlorides of tin,—stan- nous chloride or protochloride of tin (SnCl2), and stannic chloride or bichloride of tin (SnCl4). Stan- nous chloride is prepared by dissolving tin in hydro- chloric acid, the action being assisted by gentle heat. Stannic chloride is obtained by dissolving tin in nitro- hydrochloric acid (aqua regia). These two com- pounds of tin are employed in the preparation of purple of Cassius, in which process stannous chloride is added to a mixture of stannic chloride and trichlo- ride of gold (see page 146). For other compounds of tin, see works on chemistry. Discrimination.—Tin is detected before the blow- pipe by fusing the compound under examination on charcoal with sodium carbonate, when a bead of the metal is obtained. From a tin solution caustic pot- ash and soda precipitate a white hydrate, soluble in excess. Ammonia affords a similar precipitate, not soluble in excess. Hydrogen sulphide and ammonium sulphide throw down a dark-brown precipitate of monosulphide. Trichloride of gold added to a dilute solution of stannous chloride causes a purple precipi- tate (purple of Cassius). CHAPTER XXI. ELECTRO-METALLURGY. T^HE origin of electro-metallurgy was undoubtedly due to the early experiments of Wollaston and Davy, while the credit of its development belongs to the late Professor Daniell, who devised the particu- lar form of battery which bears his name. A Dan- iell cell consists of a copper vessel containing a saturated solution of sulphate of copper. In this is placed a porous cylinder containing dilute sulphuric acid. A rod of amalgamated zinc is immersed in the acid, and on the two metals being connected electrical action is immediately set up, and the zinc, which forms the positive element, is dissolved, with forma- tion of sulphate of zinc; the sulphate of copper is reduced, and metallic copper is deposited upon the sur- face of the copper vessel, which forms the negative element of the combination. It was observed that the copper thus deposited took the exact shape of the surface on which it was thrown, presenting a faithful counterpart of the slightest indentation or irregular- ity. De la Rue called attention to this fact in a paper published in 1836, but no practical application was made of it until 1839, when Professor Jacobi, of St. Petersburgh, published his discovery of a means of producing copies of engraved copper plates by the agency7 of electricity. (255) 256 DENTAL METALLURGY. In 1840 Mr. Murray announced that an electro- deposit of metal could be formed upon almost any material, provided its surface was rendered a conduc- tor of electricity by a thin coating of graphite (black- lead). Instead of copying the object in a metallic medium, it is only necessary to take a cast in plaster of Paris, wax, gutta-percha, or any convenient mate- rial, and then to coat the surface with finely-powdered graphite applied with a camel's-hair pencil. The Gramme machine, a modification of the mag- neto-electric apparatus, consists of a ring of soft iron carrying a number of coils of insulated copper wire, caused to rotate between the poles of a fixed horse-shoe magnet. The currents induced in the coils are collected by two metallic disks, whence they may be drawn off for use in electro-deposition. The core being circular, the magnetization proceeds con- tinuously, affording a uniform current. Both poles of the magnet are used, producing simultaneously two opposite continuous currents. These and similar sources of electricity enable the electro-metallurgist to deposit a metal upon a matrix or to coat one metal with another. The art of electro-metallurgy is divided into two branches, electrotypy and electro-plating. In the former the reduced metal is separated from the mold on which it is deposited, forming a distinct work of art, while in the latter the deposited metal forms an inseparable part of the plated object. Electrotypy is employed in producing copper duplicates of en- gravings on wood and of any kind of type-matter for printers' use. A cast of the object is first taken in wax or gutta-percha, and the surface of this mold is ELECTRO-METALLURGY. 257 brushed over with black-lead, and it is then, by means of a wire, suspended in a bath of sulphate of copper connected with a battery. Since the introduction of dynamic electricity, perfect copies may be produced in an hour or two, and the deposited metal made thick enough to stand any reasonable amount of wear. Electro-plating, by which silver is deposited on the surfaces of objects of copper, brass, or German- silver, was introduced soon after the discovery of the art of electro-metallurgy. The article to be plated must first be thoroughly cleansed by immersing it in a hot solution of caustic potash, and also by means of the " scratch-brush," and sometimes by "pickling " it in a bath of nitric and other acids. The surface is then given a thin film of mercury, by washing the article with a solution of mercuric nitrate. This is called "quicking." The article is then rinsed with water and transferred to the silver bath, which con- sists of a solution of cyanide of silver in cyanide of potassium—a very poisonous compound. Plates of silver are suspended from a rectangular frame con- nected with the positive pole, while the articles to be plated are suspended by wires attached to the nega- tive pole. The quantity of silver to be deposited depends upon the requirements of the case. One ounce of silver per square foot forms for ordinary purposes a sufficiently heavy coating. Electro-gilding is effected by means similar to those employed in electro-silvering. The solution employed is generally the double cyanide of gold and potassium, and it is used hot, the temperature ranging from 130° F. to 212° F., according to the ideas of the operator. 258 DENTAL METALLURGY. Nickel-plating was first introduced in 1869, by Dr. Isaac Adams, of Boston, who patented a process for depositing nickel from solutions of double salts of sulphate of nickel and ammonium. Iron may also be deposited from the double sul- phate of iron and ammonium. Practical application is now made of this discovery, and engraved copper plates are found to be much more durable when faced with electro-deposited iron. Plates for printing bank- notes are sometimes treated in this way. INDEX. Acid, nitric, action of, on silver.................................... 174 nitric, in the quartation process of refining gold........ 122 nitro-hydrochloric (aqua regia)............................... 128 sulphuric, action of, on silver................................. 