Practical Magazine 19, 1876
(Chemistry applied to the Arts, Manufactures, &c.
Metallurgy.)
For this purpose, Prof. BÖTTGER, thirty years ago, recommended a concentrated solution of sal ammoniac, in which, at boiling heat, the articles to be coated were to be treated for some time. Later experiments have shown him that the same object may be more perfectly accomplished by employing, instead of the sal ammoniac, a fluid having a powerful electric reaction. For this purpose a solution of oxide of zinc and soda, or potash, is especially fitted. Such a solution may be easily obtained by treating zinc-dust, or a grey mixture of oxidized and metallic zinc, in great excess, with a concentrated solution of caustic potash, or caustic soda, for some time at boiling heat, and then plunging the articles to be coated with zinc in the boiling fluid. By the contact of the copper or brass articles, which act electro-negatively with regard to the zinc dust, the alkaline zinc solution is decomposed, and in a few minutes of continued heat the objects are seen to be covered with a bright coating of zinc.
It may be further remarked, that the formation of the so-called tombac, red metal, or Lyonese gold, can be effected at a temperature of 284°F. If an object covered with a thin coating of zinc is carefully heated (which is best done in olive oil) to the above temperature, the thin coating of zinc combines with the copper beneath to form the gold-coloured tombac. Nothing more is necessary than to plunge the object quickly in cold water, or any other suitable fluid, to cool it, as soon as the colour that is desired becomes visible.
- Gewerbehalle, May, 1876.
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