Alloy. Compounds or mixtures which different metals form with one another are called alloys. There is an exception, however, in the case of mercury. When it is mixed with another metal, the compound is termed an Amalgam (q.v.). All alloys retain the essential properties of metals. They possess metallic lustre, and conduct heat and electricity well. On the other hand, when the metals form compounds with non-metallic elements, such as sulphur or chlorine, their general properties are quite changed. Alloys have been divided into three groups: (1) Those formed by the metals lead, tin, zinc, and cadmium, which impart to their alloys their own physical properties in the proportions in which they themselves are contained in the alloy. (2) Those formed by almost all other metals. Such alloys as belong to this group do not get imparted to them the physical properties of their constituent metals in the proportion in which they are present. (3) Those which contain metals found in both these groups of alloys.
In an alloy the Specific Heat (q.v.) and the coefficient of expansion are always the means of those of its component metals. But in other physical properties a variation takes place. This is the case with specific gravity, which, in alloys of the first group, is the mean of their constituent metals; but in those of the second group, it is always greater or less than the mean specific gravity of their constituents. The increase in density indicates that the metals have contracted; in other words, that the metallic molecules have approached each other more closely; whilst the decrease in density denotes a separation of the molecules to greater distances from each other.
Again, in alloys of the first group, the conducting power for electricity is exactly proportional to the relative volumes of the component metals; while in alloys of the second group the case is different.
If lead, tin, zinc, or cadmium be mixed with any of the metals from which alloys in the second group are formed, this alloy has its coefficient of elasticity much increased. For example, coils of copper or silver wire are made straight by weights, by which a coil of brass or gun-metal wire will scarcely be altered in shape.
In some instances, when two melted metals are mixed together to form an alloy, an evolution of heat occurs, which is believed to indicate that a chemical compound has been formed. This is the case with copper and zinc, copper and aluminium, platinum and tin, &c. Many alloys, however, can be obtained in well-defined crystalline forms, which is usually considered a test of a definite chemical compound; yet a number of these—copper-zinc alloys, for example—crystallise in the same form, even when the proportions of the component metals vary considerably.
A curious fact may be mentioned in regard to the solubility of alloys. Platinum by itself is quite insoluble in nitric acid, but if it be alloyed with silver the compound is completely dissolved. Silver, on the other hand, readily dissolves in nitric acid, but it will not do so when mixed with a large quantity of gold.
The strength or cohesion of an alloy is generally greater than that of the mean cohesion of the metals contained therein, or even of that of the most cohesive of its constituents. Thus, the breaking weight of a bar of copper or tin is very much lower than the breaking weight of a bar of the same size composed of certain alloys of tin and copper.
The most useful alloy in the arts is brass. This compound metal is next to iron in importance. Several kinds are made varying in composition from equal parts of copper and zinc, to five parts of copper with one of zinc. According to the proportions of these metals in the alloy it is called sheet-brass, Pinchbeck Brass (q.v.), Dutch brass or Dutch Metal (q.v.), ordinary yellow brass, Muntz's metal or ship-sheathing brass, and by several other names (see BRASS).
There are some important alloys of copper and tin. Among them Bronze (q.v.), Gun-metal (see CANNON, p. 714), Bell-metal (See BELL), and Speculum Metal (q.v.). In these the proportions vary from equal parts of copper and tin, to ten parts of copper with one of tin. The most cohesive, that is, the strongest, of them is a bronze consisting of six parts of copper to one of tin. Phosphor-bronze is an invention of recent years. The addition of from 0.25 to 2.5 per cent. of phosphorus to a bronze containing from 7 to 8 per cent. of tin, gives it greater hardness, elasticity, and toughness. This alloy is now much used for parts of machinery.
German Silver (q.v.) is an alloy composed, in its best quality, of two parts of zinc, four of copper, and one of nickel. Britannia Metal (q.v.) generally consists of about ninety-two parts of tin, eight of antimony, and two of copper. This is a softer metal than German silver, but both are largely manufactured into such objects as teapots, jugs, spoons, and the like, many of them being plated with silver. Nickel-copper alloys are used in the United States, Belgium, and Germany, for coins.
Pewter is a tin alloy which was more used formerly than now. Its composition varies. Commonly, it consists of four parts of tin to one of lead, but sometimes it is tin with a little copper. Type metal (see TYPE) is a compound of fifty parts of lead, twenty-five of antimony, and twenty-five of tin, but it varies slightly. Fusible metal melts at low temperatures; one kind is composed of three parts of tin, five of lead, and eight of bismuth, and melts in hot water. This alloy is now a good deal employed in stereotyping, and in obtaining copies of woodcuts. 'Albion metal,' which is largely used in some minor Birmingham manufactures, is an example of two metals combined by pressure, and therefore is not, strictly speaking, an alloy. It consists of tin laid on lead, the two metals being made to cohere by passing them between rollers. White, or anti-friction, metal, recently much employed for certain kinds of machinery bearings, has, in one variety, a composition of eighty-five parts of tin, ten of antimony, and five of copper.
Aluminium-bronze (see ALUMINIUM), an alloy very closely resembling gold in appearance, which is much used for pencil-cases, chains, and some larger objects, varies in composition from ninety-five of copper and five of aluminium, to ninety of copper and ten of aluminium. A compound of silver and aluminium is sometimes used for watch-springs, and for spoons and forks. Dentists use a very ductile alloy composed of two parts by weight of silver and one of platinum. A metal formed of nine parts of platinum and one part of iridium, has recently been employed for the stan- dard metre-measures by the Parisian commission for the international metrical system. An alloy of osmium and iridium, which is not attacked by acids, is employed for tipping gold pens, and sometimes also for the bearings of the mariner's compass.
Sterling silver consists of 11 oz. 2 dwt. of silver, and 18 dwt. of copper in the troy pound. That is, it contains 7.5 per cent. of copper. With certain exceptions, the English law requires that all silver wares, as well as coins, shall be of this standard. The proportion of alloy in gold coin and plate is also regulated by law. It is well to state that an assayer or bullion-dealer uses the term alloy in a different sense from what may be called its common meaning. He would say that standard silver contains 18 dwt. of alloy in the pound troy; but the more general, or at least, the more scientific, way is to call it an alloy of silver and copper. It is the same with gold. Pure silver is too soft to be used for anything which is to be much handled. A little copper imparts to it greater hardness and toughness, and makes it more easily fusible. The alloy used for English is said to wear better than that used for any foreign silver coin, although the difference in one or two cases is small.
When gold is to be used for coins, jewelry, or plate, it requires to be alloyed with copper or silver, or with both, in order to harden it. Like silver, it is too soft when pure. There are five legal standards in Great Britain for articles made of gold—i.e. alloyed gold, apart from coin. These are called 22, 18, 15, 12, and 9 carat gold. That is to say, these figures represent the number of parts of pure gold in every twenty-four parts of the alloy used by the goldsmith or jeweller. English sovereigns are made of a mixture of twenty-two parts gold to two of copper, and this is called 22-carat or standard gold. In Germany, Italy, and the United States, standard gold for the coinage is 21.6 carats. Gold jewelry usually contains both copper and silver, and, according to the proportion of the constituents, the objects have different shades of yellow.
In the United States, it is declared by law that the standard for both gold and silver coins shall be such, that of a thousand parts by weight, nine hundred shall be of pure metal, and one hundred of alloy. Until lately, it was provided that the alloy of gold coins might be of either copper or silver; but by a recent regulation, only copper is used in the alloy either of silver or gold coins.