167 sulphuric, in the quartation process of refining gold... 122 Acids, action of, on alloys............................................ 45 Alloying, influence of..............................................32, 39 the purpose of....................................................... 35 Alloys as definite compounds........................................ 36 color of............................................................... 38 composition of...................................................... 41 conductibilityof................................................... 26 decomposition of................................................... 42 density of............................................................ 37 for solders............................................................ 35 fusibility of......................................................... 40 influence of constituent metals in............................ 42 Matthiesen's definition of....................................... 37 of gold employed in dentistry as solders................... 138 of silver employed in dentistry as solders.................. 181 oxidizability of...............................,.................... 45 preparation of...................................................... 44 properties of........ ................................................ 35 specific gravity of................................................. 38 study of............................................................... 34 table of................................................................ 41 temper of............................................................ 44 Von Eckart's........................................................ 181 Alumina................................................................... 245 Aluminum................................................................ 238 alloys of............................................................... 239 bronze............................................................41, 239 " preparing, Cowles's method of....................... 240 " solders for................................................... 240 259 260 INDEX. Aluminum, casting, Bean's method of............................ 242 casting, Carroll's method of.................................... 242 solder for............................................................. 245 solvents of........................................................... 239 swaged plates of................................................... 244 Amalgams................................................................. 46 composition of some of the well-known.................68, 69 definition of....................................................... ... 46 discoloration of..................................................... 48 expansion of........................................................ 47 experiments with.................................................. 49 for dental purposes................................................ 46 formation of......................................................... 46 forming alloys for.............................................50, 59 formula for, Dr. Ambler Tees's............................... 70 " " Dr. L. Jack's...................................... 61 influence of different metals in................................ 50 introducing fillings of............................................ 57 mixing, methods of............................................. 56 palladium..........................................................61, 62 qualitative and quantitative examinations of............. 63 quantity of mercury to be used in........................... 55 shrinkage of.................................................... .... 47 Sullivan's............................................................ 62 Ammonium............................................................... 19 Argentiferous galena................................................... 170 Argentum (silver).............................................,........ 166 Arsenic in alloys......................................................... 43 Bloxam's classification of....................................... 18 odor of................................................................ 20 Assay of amalgam alloys............................................. 63 Assaying gold............................................................ 152 Atomic weights of metallic elements..........................14, 16 Auric chloride............................................................ 146 oxide.................................................................. 145 iodide.................................................................. 146 sulphide............................................................... 152 Aurous chloride......................................................... 145 oxide.................................................................. 146 INDEX. 261 Aurum (gold)............................................................ 113 Babbitt-metal............................................................ 252 Beating gold.............................................................. 162 Bellows for blow-pipe..........................................76, 77, 78 Bessemer steel............................................................ 207 Black-lead crucibles.................................................... 87 Blast-furnaces...........................................................82, 87 Blistered steel............................................................ 206 Blow-pipes................................................................. 73 hot-blast.............................................................. 73 Knapp's.............................................................. 79 oxyhydrogen...................................................188, 189 supports for use with............................................. 95 Bone-ash cupels........................................................... 172 Borax........................................................................ 92 Brass........................................................................ 231 Britannia-metal.......................................................... 251 Brittle gold, treatment of............................................. 126 Bromides, metallic...................................................... 101 Bronze...................................................................... 252 Cadmium.................................................................. 236 precipitation of..................................................64, 237 Calamine.................................................................... 