When two or more metals of different specific gravities are melted together and intimately mixed, they frequently enter into chemical union and form a new compound, called an alloy of the metals. These alloys often possess important properties which their constituents singly do not, and hence become valuable acquisitions to the arts. The metals thus combined may be fused together in any proportion; but if one of them greatly exceed the other in specific gravity, their intimate union is sometimes rendered difficult and even impracticable, partly from the weak affinity and partly from the gravitating principle causing the metal of least specific gravity to arise to the surface.
Notwithstanding this union of metals in seemingly indefinite proportions, there are only a few proportions in which the alloys possess peculiar excellences so as to entitle them to the attention of artists. These proportions have in many instances been discovered by experience; and it only remains for theory to point out the reason for such proportions, and to suggest other proportions which may bid fair to possess desirable qualities, and thereby diminish the unsuccessful attempts for improvement in these combinations.
That the metals thus alloyed constitute true chemical compounds and not merely mechanical mixtures, may be inferred from the change made in their primary qualities; such as
1. Tenacity, hardness, &c. Some alloys are much superior to their ingredients in tenacity and hardness, whilst others affect a kind of medium between them. This last is often the case too in regard to ductility and malleability.
2. Fusibility. Several alloys fuse at temperatures intermediate between the fusing temperatures of their ingredients, but mostly lower than the mean; there are others which fuse below the temperature of the lowest, and few if any require a temperature above the mean for their fusion.
3. Colour. In many cases the colour of alloys is such as would be produced by the mixture of the colours of the metals; but in others, remarkably different; for instance, the alloys of copper and zinc,—forming the various kinds of brass.
4. Specific gravity. This is not always what might be inferred from a mixture of the two ingredients. Sometimes it is greater and other times less; but this is not a decisive mark of chemical union, as the same metal varies in specific gravity, by hammering, rolling, tempering, &c. very considerably. Besides, it is more than probable that the differences said to have been observed, have in some instances arisen from inaccurate experiments; as it is a delicate operation to find the specific gravity of small pieces of metal with sufficient precision for comparisons of this kind.
Many of the simple metals, when fused and exposed to the air for some time, without a covering of charcoal, or some similar principle, acquire less or more of oxygen, and retain it even in a fluid state, as is proved from Mr. Lucas’s interesting communication in the 3d Vol. of the Manchester Society’s Memoirs (new series). Hence by frequent fusions of the same metal its quality becomes impaired in regard to tenuity and other properties.
This is more eminently the case with regard to alloys. Thus, zinc at the temperature in which brass melts is combustible; and hence a portion of it escapes by combustion. Hence the proportions of brass are changed less or more at each fusion, unless fresh zinc be added. The same observation applies to alloys of copper and tin with regard to the tin. The mixtures of lead, tin, bismuth and other soft and easily fused metals, are still more remarkable in this respect. They should be fused under a cover of oil or tallow in order to keep them of the same proportions; otherwise, some of them, particularly the tin, is liable to great oxidation, and no two successive fusions will present the same alloy. Hence in some degree the use of fluxes in metallurgy which serve to cover the surface of the metals and prevent oxidation from the atmosphere.
When an alloy is made, it seldom happens that the metal is perfect and compact the first fusion; it is more or less porous, especially when the two metals fuse at very different temperatures. By a second fusion, which usually takes place at a much lower temperature than that requisite for the first, the metal becomes compact and free from pores. This is particularly the case with speculum metal; and I have little doubt it is so with regard to many other alloys.