A combination of two or more metals. The term is sometimes employed to denote the inferior metal combined with gold or silver. Thus it is said the standard gold of jewellers is 18 carats of gold and 6 of alloy, whatever metal the alloy may be. When metals are combined either by fusion or cementation, the alloy formed generally possesses properties and characters very different from those of the respective components. The density is sometimes greater, sometimes less; the fusing point in some cases is considerably lower than the mean. Elasticity is sometimes communicated, sometimes destroyed; and the malleability and ductility of the alloy seldom correspond with those of the metals forming it. These important changes would lead to the inference, that alloys are chemical combinations, and not mechanical mixtures; but there are many objections to this supposition, the most important of which are, that metals may be combined in any proportions, and that they may be separated by the process called eliquation, if there is a great difference in the respective temperature of their fusing points.
Thus, silver and lead may be separated from copper by heat, the copper requiring a higher temperature for its fusion than the other two metals combined; and an alloy containing a volatile metal, as mercury, or zinc, may be decomposed by a strong heat, the fixed metal remaining when the more volatile is expelled. In many cases, a very small proportion of one metal is sufficient to change the most important characters of another. A quarter of a grain of lead will render an ounce of gold perfectly brittle, although neither gold nor lead are brittle metals. If a crucible containing arsenic be placed in the same fire with a crucible containing gold, the fumes of the arsenic will render the gold brittle. Some of the changes thus produced are of the utmost importance in the arts, as many of the alloys are far more valuable on account of the newly-acquired properties, than any of the simple metals. Gold and silver, in their pure state, would be totally unfit for the useful purposes to which they are applied, if they were unalloyed, on account of their softness. Even the standard current coin of the realm is alloyed, to render it hard, otherwise the impression would be speedily effaced, and the coin, by abrasion, would soon become deficient in weight.
Pure copper would be unfit for many of the purposes to which it is so extensively applied in the arts, if it were not alloyed by some metal to give it hardness; and it is singular that the metals employed for this purpose are all soft metals. Brass, bell-metal, gun-metal, etc. are all alloys of copper with soft metals. Some metals which will not combine together immediately, may be united by the intervention of a third. Thus, mercury will not combine directly with iron; but if zinc or tin is first added to the iron, an amalgam may be formed of it with mercury. It may here be observed, that when mercury is united to any other metal, the compound is called an amalgam. In order to make a perfect alloy, a very intimate admixture, by mechanical agitation, should be effected while the metals are in the fluid state. They should, therefore, be either constantly stirred with an infusible rod, or repeatedly poured from one hot crucible to another. Mr. Hatchett found that the lower end of a bar of standard gold was of inferior specific gravity and value to the upper extremity, which would be formed by the last portions of the metal in the crucible.
The surface of metals, also, should be carefully defended, while in the fluid state, from the action of the atmosphere, by a stratum of wax, pitch, or resin, if the fusing point be low; or by a layer of salt, pounded glass, borax, etc, if it be high. In this article we shall merely give a brief account of the nature and composition of the most important alloys, and their respective uses. Where the mode of manufacture is complicated, and requires peculiar processes, we shall more fully describe them under their several heads.
Brass is composed of variable proportions of zinc and copper, according to the use for which it is required. In general, about 9 parts of zinc are added to 16 of copper when melted. The best brass is not made by the direct combination of the two fluid metals, but by the process called cementation (See Cementation). The vapour of the zinc ore by this mode combines more intimately with the copper