5. Copper and zinc. Copper and zinc combined form brass, one of the most useful of all alloys. Though this is a general name for such combinations, yet several of the proportions form compounds to which peculiar names are given, some of which will be noticed below.
It may be proper to remark that copper is estimated by Mr. Kirwan at 7½° in hardness, whilst zinc is 6½. The former metal is highly tenacious and malleable; the latter is brittle and malleable only in a small degree. According to Lewis a very small proportion of zinc dilutes the colour of copper and renders it pale; when the copper has imbibed ¹/₁₂ of its weight, the colour inclines to yellow. The yellowness increases with the zinc till the weight of that metal in the alloy equals the copper. Beyond this point if the zinc be increased the alloy becomes paler and paler and at last white, like zinc.
The tenacity of brass is greater than that of either copper or zinc according to Muschenbroek. His experiments give brass nearly twice as strong as copper, and 18 times as strong as zinc. It seems to me most probable that the tenacity of brass increases with the increase of zinc in the alloy to a certain proportion, when it becomes a maximum, and thence diminishes with the further increase of zinc, but experiments are yet wanting, I presume, to ascertain what proportion of the two metals must be taken to form the alloy of greatest tenacity. The same observation may be made as to the maximum hardness; it is not improbable that the two maxima may be found in different kinds of brass.
The point of temperature at which copper fuses is stated to be 27° of Wedgwood’s thermometer, whilst that of zinc is much lower, namely, 680° of Fahrenheit. Common brass is stated to melt at 21° of Wedgwood. It is very probable that all kinds of brass melt at temperatures intermediate between those of copper and zinc; and that the more of zinc the lower will be the fusing temperature; but there have not been direct experiments to ascertain the degrees, as far as I know.
In enumerating the different proportions of such alloys as have come under my notice I shall begin with that containing the maximum of copper, and proceed in gradation to that with the maximum of zinc.
(a). Brass for the manufacture of plated goods. This alloy is composed, judging from one specimen I analysed, of 12 atoms of copper and 1 of zinc; or of nearly 28 parts of copper by weight and 1 of zinc. The atom of copper is here estimated at 56 and that of zinc at 29, or very nearly ½ that of copper. This alloy had much the same qualities apparently as copper itself, only a little more yellow.
(b). Dutch gold, gilding metal. This is the alloy which may be beaten out into thin leaves, after the manner of gold leaf. I have not been able to find any proportions for this compound in books. It seems to have been kept as a secret by the manufacturers. By analysis however I find it composed of 6 atoms of copper and 1 of zinc, or nearly 12 parts copper and 1 zinc by weight. This alloy is probably the most malleable of all the kinds of brass. A leaf containing 12 square inches weighs about ⁷/₁₀ of a grain. The colour, as is well known, makes a good approach to that of gold. It is the composition used for making articles to be gilt, as buttons, &c.
(c). Dipping metal for stamped brass goods. This is a well known article of Birmingham manufacture. It is an alloy both tenacious and malleable, as is manifest from the perfection of the articles. It possesses a beautiful gold colour. A specimen was composed, by my analysis, of 4 atoms of copper to 1 of zinc; or of 8 lbs. of copper and 1 of zinc; or of 4 lbs. copper and 3 of common brass; but it is varied according to the colour wanted.
(d). Soft, fine coloured brass. According to M. Sage, a very fine kind of brass may be made by mixing oxide of copper, calamine, black-flux and charcoal powder together and fusing the mixture in a crucible till the blue flame disappears. The brass is found to weigh ⅙ more than the copper resulting from the weight of oxide. He says when the copper retains ⅕ of zinc the colour is not so fine; and the excess of zinc will be burned off by heat, but the zinc cannot be reduced by burning below ⅙; so that this appears to be a natural limit. Hence this compound, being formed of 6 parts copper and 1 of zinc, must be constituted of 3 atoms of copper and 1 of zinc.
(e). Soft brass preferred for watch movements. There is a kind of brass greatly preferred by watch-makers on account of its working well with steel. I have not met with a specimen; but Dr. Thomson has analysed one and found it to consist of 2 atoms of copper and 1 of zinc;[22] or 4 parts copper and 1 of zinc by weight nearly.