Lord Rosse, however, in casting specula, preferred using copper and tin in their atomic proportions, or 68·21 per cent. of copper and 31·79 of tin.
The addition of tin, while increasing the hardness of copper, also renders it more fusible. In small proportions it but little affects the colour of the copper,[1603] and it is difficult to recognise its presence from the physical characters of the copper, except from that of increased hardness. What appear, therefore, to be copper instruments may, and indeed often do, contain an appreciable admixture of tin, which, however, can only be recognised by analysis.
Besides the superiority of one alloy over another, it appears probable that the method of treatment of the metal may somewhat affect its properties. M. Tresca[1604] found that a gun-metal cast by Messieurs Laveissière, consisting of—
| Copper | 89·47 |
| Tin | 9·78 |
| Zinc | 0·66 |
| Lead | 0·09 |
was superior in all respects to either the common gun-metal A or the phosphor-bronze B cast at Bourges, the constituents of which were as follows:—
| A | B | |
| Copper | 89·87 | 90·60 |
| Tin | 9·45 | 8.82 |
| Zinc | 0·31 | 0·27 |
| Lead | 0·37 | 0·31 |
| ——– | ——– | |
| 100·00 | 100·00 |
The results of both ancient and modern experience as to the proportions in which copper and tin should be mixed, in order to produce a tough and hard though not brittle metal, appear to be nearly the same; and nine parts of copper to one part of tin may be regarded as the constituents of the most serviceable bronze or gun-metal.
In the following table I have given the results of some of the more recent analyses of bronze antiquities found in the United Kingdom, and have omitted the early analyses of Dr. Pearson[1605] in 1796 as being only approximative. I have arranged them so far as practicable in accordance with the different forms of the objects analyzed; and one feature which is thus brought out tends strongly to confirm the conclusion which has been arrived at from other premises, that certain forms of bronze weapons and other instruments and utensils are of later date than others.
It will be seen, for instance, that in the flat and flanged celts, the palstaves, and even spear-heads, lead, if present at all, exists in but very minute quantity; whereas in the socketed celts and swords, which are probably later forms, and especially in those from Ireland, this metal occurs in several cases in considerable proportions.