Britannia metal is an alloy consisting principally of tin and antimony. Many varieties contain only these two metals, and may be considered simply as tin hardened with antimony, while others contain, in addition, certain quantities of copper, sometimes lead, and occasionally, though rarely on account of its cost, bismuth. Britannia metal is always of a silvery-white color, with a bluish tinge, and its hardness makes it capable of taking a high polish, which is not lost through exposure to the air. Ninety per cent of tin and 10 per cent of antimony gives a composition which is the best for many purposes, especially for casting, as it fills out the molds well, and is readily fusible. In some cases, where articles made from it are to be subjected to constant wear, a harder alloy is required. In the proportions given above, the metal is indeed much harder than tin, but would still soon give way under usage.
A table is appended, giving the composition of some of the varieties of Britannia metal and their special names.
| Tin | Antimony | Copper | Zinc | Lead | |
|---|---|---|---|---|---|
| English | 81.90 | 16.25 | 1.84 | — | — |
| English | 90.62 | 7.81 | 1.46 | — | — |
| English | 90.1 | 6.3 | 3.1 | 0.5 | — |
| English | 85.4 | 9.66 | 0.81 | 3.06 | — |
| Pewter | 81.2 | 5.7 | 1.60 | — | 11.5 |
| Pewter | 89.3 | 7.6 | 1.8 | — | 1.8 |
| Tutania | 91.4 | — | 0.7 | 0.3 | 7.6 |
| Queen’s metal | 88.5 | 7.1 | 3.5 | 0.9 | — |
| German | 72.0 | 24.0 | 4.0 | — | — |
| German | 84.0 | 9.0 | 2.0 | 5.0 | — |
| German (for casting) | 20.0 | 64.0 | 10.0 | 6.0 | — |
| Malleable (for casting) | 48.0 | — | 3.0 | 48.0 | 1.0 |
Britannia metal is prepared by melting the copper alone first, then adding a part of the tin and the whole of the antimony. The heat can then be quickly moderated, as the melting point of the new alloy is much lower than that of copper. Finally, the rest of the tin is added, and the mixture stirred constantly for some time to make it thoroughly homogeneous.
An alloy which bears a resemblance to Britannia metal is Ashberry metal, for which there are two formulas.
| I | II | |
|---|---|---|
| Copper | 2 | 3 |
| Tin | 8 | 79 |
| Antimony | 14 | 15 |
| Zinc | 1 | 2 |
| Nickel | 2 | 1 |
Bronzes.
The composition of bronze must be effected immediately before the casting, for bronze cannot be kept in store ready prepared. In forming the alloy, the refractory compound, copper, is first melted separately, the other metals, tin, zinc, etc., previously heated, being then added; the whole is then stirred and the casting carried out without loss of time. The process of forming the alloy must be effected quickly, so that there may be no loss of zinc, tin, or lead through oxidation, and also no interruption to the flow of metal, as metal added after an interval of time will not combine perfectly with the metal already poured in. It is important, therefore, to ascertain the specific weights of the metals, for the heavier metal will naturally tend to sink to the bottom and the lighter to collect at the top. Only in this way, and by vigorous stirring, can the complete blending of the two metals be secured. In adding the zinc, great care {56} must be taken that the latter sinks at once to the level of the copper, otherwise a considerable portion will be volatilized before reaching the copper. When the castings are made, they must be cooled as quickly as possible, for the components of bronze have a tendency to form separate alloys of various composition, thus producing the so-called tin spots. This is much more likely to occur with a slow than with a sudden cooling of the mass.