CHEMICAL GILDING This embraces those processes in which the gold used is at some stage in a state of chemical combination. Of these the following are the principal:— Cold Gilding.—In this process the gold is obtained in a state of extremely fine division, and applied by mechanical means. Cold gilding on silver is performed by a solution of gold in aqua regia applied by dipping a linen rag into the solution, burning it, and rubbing the black and heavy ashes on the silver with the finger or a piece of leather or cork.
Wet Gilding is effected by means of a dilute solution of chloride of gold with twice its quantity of ether. The liquids are agitated and allowed to rest, when the ether separates and floats on the surface of the acid. The whole mixture is then poured into a funnel with a small aperture, and allowed to rest for some time, when the acid is run off and the ether separated. The ether will be found to have taken up all the gold from the acid, and may be used for gilding iron or steel, for which purpose the metal is polished with the finest emery and spirits of wine. The ether is then applied with a small brush, and as it evaporates it deposits the gold, which can now be heated and polished. For small delicate figures a pen or a fine brush may be used for laying on the ether solution.
Fire-gilding or Wash-gilding is a process by which an amalgam of gold is applied to metallic surfaces, the mercury being subsequently volatilized, leaving a film of gold or an amalgam containing from 13 to 16% of mercury. In the prepa ration of the amalgam the gold must first be reduced to thin plates or grains, which are heated red hot, and thrown into mercury, previously heated, till it begins to smoke. Upon stirring the mercury with an iron rod, the gold totally disappears. The proportion of mercury to gold is generally six or eight to one. When the amalgam is cold it is squeezed through chamois leather for the purpose of separating the superfluous mercury ; the gold, with about twice its weight of mercury, remains behind, forming a yellowish, silvery mass of the consistency of butter. When the metal to be gilded is wrought or chased, it ought to be covered with mercury before the amalgam is applied, that this may be more easily spread ; but when the surface of the metal is plain, the amalgam may be directly applied to it. When no such prepa ration is applied, the surface to be gilded is simply bitten and cleaned with nitric acid. A deposit of mercury is obtained on a metallic surface by means of "quick-silver water," a solution of nitrate of mercury, the nitric acid attacking the metal to which it is applied, and thus leaving a film of free metallic mercury. The amalgam being evenly spread over the prepared surface of the metal, the mercury is then sublimed by a heat just sufficient for that purpose; for, if it is too great, part of the gold may be driven off, or it may run together and leave some of the sur face of the metal bare. When the mercury has evaporated, which is known by the surface having entirely become of a dull, yellow colour, the metal must undergo other operations, by which the fine gold colour is given to it.
First, the gilded surface is rubbed with a scratch brush of brass wire, until its surface is smooth ; then it is covered with a preparation called "gilding wax" and again exposed to the fire until the wax is burnt off. This wax is composed of bees-wax mixed with some of the following substances : viz., red ochre, verdigris, copper scales, alum, vitriol, borax. By this operation the colour of the gilding is heightened; and the effect seems to be produced by a perfect dissipation of some mercury remaining after the former operation. The dissipation is well effected by this equable application of heat. The gilt surface is then covered over with nitre, alum or other salts ground together, and mixed up into a paste with water or weak ammonia. The piece of metal thus covered is exposed to a certain degree of heat, and then quenched in water. By this method its colour is further improved and brought nearer to that of gold, probably by removing any particles of copper that may have been on the gilt surface. This process, when skilfully carried out, produces gilding of great solidity and beauty; but owing to the exposure of the workmen to mercurial fumes, it is very dangerous, and besides there is much loss of mercury.