GILDING. The application of a su perficial coat of gold on wood, metal, and other materials. The beauty and dura bility of gold render it the most valuable of all ornamental substances ; but, on ac count of its weight and high price, its use in these respects would be exceed ingly limited, were it not the most ex tensible and divisible form of matter, so that it may be made to cover a larger surface than an equal quantity of any other body. Metals are usually covered with gold by the process of water gilding. It consists in perfectly cleaning their surface, and then, in the case of silver, for instance, rubbing it over with a solu tion of gold in mercury, called amalgam of gold: the vessel is then heated over a clear charcoal fire, by which the mercury is driven off, and the gold left adhering to the silver surface, upon which it is afterwards burnished. The surface of copper or brass is usually prepared by cleaning and rubbing it over with a so lution of nitrate of mercury, which amal gamates the surface, and enables the gold amalgam, when subsequently ap plied, to adhere ; heating and burnish ing are then resorted to as before. Brass and copper buttons are gilt in this way ; and the requisite quantity of gold is so small that twelve dozen buttons of one inch diameter may be completely gilt upon both surfaces by five grains of gold. Other kinds of gilding are per formed by gold leaf, which, if intended for out-door work, is laid on by the help of gold size, which is drying oil mixed with calcined red ochre ; or, if for pic ture and looking-glass frames, they are prepared by a size made by boiling parch ment clippings to a stiff jelly, and mixed with fine Paris-plaster or yellow ochre. The leaves of books are gilt upon the edges by brushing them over, while in the binder's press, with a composition of four parts of Arminian bole and one of powdered sugar candy mixed up with white of egg ; this coating, when nearly dry, is smoothed by the burnisher, then slightly moistened, and the gold-leaf ap plied and burnished. To impress gilt
figures on book covers, the leather is dusted over with finely-powdered mastic : the iron tool by which the figure is made is then moderately heated and pressed upon a piece of leaf-gold, which slightly adheres to it.
In gilding wood, the operator should be provided with a cushion, made of a board about 10 inches square, covered with leather, and lightly stuffed with cotton, and a thumb-piece at the back ; also, with a tip-brush, a pallet-knife, and a dabber, or silk bag filled with cotton. The pattern to be gilt is then exactly washed with jappanner's gold size, (lin seed-oil and gain animi, thinned with oil of turpentine,) and the gold leaves being cut on the cushion with the pallet knife, are transferred by the tip-larush to the sized surface and tapped with the silk bag, and left to dry.
In covering surfaces with gold, the size is made of 3 boiled oil, 1 japanner's gold size, with yellow ochre ground in boiled oil. Two separate washes of this must be applied to the pattern, and be fore quite hard, the gold-leaf laid 'on. Other size is made by grinding red lead with thick drying-oil, and diluting with turpentine ; and, in other eases, with mere glue.
Iron and steel are gilt by simply dip ping. The solution in aqua-regia is to be evaporated till it crystallizes, and then, if dissolved in water and alcohol, the iron may be dipped. But if sulphuric ether be added, polished steel will be gilt by simple immersion.
Silver is gilt by a solution of the gold in a menstruum of nitric acid, sal ammo niac, and corrosive sublimate. It black ens the silver, but a red heat restores the gold color.
The mercurial amalgam can be applied to copper, or brass, or silver, by washing the surface with a solution of dilute sul phuric acid and mercury. The amalgam is then evenly applied with a wet brush of brass wire. The heat of a furnace evaporates the mercury, and leaves the gold. Rub with gilders' wax, and bur nish with steel.