228 Calomel..................................................................... 216 Carbon, proportion of, in cast-iron and steel.................... 206 Case-hardening........................................................... 209 Cast-iron................................................................... 206 Cast-steel..............................................................206, 207 Charcoal as a reducing agent......................................... 108 Chlorides, metallic...................................................... 100 preparation of...................................................... 101 reduction of......................................................... 108 Cinnabar................................................................... 216 Coke for supports in soldering....................................... 96 Color of metals........................................................... 19 influence of alloying on......................................... 38 23 262 INDEX. Copper...................................................................... 221 alloys of............................................................... 223 amalgam............................................................ 223 as a constituent in amalgams.................................. 224 discrimination of................................................... 226 properties of......................................................... 222 solvents of............................................................ 223 Corundum................................................................. 241 Crucibles................................................................... 86 Crystallization.,.......................................................... 31 Cupellation..........................................................173, 187 Cupels...................................................................... 172 Cyanides, metallic....................................................... 100 Ductility of metals....................................................27, 28 influence of alloying on......................................... 39 Electro-deposit of metals............................................. 257 Electrolysis................................................................ 112 Electro-metallurgy...................................................... 255 Electrotypy, origin of.................................................. 255 Elements, metallic...................................................... 13 Emery...........................................,........................... 245 Flame, blow-pipe, management of................................. 73 Fluorides, metallic...................................................... 102 Flux....................................................................148, 150 Forging platinum....................................................... 190 Fulminating gold....................................................... 151 silver.................................................................. 176 Furnace, lime, for melting platinum.............................. 186 Cowles's electrical............................................106, 240 Furnaces.................................................................... 82 Fusible metal................................«............................ 246 Fusing-points of metals................................................ 20 Galena, argentiferous................................................... 170 Gauge-plate............................................................... 90 Gold......................................................................... 113 INDEX. 263 Gold, alloys of............................................................ 134 chlorides of.......................................................... 145 clasps for partial artificial dentures.......................... 137 coin.................................................................... 134 compounds of....................................................... 145 containing iridium................................................ 127 discrimination of................................................... 151 foil, cohesive................#........................................ 158 foil, non-cohesive.................................................. 159 in combination with tin in filling teeth..................... 253 Lamm's shredded.................................................. 131 oxides of.............................................................. 145 precipitants for..................................................... 131 precipitated, different forms of................................. 130 precipitation of, by ferrous sulphate......................... 132 precipitation of, by oxalic acid............................... 130 precipitation of, by sulphurous acid.......................... 132 properties, occurrence, and distribution of................. 114 pure, preparation of........................................ ...... 128 quartation process of refining.................................. 121 reducing, to a higher or lower carat.......................... 140 refining............................................................... 121 volatilizing......................................................33, 117 swaging, when alloyed with platinum..................... 137 Watts's crystal..................................................... 133 welding properties of............................................. 116 Gum frit................................................................... 148 Gun-metal......... ....................................................... 251 Hot-blast blow-pipes.......................,........................... 73 Hydrogenium............................................................ 18 Ingot-molds............................................................... 81 Iodides, metallic......................................................... 102 Iridium...............................................................192, 196 Iron.......................................................................... 203 compounds of.........................-............................. 205 fusing-point of...................................................... 204 meteoric............................................................... 203 264 INDEX. Iron, native............................................................... 203 properties of......................................................... 204 solvents of............................................................ 204 Kaolin...................................................................... 