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.
Annealing Bronze.
—This process is more particularly employed in the preparation of alloys used in the manufacture of cymbals, gongs, bells, etc. The alloy is naturally brittle, and acquires the properties essential to the purpose for which it is intended only after casting. The instruments are plunged into cold water while red-hot, hammered, reheated, and slowly cooled, when they become soft and sonorous. The alloy of copper and tin has the peculiar property that, whereas steel becomes hard through cooling, this mixture, when cooled suddenly, becomes noticeably soft and more malleable. The alloy is heated to a dark-red heat, or, in the case of thin articles, to the melting point of lead, and then plunged in cold water. The alloy may be hammered without splitting or breaking.
Aluminum Bronze.
—This is prepared by melting the finest copper in a crucible, and adding the aluminum. The copper is cooled thereby to the thickly fluid point, but at the moment of the combination of the two metals, so much heat is released that the alloy becomes white hot and thinly fluid. Aluminum bronze thus prepared is usually brittle, and acquires its best qualities only after having been remelted several times. It may be remarked that, in order to obtain a bronze of the best quality, only the very purest copper must be used; with an inferior quality of copper, all labor is wasted. Aluminum bronze is not affected by exposure to the air; and its beautiful color makes it very suitable for manufacturing various ornamental articles, including clock cases, door knobs, etc.
Aluminum bronze wire is almost as strong as good steel wire, and castings made from it are almost as hard as steely iron; its resistance to bending or sagging is great.
I.—A good formula is 90 to 95 per cent of aluminum and 5 to 10 per cent of copper, of golden color, which keeps well in the air, without soon becoming dull and changing color like pure copper and its alloys with tin and zinc (bronze, brass, etc.). It can be cast excellently, can be filed well and turned, possesses an extraordinary hardness and firmness, and attains a high degree of polish; it is malleable and forgeable. On the latter quality are founded applications which were formerly never thought of, viz.: forged works of art for decorative purposes. An alloy of 95 parts aluminum and 5 parts copper is used here. The technical working of bronze is not materially different from that of iron. The metal, especially in a hot condition, is worked like iron on the anvil, with hammer and chisel, only that the temperature to be maintained in forging lies between dark and light cherry red. If the articles are not forged in one piece and the putting together of the separate parts becomes necessary, riveting or soldering has to be resorted to. Besides forging, aluminum bronze is well suited for embossing, which is not surprising considering the high percentage of copper. After finishing the pieces, the metal can be toned in manifold ways by treatment with acid.
II.—Copper, 89 to 98 per cent; aluminum and nickel, 1 to 2 per cent. Aluminum and nickel change in the opposite way, that is to say, in increasing the percentage of nickel the amount of aluminum is decreased by the equal quantity. It should be borne in mind that the best ratio is aluminum, 9.5 per cent; nickel, 1 to 1.5 per cent at most. In preparing the alloy a deoxidizing agent is added, viz., phosphorus to 0.5 per cent; magnesium to 1.5 per cent. The phosphorus should always be added in the form of phosphorous copper or phosphor aluminum of exactly determined percentage. It is first added to the copper, then the aluminum and the nickel, and finally the magnesium, the last named at the moment of liquidity, are admixed.
