Alloys containing 94 to 88 per cent copper and 6 to 12 per cent tin can be rolled and hammered at ordinary temperatures, and are not increased in density by slow or sudden cooling; if they are at the same time subjected to mechanical treatment their specific gravities are slightly increased. A bronze containing 6 per cent tin had its density increased from 8.924 to 8.932, by 72 blows alternating with 24 annealings; and by similar treatment, substituting quick for slow cooling, the density was increased from 8.928 to 8.935. According to the amount of tin present in bronze the colour varies between red and white, and with a large excess of tin it becomes steel-gray. Generally speaking, tin whitens copper more than zinc, 73 parts copper and 27 parts tin forming a white alloy. Alloys containing 89 per cent of copper and upwards are red or reddish-yellow, 88 per cent copper and 12 per cent tin is orange-yellow, and the alloy containing 85 per cent of copper is pure yellow; from 85 to 74 per cent copper the yellow colour becomes fainter, and disappears with 72 per cent of copper. Alloys with 1 to 2 per cent of tin are malleable, ductile, and tough, but less so than pure copper. With a greater content of tin the metal becomes less malleable, an alloy with 5 per cent tin can only be worked hot, cracking when hammered cold. Alloys with 15 per cent tin cannot be forged hot or cold. The greatest strength, as previously remarked, being found in bronze used as gun-metal containing about 90 per cent copper and 10 per cent tin. From practical experience it has been found that the greatest strength is obtained by so working as to produce the crystals of the alloy as small as possible, even the kind of mould in which the casting is effected exerting an influence upon the grain, and through this upon the strength. Articles must be cast at a higher temperature in iron moulds than in sand moulds, 1600° C. being required in the former, while 1400° C. will suffice for the latter, especially for large castings. § 67. The following table of copper-tin alloys was prepared for the United States Board by the Committee on Alloys (Report, vol. i. 1879, p. 390). :: W. U.S.B. : : : : ::: ::::: :: : : : : ::: : : : : ::: 7.3 ::::: :: 8.648 grayish- earthy yellow 53 SnCu12 86-57 13:43 87-15 12-73 8-681a mottled 8.9436 white 31,100 finely 29,430 vesicular 89-29 10.71 89.29 10.71 89.23 10.77 45 SnCu15 89 11 46 SnCu15 88.97 11.03 88.89 11.11 47 88.39 11.61 49 88 12 87.65 12.35 51 87.5 12.5 52 86.8 13.2 |