The object of the present chapter, is to explain in a general way some of the peculiarities and differences amongst alloys, in the manner of a supplement to the list; prior to entering in the next chapter on the means of melting the metals, without which process alloys cannot be made: yet notwithstanding that the list contains the greater number of the alloys in ordinary use, and many others, it is merely a small fraction of those which might be made, for, says Dr. Turner, "It is probable that each metal is capable of uniting in one or more proportions with every other metal, and on this supposition the number of alloys would be exceedingly numerous"*
It is also stated by the same distinguished authority, that "Metals appear to unite with one another in every proportion, precisely in the same manner as sulphuric acid and water. Thus there is no limit to the number of alloys of gold and copper." The same might be said of many other metals, and when the alloys compounded of three, four, or more metals, are taken into account, the conceivable number of alloys becomes almost unlimited. "It is certain, however, that metals have a tendency to combine in definite proportion; for several atomic compounds of this kind occur native." "It is indeed possible that the" variety of proportions in alloys is rather apparent than real, arising from the mixture of a few definite compounds with each other, or with uncombined metal; an opinion not only suggested by the mode in which alloys are prepared, but in some measure supported by observation."†
It appears to be scarcely possible to give any sufficiently general rules, by which the properties of alloys may be safely
* Dr. Turner's Chemistry, Seventh Edition, 1841, p. 558. † Ibid., p. 559.
Inferred from those of their constituents; for although, in many cases, the working qualities and appearance of an alloy may he nearly a mean proportional between the nature and qualities of the metals composing it; yet in other and frequent instances the deviations are excessive, as will be seen by several of the examples Inferred to.
Thus, when lead, a soft and malleable metal, is combined with antimony, which is hard,brittle, and crystalline, in the proportions of from twelve to fifty parts of lead to one of antimony; a flexible alloy is obtained, resembling lead, but somewhat harder, and which is rolled into sheets for sheathing ships. Six parts of lead and one of antimony, are used for the large soft printers' types, which will bend slightly but are considerably harder than the foregoing; and three parts of lead and one of antimony are emploved for the smallest types, that are very hard and brittle, and will not bend at all; antimony being the more expensive metal, is used in the smallest quantity that will suffice. The difference in specific gravity between lead and antimony constantly interferes, and unless the type metal is frequently stirred, the lead, from being the heavier metal, sinks to the bottom and the antimony is disproportionally used from the surface.† In the above examples, the differences arising from the pro-portions appear intelligible enough, as when the soft lead prevails, the mixture is much like the lead; and as the hard, brittle antimony is increased, the alloy becomes hardened, and more brittle: with the proportion of four to one, the fracture is neither reluctant like that of lead, nor foliated like antimony, but assumes very nearly the grain and colour of some kinds of steel and cast-iron. In like manner, when tin and lead are alloyed, the former metal imparts to the mixture some of its hardness, whiteness, and fusibility, in proportion to its quantity, as seen in the various qualities of pewter, in which however copper, and sometimes zinc or antimony, are found.
* In this alloy the antimony fulfils another service besides that of imparting hardness: antimony somewhat expands on cooling, whereas lead contracts very much, and the antimony, therefore, within certain limits, compensates for this contraction, and causes the alloy to retain the full size of the moulds.
Sometimes from motives of economy the neighbouring parts of machinery are not wrought accurately to correspond one with the other, but lead is poured in to fill up the intermediate space, and to make contact. As around the brass nuts in the heads of some screw presses, in the guides or followers for the same, and some other parts of either temporary or permanent machinery. Antimony is quite essential in all these cases to prevent the contraction the lead alone would sustain, and which would defeat the intended object, as the metal would otherwise become smaller than the space to be filled.
† A little tin is commonly introduced into types, and likewise copper in minute quality; iron and bismuth are also spoken of; the last is said to be employed on account of its well-known property of expanding in cooling, so as to cause the typese to swell in the mould, and copy the face of the letter more perfectly, but although I find bismuth to have been thus used, it appears to be neither common not essential in printing types.
The same agreement is not always met with: as nine parts of copper, which is red, and one part of tin, which is white, each very malleable and ductile metals, make the tough, rigid metal used in brass ordnance, from which it obtains its modern name of gun-metal, but which neither admits of rolling nor drawing into wire; the same alloy is described by Pliny as the soft bronze of his day. The continual addition of the tin, the softer metal, produces a gradual increase of hardness in the mixture; with about one-sixth of tin the alloy assumes its maximum hardness consistent with its application to mechanical uses: with one-fourth to one-third tin it becomes highly elastic and sonorous, and its brittle-ness rather than its hardness is greatly increased.
When the copper becomes two, and the tin one part, the alloy is so hard as not to admit of being cut with steel tools, but crumbles under their action; when struck with a hammer or even suddenly warmed, it flies in pieces like glass, and clearly shows a structure highly crystalline, instead of malleable. The alloy has no trace of the red colour of the copper, but it is quite white, susceptible of an exquisite polish, and being little disposed to tarnish, it is most perfectly adapted to the reflecting speculums of telescopes and other instruments, for which purpose it is alone used.
Copper, when combined in the same proportions with a different metal, also light-coloured and fusible, namely two parts of copper, with one of zinc, (which latter metal is of a bluish-white, and crystalline, whereas tin is very ductile,) makes an alloy of entirely opposite character to the speculum metal; namely, the soft yellow brass, which becomes by hammering very elastic and ductile, and is very easily cut and filed.
Again, the same proportions, namely, two parts of copper, and one of lead, make a common inferior metal, called pot-metal, or cock-metal, from its employment in those respective articles. This alloy is much softer than brass, and hardly possesses malleability; when for example, the beer-tap is driven into the cask, immediately after it has been scalded, the blow occasionally breaks it in pieces, from its redued cohesion.
Another proof of the inferior attachment of the copper and lead, exists in the fact that if the moulds arc opened before the castings are almost cold enough to be handled, the lead will ooze out, and appear on the surface in globules. This also occurs to a lees extent in gun-metal, which should not on that account he too rapidly exposed to the air; or the tin strikes to the surface, as it is called, and makes it particularly hard at those parts, from the proportional increase of the tin. In casting large masses of gun-metal, it frequently happens that little hard lumps, con-sisting of nearly half tin, work up to the surface of the runners or pouring places, during the time the metal is cooling.
In brass, this separation scarcely happens, and these moulds may be opened whilst the castings are red-hot, without such occurrence; from which it appears that the copper and zinc are in more perfect chemical union, than the alloys of copper with tin, and with lead.