Copper, and the alloys formed by its combination with zinc, are gilded nearly in the same manner as silver; but as their affinity for mercury is coniderably less that that of silver, it would be difficult to made the amalgam of gold adhere to the burnished surface of these metals by the same means, and with the same evenness, as takes place in the case last described. To obviate this incOnvenience, advantage is very ingeniously taken of the action of nitric acid to facilitate the adhesion of the copper and mercury, in the following manner. A piece of copper, a button for example, is cleaned, by steeping it in acid, and subsequent washing, and is then burnished, either in a lathe, or by any other means : after this it is dipped in a neutralized solution of nitrate of mercury, and in the space of a few minutes, owing to the strong affinity of nitric acid for copper, the mercurial salt is decomposed, the copper takes the place of the mercu ry, and at the same time the mercury is deposited in the metallic state on the surface of the copper, covering it entire ly, and strongly adhering to it ; the gold amalgam is now applied, and the rest of the process goes on as already described. By this method of proceeding, a given quantity of gold may be made to cover a larger surface than in any other way of gilding on metals ; five grains of gold will completely gild both the upper and un der surfaces of one hundred and forty four copper buttons, each of them an inch in diameter.
There is no metal, the gilding of which presents so many difficulties as iron, or rather steel. If the method of simple burnishing down is had recourse to, the heat requisite for this purpose will, in ma ny cases, bring the temper of the steel too low : on such occasions, the way al ready described for gilding on copper is sometimes practised ; that is, the parts of the steel to be gilded are pencilled over with nitrate of mercury, by which they are covered with a slightly adhering coat of mercury ; then the amalgam is ap plied, and the gilding finished in the usual way. The objections to this mode of pro ceeding are, first, that a considerable heat is required, though inferior to that requi site for burnishing down ; and, secondly, that, even with all possible care, the gild ing is apt to be rough and scale off. A
very considerable improvement in this way of gilding is, to trace the figure of the gilding on the steel first of all, with a brush charged with a strong solution of sulphated copper, in consequence of which a pretty thick plate of this metal is deposited on the steel, to which it may be made to adhere, with considerable firmness, by means of the burnisher ; thus the gilding is, in fact, performed upon the copper.
A new method of gold-gilding upon steel has lately been published, possessed of many advantages over the others, and probably in time may attain to a very high degree of perfection. It depends upon the well known fact, that, if sulphuric ether and nitro-muriate of gold are mix ed together, the ether will, by degrees, separate from the acid nearly the whole of the gold, and retain it for some time in solution, in nearly a metallic state. If ether, thus charged with gold, is spread, by means of a pen or fine brush, on the surface of highly polished steel, the ether presently evaporates, leaving the gold be hind in close contact with the steel, and the adhesion is considerably improved by the subsequent application of the bur nisher. The dearness, and especially the rapid volatility of ether, are, at first, ob jections of some moment, but may be got over by using the best oil of turpentine instead of ether, which- has nearly the same efficacy in decomposing the nitro muriate of gold, midis both cheaper, and not so very quickly evaporable.
Gold-gilding upon silver is, we believe, at present entirely disused. It was per formed in the following manner : a satu rated solution of gold, in nitro-muriatic acid, was poured upon some linen rags, and, when they were become dry, they were heaped in a plate, and touched with a hot coal. The fire gradually spread through the mass, and reduced it to a heavy black ash. A soft cork, being moistened in water, was dipped in this ash, to which a part of it adhered, and was then rubbed on the surface of polished silver, upon which the minute particles of gold became fixed, and covered it with an extremely thin coating, which, when burnished, exhibited the genuine colour and lustre of the precious metal. Aikin's Diet.