False Gilding, although an old invention, has become in recent years an important trade in Germany. The moulding intended to be 'gilt' in this way is first covered with bright silver-leaf or tinfoil on a surface prepared as above, and then coated with a yellow varnish. Other substitutes for genuine gilding that are largely used consist in applying 'Dutch gold,' which is copper beaten out like gold-leaf, as in genuine gilding, or in using so-called 'gold paint,' which is finely powdered brass or other similar alloy.
Chemical Gilding.—Metals are now usually gilded by the process of electro-gilding (see ELECTROMETALLURGY); but, besides this, various methods of chemical gilding have been adopted, and some are still in use.
Water or Wash Gilding, as it is somewhat inappropriately termed, consists in applying to metal a paste formed of an amalgam of gold, and afterwards evaporating the volatile mercury by heat, which leaves the gold firmly adhering to the surface of the metal.
Gilding by Immersion.—For this purpose a solution of gold in nitro-muriatic acid is used which slowly attacks the metal to be gilded, and at the same time deposits on its surface an equivalent of gold. The method called Grecian Gilding is another similar process, in which gold is used dissolved in a solution of sal-ammoniac and corrosive sublimate in nitric acid.
Most articles that are gilded by either of the above chemical methods, or by electro-gilding, are submitted to an after-process of colouring. This consists either in acting upon the surface with a saline solution, and heating the article afterwards, or in coating it with a kind of varnish of beeswax and yellow ochre, and then burning this off. The colouring of jewelry, &c., made of gold alloyed with copper or brass, is performed by submitting the article to the action of a mixture of nitre, alum, and common salt, either dry or dissolved in water, heat being applied in either case. The baser alloy is thus removed from the surface, which becomes covered with a richly coloured film of nearly pure gold.
Sword-blades, lancets, and other steel articles are gilded in fancy devices by drawing the design with a camel-hair pencil moistened in a solution of gold, prepared by agitating ether with a solution of tetrachloride of gold, and decanting the light liquid which floats on the top.
Silks, artificial flowers, ivory, bone, &c. may be gilded by immersing them in, or painting them with, a neutral solution of one part of tetrachloride of gold to four or five of water, and then exposing them in a vessel containing hydrogen gas, which readily combines with the chlorine, and reduces the gold to the metallic state.
Encaustic Gilding is usually applied to glass and porcelain. The gold is first obtained in a finely divided state by precipitating from the chloride with protosulphate of iron, or by simply heating the chloride. This powder is ground up with th of its weight of oxide of bismuth and some borax and gum water, and then painted on the ware. It is then heated till the borax is vitrified and the gold thereby fixed. Sometimes the gold is ground with turpentine, or an amalgam of gold is used. This has a brown dingy appearance when it leaves the kiln; the gold lustre is brought up by burnishing.
Gilding Metal.—The metal of which gilded goods are made is required to have as nearly as possible the colour of gold, so that when the surface-gilding is worn off at the more exposed parts the difference of colour will not be readily apparent. This is obtained by making a kind of brass having a much larger proportion of copper than common brass. The following are three receipts from among a variety in use: (1) 6 parts copper, 1 common brass; (2) 4 parts copper to 1 Bristol brass; (3) 13 parts copper, 3 parts brass, 12 parts tin. The last is much harder than No. 1 or 2.