The American Cyclopædia (1879)/Alloy

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1540166The American Cyclopædia — Alloy

ALLOY (Fr. aloi, standard of coin, from à la loi, according to law), a compound of two or more metals fused together. When one of the metals is mercury, the compound is called an amalgam. (See Amalgam.) By the alchemists metals were called “noble” and “base,” and when one of the latter was brought into combination with one of the former, the nobility of this was said to be “allayed” or “alloyed,” and assayers at the present day still use the term in this sense. Most alloys are mixtures of no exact proportions; the metals dissolve in one another indefinitely, as sulphuric acid unites with water. Some, however, appear to be combinations in equivalent proportions, and of these there are found examples in nature, as of the native gold, which occurs combined with silver—4, 5, 6, or 12 atoms of gold to one of silver, but never a fractional part of an atom of gold. The tendency of some alloys to take crystalline forms also indicates definite combinations. This is verified by cooling a melted mixture slowly, and when partially solidified pouring off the liquid remnant, when crystals are left which are always combinations in the proportion of the atomic weights of the metals; for instance, in the mixtures of copper and tin (bronze), copper and zinc (brass), copper and nickel (German silver), or copper and aluminum (aluminum bronze), the proportions of the crystals are found to be either in the ratio of the numbers 64, 118, 65, 59, 27, which are the respective atomic weights of copper, tin, zinc, nickel, and aluminum, or of a multiple the one of the other. The metals of many alloys are with difficulty brought into combination, and even tend to separate from each other while in the melted state, and in some instances form layers which contain different proportions of the metals.—The changes in the physical properties of metals effected by their combinations are of great variety, and cannot before experiment be at all anticipated. Even slight variations in the proportions of the metals involve great changes in the product of their union. The specific gravity of the alloy may be greater or less than the mean of that of the component parts. In the alloy of gold and tin it is greater; also of silver with zinc, lead, tin, bismuth, or antimony; copper with zinc; lead with palladium; bismuth with antimony; lead with bismuth; and zinc with antimony. The specific gravity is less in the alloy of gold with silver, lead, iron, copper, nickel, or iridium; also of iron with bismuth, zinc, antimony, or lead; tin with lead; zinc with palladium or antimony; and zinc with antimony. The alloy of silver and copper as used in coins is also of less specific gravity when cast; but Karnmarsch found that by rolling and coining it is so far compressed that the specific gravity is the same as the mean obtained by calculation. Alloys are always more fusible than the metal most difficult to melt that enters into their combination, and generally more so than the most easily melted one. The fusible metal discovered by Sir Isaac Newton melts at different temperatures between 198° and 210° F. It is composed of bismuth 5 or 8 parts, lead 2 or 5 parts, and tin 3 parts. These metals melt, the first at a temperature of 476°, the second at about 600°, and the last at 442°. The addition of one part of mercury lowers the melting point of this alloy to 167°. Wood's fusible alloy, discovered in more recent times, consists of 2 parts cadmium, 2 tin, 1 lead, and 3 bismuth; it melts at the low temperature of 150° F. The alloy fusible at the lowest temperature is that of sodium and potassium; the first melts at 194°, the second at 128°, while the alloy melts at 80°, and is thus liquid at the common summer temperature. Alloys conduct heat and electricity less perfectly than their pure metals; they are also generally more brittle. But their power of cohesion is usually greater than that of either of the metals, the alloy resisting more strongly the force applied to draw a bar apart than does a bar of either one of the metals composing it. The color which the alloy will take is as uncertain as any of its other properties. A large addition of zinc will not make its alloy with copper whiter, but will give it the rich pinchbeck hue. Tin makes copper more pale, but especially nickel, the addition of one eighth of which is sufficient to make it almost white. Aluminum acts in a similar way, while silver possesses the power of destroying the red color of the copper in so high a degree, that it may be largely alloyed with it without materially impairing its whiteness. Alloys composed of metals of different degrees of fusibility may sometimes be separated into their distinct metals by heating to the melting temperature of one of them. An alloy of tin and copper may be thus treated, the tin melting at 442°, and the copper at 1,996°. This “sweating process,” called liquation, is used to separate silver from copper. Lead is first melted in with the other metals, and when sweated out it takes the silver along with it. This alloy is then separated by another process, depending on the easy oxidation of the lead. An interesting property of the metals, which may seem somewhat opposed to the one just described, is the tendency of one, when melting, however fusible it may be, to cause any other in contact with it, however infusible, to dissolve in the melted metal; its surfaces are washed away, till nothing solid is left. Platinum, which is among the most difficult metals to melt, is