The art of applying to various substances an extremely thin coating of gold. If the substances to be gilt be metallic, this is effected by simple adhesion of the surfaces; but if not, the gold is attached by means of some adhesive medium. The simplest of all the kinds of gilding on metal is that by which copper or silver wire is gilt. The bar, before it is given to the wire drawer, is plated with gold by having several layers of gold leaf burnished down upon them whilst hot; and being then subjected to the stronger compression which takes place in wire drawing, the gold and the other metal become so perfectly united as to form, in fact, one substance; but the most usual way of covering the face of metals with gold is by means of an amalgam, or, as it is technically termed, water gilding. If the metal to be gilt be silver, it is first soaked in warm muriatic acid, that the surface may be rendered perfectly clean, and then washed in clean water, changed two or three times to get rid of the whole of the acid; being afterwards dried and made moderately warm, a little gold amalgam, also warm, is to be carefully and evenly spread upon the silver, to which it will immediately adhere.
The plate is then placed upon a convenient support over a charcoal fire, and the mercury is driven off by heat, when the plate will be found entirely covered with a thin coating of pale dull gold. The small roughnesses are now to be removed with a scratching brush, composed of extremely fine brass wire, which renders the surface perfectly smooth and bright; after which the colour is heightened by warming the piece and smearing it over with gilder's-wax, which is a composition of bees'-wax, red ochre, verdigris, and alum. The wax being burnt off over a charcoal fire, and the piece quenched in urine, the colour of the gilding will be found to be much heightened, after which it may be burnished or not, as may be desired. The affinity of copper and its alloys not being so great as that of silver for mercury, the adhesion of the amalgam is promoted by the action of nitric acid in the following manner: - the piece of copper, a button for instance, after being cleaned and burnished, is dipped in a solution of nitrate of mercury, which, owing to the superior affinity of the copper for the nitric acid, is quickly decomposed, and the mercury becomes deposited in a metallic state over the whole surface of the copper, to which it strongly adheres; the gold amalgam is now applied, and the rest of the process goes on as already described.
Gilding is rarely applied to other metals than silver, copper, and the alloys of the latter metal. There are two methods of gilding wood, viz., oil gilding and burnished gilding. Oil gilding is thus performed: - the wood is first primed or covered with two or three coatings of boiled linseed oil and white lead, to fill up the pores of the wood, and to render the surface smooth and even. When the priming is quite dry, a thin coat of gold size must be laid on; this is prepared by grinding together some strongly calcined red ochre with the thickest drying oil that can be procured; and previous to using it, it must be mixed with a little oil of turpentine, that it may work freely. When the gold size is sufficiently dry, leaf gold, cut into strips, is taken up by the point of a fine brush and applied to the parts to be gilded, and is then gently pressed down by a ball of soft cotton; the gold instantly adheres to the sticky surface, and, after a few minutes, the dexterous application of a camel's-hair brush sweeps away the loose particles of gold leaf without disturbing the rest. In a day or two the size will be perfectly dry, and the operation is then finished.
This method is simple and durable, but will not admit of burnishing, and therefore wants the high lustre produced by the next process: it is chiefly used for out-door work. Burnished gilding, or gilding in distemper, is thus performed: - the surface to be gilt must be first covered with a thick coating of strong parchment size; this coating being dried, eight or ten more must be applied, consisting of the same size, mixed with fine plaster of Paris or washed chalk; and when the whole is perfectly dry, a moderately thick layer must be applied of size, mixed with bole or yellow ochre. While this last is yet moist the gold leaf is to be put on in the usual manner; it will immediately adhere on being pressed by the cotton ball, and before the size is perfectly dry, those parts which are intended to be most brilliant are to be carefully burnished with an agate or dog's tooth. This kind of gilding will not withstand rain or even damp, and is therefore only applied to in-door work, as picture frames, etc.; it may be cleaned with a soft brush and hot spirit of wine, or oil of turpentine.