This alloy has been known and employed since very remote ages. It was used exclusively by the ancients for making swords and other sharp instruments, for coinage, statues, and many other useful and ornamental purposes. It is composed of copper and tin, sometimes with the addition of a little zinc and lead. Great variations are made in the proportions of the two chief constituents, according to the nature of the application for which it is destined. For statuary, the proportions used by the Brothers Keller, the most noted bronze-founders of modern times, were copper, 91.40; zinc, 5.53; tin, 1.70; and lead, 1'37. The bronze coinage of this country contains 95 parts copper, 4 of tin, and 1 of zinc The addition of a little zinc to the alloy is an advantage, but too much diminishes its tenacity; lead is objectionable, owing to its tendency to sink after casting, thus destroying the homogeneity of the alloy. The metals should be melted rapidly to prevent loss of metal by oxidation, and the melted mass should be covered with a layer of charcoal, and kept constantly stirred.
The operation is generally carried on in refractory crucibles, heated in a rever-beratory furnace of suitable form. The cooling in the moulds must be as rapid as possible, in order to prevent the separation of the metals.
The composition of different kinds of bronze is shown below:-
For gilding: (1) copper, 82 parts; zinc, 18; tin, 3; lead, 1 1/2; (2) copper, 83; zinc, 17; tin, 2; lead, 1; (3) copper, 70; zinc, 25; tin, 2; lead, 3. Nos. 2 and 3 represent extremes.
For mortars: copper, 93 parts; lead, 5; tin, 2.
For statuary: (1) copper, 88 parts; tin, 9; zinc, 2; lead, 1; (2) copper, 82 1/2; zinc, 10 1/2; tin, 5; lead, 2; nearly the proportions of the celebrated statue of Louis XV.; (3) copper, 90; tin, 9; lead, 1; (4) copper, 91; tin, 9; (5) copper, 91*4; zinc, 5*6; tin, 1*6; lead, 1.4; (6) copper, 89.35; tin, 10-05; zinc, 0*5; lead, 0.1.
In a paper lately read before the American Society of Civil Engineers, Prof. R. H. Thurston describes a new bronze alloy of maximum strength. The properties of this alloy were ascertained by Thurston in the coarse of his examination (in the mechanical laboratory of the Stevens Institute of Technology) of a series of 36 alloys of copper, tin, and zinc, in which the proportions of the copper were varied from 10 to 80 per cent.; of the tin, from 10 to 80 per cent.; and of the zinc, from 10 to 70 per cent. The results of these experiments pointed to an alloy of the proportions of 55 copper, 43 zinc, and 2 tin, as likely to be that possessing maximum strength, and on Thurston making the alloy, he found it to be of a good colour, close grained, and susceptible of high polish. It was also found to have immense strength, considerable hardness, and moderate ductility, while it, could also be forged if carefully heated. For purposes demanding toughness as well as strength, Thurston found, however, an alloy with less tin to be preferable, and he gives the proportions of 55 copper, 0.5 tin, and 44.5 zinc, as affording the best results.
This alloy, he states, has a tensile strength of 68,900 lb. per sq. in. of original area, and 92,136 lb. per sq. in. of fractured area, while it elongated 47 to 51 per cent. (length of test sample not stated), and reduced to 0'69 to 0.71 of its original diameter before fracture. He also states that the shavings produced by the action of the turning tool on this alloy curled closely, and were tough and strong, like those of good iron. Thurston also refers to an alloy discovered several years ago by J. A. Tobin, but which appears not to be generally known. This alloy, which consists of 58*22 copper, 2*3 tin, and 39*48 zinc, had, when cast, a tensile strength of 66,500 lb. per sq. in. of original section, while when rolled hot its tenacity rose to 79,000 lb. per sq. in., and when moderately and carefully rolled cold, to 104,000 lb. per sq. in. It could also be bent double either hot or cold, and was found to make excellent bolts and nuts, while it could be forged at a low red heat.