245 Lamps, soldering......................................................... 71 Lead.......................................,................................. 246 alloys of.............................................................. 246 amalgamation of...............................................246, 247 desilvering of...................................................171, 247 discrimination of................................................... 248 properties of......................................................... 247 reduction of......................................................... 246 Magnetic iron............................................................. 205 Malleability............................................................27, 89 influence of alloying on......................................... 39 Mercury.................................................................... 211 adulterations of...........................................,......... 212 chlorides.............................................................. 216 compounds........................................................... 216 discrimination of................................................... 219 filtration of.......................................................... 214 native.................................................................. 211 occurrence of...................................................211, 212 ores, reduction of................................................... 211 oxides of.............................................................. 216 properties of......................................................... 214 pure, to obtain...................................................... 213 purification of, by digestion.................................... 212 purifying, Priestley's plan of.................................. 214 quantity of, to be used in amalgams.......................... 55 redistillation of..................................................... 212 sources of....................................,........................ 211 sulphides of.....................................................216, 217 Metallurgy, definition of............................................. 9 Metallic bromides....................................................... 101 INDEX. 265 Metallic chlorides........................................................ 100 cyanides............................................................... 100 fluorides............................................................... 102 iodides................................................................. 102 oxides.................................................................. 102 sulphides............................................................. 105 Metals, base............................................................... 16 capacity for heat of................................................ 22 color of............................................................... 19 conductidn of electricity of.................................... 25 conduction of heat of............................................. 25 effects of alloying on............................................. 35 elasticity and sonorousness of................................... 32 expansion of, by heat............................................. 23 fusibility of........................ ................................ 20 fusing-points, table of........................................... 21 malleability, ductility, and tenacity of...................... 27 noble.................................................................. 16 odorand taste of.................................................. 20 properties of........................................................ 17 table of............................................................... 14 volatility of.......................................................32, 33 volatility of, agents which induce............................ 33 Nickel-plating................. ......................................... 258 Oxides, metallic.......................................................... 102 Oxides, reduction of.................................................... 110 Palladium................................................................. 199 alloys of............................................................. 200 amalgams...................................................61, 62, 201 cost of................................................................. 202 discrimination of................................................... 202 influence of, in amalgams....................................... 61 properties of........................................................ 200 sources of............................................................ 199 Pewter....................................................................... 251 Phosphor-bronze.....................................................43, 225 23* 266 INDEX. Phosphor-iridium...................................................197, 198 Platinum.................................................................. 184 alloys of.............................................................. 191 amalgamation of.................................................. 192 associate metals with............................................. 184 chlorides of.......................................................... 194 melting, Deville's furnace for.................................. 187 preparing, Wollaston's method of........................... 184 properties of........................................................ 189 pure.................................................................... 189 solvents of.......................................................... 191 welding............................................................... 186 Polishing-putty.......................................................... 253 Purple of Cassius....................................................... 146 Quartation process of refining gold................................ 121 Reduction of metals.................................................... 106 Refining gold............................................................. 121 Reinsch's test.......................................................112, 219 Rolling mills.............................................................. 89 Ruby........................................................................ 245 Sapphire................................................................... 