very susceptible of injury from this cause. The costly crucible and other vessels of the chemist may be ruined in an unguarded moment by contact with other metals highly heated. On this property is based the principle of soldering two pieces of metal by means of a third. Their surfaces are fixed together by interposing an alloy which is more fusible than either of the metals to be joined; and this must also consist of metals which are disposed to unite and form a new alloy with them. Pieces of gold are soldered together with an alloy of gold with silver or with copper; articles of silver with an alloy of silver and copper; of copper, with an alloy called hard solder, which is brass containing a large proportion of zinc. Another interesting property of alloys is the different effects produced by the order in which their component parts have been mixed, the proportions continuing the same. Ten parts of antimony added to 90 of tin and 10 of copper, make a compound of very different physical properties from that produced by adding 90 parts of tin to 10 of copper and 10 of antimony. This appears to be analogous to what we witness in vegetable chemistry, as in the identity of composition in starch and sugar.—The alloys already in use are very numerous, and new valuable combinations are continually discovered. Those alone of copper with zinc form a long list, in which we find the names of many very useful compounds, some of them known from the time of Tubal Cain. Pewter has long been a useful, though a very homely alloy. It is made of different combinations of lead and tin, sometimes with additions of antimony, bismuth, and copper, and in this case is known in trade under different fanciful names, as britannia, &c. German silver, composed of copper, nickel, and usually zinc, has in part displaced it, and is likely to be itself displaced by some improved combinations. Muntz's yellow metal is an alloy of 60 parts of copper to 40 of zinc. These proportions may be slightly varied, but they are the ones specially recommended in the patent, as producing a composition more easily rolled into sheets while hot. It is used for sheathing the bottoms of ships. In importance, no alloys can rank higher than those of which printers' types are made, and no known metal possesses the properties essential to them. They consist of lead and antimony, in proportions varying with the kind of types. For very fine types tin is added, to increase the fusibility and consequently to make the metal flow better, so as to fill the finest details of the mould. Many type founders introduce also some copper, by first alloying it with the antimony; this increases the durability of the type considerably. The noble metals, gold and silver, are too soft to be used in a pure state. They are alloyed with copper to give them hardness, and gold also with silver. The standard silver of Great Britain consists of silver 11.10, and copper 0.90. The French silver plate contains 9.5 parts of silver and 0.5 copper; trinkets, 8 parts of silver to 2 of copper. In the United States these alloys are made as rich or as poor as the individual manufacturer judges best for his interest. His reputation is the only guarantee that his work is what it is sold for. There is no test but actual analysis, and this is not applicable to the articles without destroying part of them. Specific gravity may be employed to some extent, but as the alloy often has a somewhat different density from that of the mean of its metals, the calculation gives an approximation more or less correct according to circumstances. The following rule given by Dr. Van der Weyde may be used to find the specific gravity of an alloy made of any number of metals, mixed in whatever proportion: “Find the relative volume of each metal by dividing its weight by its specific gravity; the sum of all the weights divided by the sum of the volumes gives the specific gravity of the alloy.”—An alloy which closely resembles gold in color, specific gravity, and ductility, is made of 16 parts of platinum, 7 parts of copper, and 1 of zinc. These are put into a crucible, covered with charcoal powder, and melted. Its cost is scarcely one fourth of that of gold. The so-called oroide gold is a very base alloy, only resembling gold in color if kept clean, and is easily distinguished from it by having scarcely half its specific gravity. It is said to be made by melting copper 100 parts, tin 17, magnesia 6, carbonate of potash 9 or salt of antimony 3.6, and quicklime 1.6. The latest improvement is the so-called sterrometal, invented by Rosthorn in Vienna; it is made by melting 600 lbs. copper with 2 lbs. cast iron; when fluid there is added 36 lbs. zinc and 4 oz. borax. It is asserted that it is 8,000 lbs. stronger per square inch than the best wrought iron. Alexander Birchholz of Hartford, Conn., has patented the same alloy, and erected a factory in Providence, R. I.—Among interesting applications of alloys we must mention the plates of easily fusible metals with which steam boilers are sometimes provided, offering an additional safety besides the safety valve, as they will melt at a temperature corresponding with too high a pressure. Another application is founded on the fact that alloys are much more imperfect conductors of electricity than the separate metals, from offering more resistance. Small coils of wire are made of an alloy similar to German silver, in which the resistance is equal to many miles of telegraph wire; they are used in connection with voltameters to measure the strength of batteries, and to detect imperfections or breaks in telegraph wires.