245 Scorification............................................................... 154 Silicate of gold........................................................... 150 Silver........................................................................ 166 alloys of.............................................................. 179 chloride of........................................................... 175 compounds......................................................174, 175 cupellation of....................................................... 173 deposition of, by battery........................................ 183 discrimination of.................................................. 175 estimation of........................................................ 176 native................................................................ 168 nitrate of............................................................ 174 oxide of............................................................... 174 precipitation of, by copper...........................,......... 179 precipitation of, by iron......................................... 178 INDEX. 267 Silver, precipitation of, by sodium chloride..................... 178 precipitation of, by zinc........................................ 178 properties of......................................................... 166 pure................................................................... 178 separation of, from ores.......................................... 169 solders for.......................................................181, 182 solvents of......................................................167, 178 spitting of, during fusion....................................... 167 sulphate of........................................................... 175 sulphide of........................................................... 175 Solders for aluminum.................................................. 244 gold..............................................................138, 139 silver.............................................................181, 182 soft..................................................................... 251 Specific gravity of alloys............................................. 38 Speculum-metal.......................................................... 252 Stannic chloride.......................................................... 249 Stannous chloride....................................................... 254 Stannum (tin)............................................................ 249 Steel......................................................................... 205 Bessemer............................................................ 207 blistered.............................................................. 206 cast....................................„................................ 206 discrimination of................................................... 210 hardening......................................................207, 208 shear.................................................... .............. 206 tempering.......................................................207, 208 Sulphides, metallic...................................................... 105 reduction of......................................................... 109 Supports for use in melting and soldering....................... 95 Tellurium.................................................................. 17 Tenacity...............................................................28, 39 influence of alloying on......................................... 39 Tin, alloys of.............................................................. 249 amalgamation of................................................... 249 chlorides of.......................................................... 254 discrimination of................................................... 254 268 INDEX. Tin, estimation of........................................................ 63 foil..................................................................... 253 in amalgams......................................................... 50 oxide of................................................................ 63 pure, preparation of............................................... 249 refining...........................................................252, 253 solvents of............................................................ 253 Titanium................................................................... 19 Type-metal................................................................ 251 Vermilion.................................................................. 217 Von Eckart's alloy...................................................... 181 Welding properties of platinum.................................... 190 Wire-drawing............................................................ 90 Wood's metal............................................................. 251 Zinc.......................................................................... 228 alloys in dentistry................................................. 229 alloys of............................................................... 229 counter-dies.....................................................231, 232 dies for swaging plates........................................... 230 discrimination of................................................... 235 expansibility of..................................................... 23 in amalgams...............................................60, 61, 229 oxychloride of..................................................... 234 properties of....................................................228, 229 vn 3noio3w jo Aavaan tvnouvn snqiqsw jo Aavaan tvnouvn IE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE LVN 3NIDI03W JO AavaBIT IVNOIIVN 3NIDI03W JO Aavaan TVNOUVN INE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE ivn 3Nioia3w jo Aavaan tvnouvn snoiosw jo Aavaan ivnoiivn 0 :iNE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE iiivn 3nidio3w jo Aava8ii ivnouvn 3NOia3w jo Aavaan TVNOUVN CINE NATIONAL LIBRARY OF MED' /aan tvnouvn oigjvv jo Aavaan ivnoiivn hiw jo Aavaan v. vaan tvnouvn 3NI3I03W jo Aavaan tvnouvn Jnidioiw jo Aavaan im V (Y OF MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF M aan tvnouvn snidichw jo Aavaan tvnouvn 3noiQ3w jo Aavaan r OF MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY Of M svsan ivnoiivn ;3w jo Aavaan tvnouvn jnoiojw jo Aavaan s,RY OF MEDICIN[ NATIONAL LIBRARY Or ML'.: .-.T'O'J.M LIBP Ar'Y Of t V~~ VZ^ NLM052297970