GILDING, art of. The art of gilding, or of laying a thin superficial coating of metal on wood, metal, and other sub stances, has been long practised and highly esteemed, both for its utility, and the splendid effect which it produces. Geld, from the extreme beauty of its colour, and from the length of time dur ing which itmay be exposed to the actiun of the air without tarnishing, is perhaps the most valuable of all substances for the purpose of decoration ; but on ac count of its dearness and weight, it can very seldom be employed in substance, and its ornamental use would be limited, indeed, if it were not at the same time the most extensible of all substances ; so that a given weight of gold, notwithstand ing its high specific gravity, may, by beat ing, be made to cover a larger surface than an equal quantity of any other body. Among the ancients, the Romans, and, among the moderns, the French have been remarkable for their large and pro fuse consumption of gold ; not only the temples, theatres, and other public build ing, being adorned with gilding, but even the private houses of the wealthier classes.
The materials for gilding, or rather the different states in which gold is used for the purpose, are the following : leaf gold of different thicknesses, and formed either of the pure metal, or of an alloy of this with silver, amalgam of gold, and gold-powder. The leaf-gold is procured by the gilder from the gold-beater, for an account of Which we shall refer the reader to the article GOLD; but the other two substances being prepared by the gilder himself, may be with propriety described here. The amalgam of gold is made, by heating in a crucible some pure quicksilver ; and when it is nearly boiling, adding to it about a sixth of its weight of fine gold in thin plates, heated red hot ; the mixture, after being kept hot for a few minutes, becomes of a perfectly homogeneous consistence, and may then be allowed to cool : when cold, it is to be put in a piece of soft leather, and, by gradual pressure, the fluid part of the amalgam, consisting almost wholly of mercury, may be forced through the pores of the leather, while the gold com bined with about twice its weight of mer cury will remain behind, forming a yel lowish silvery mass of about the consis tency of soft butter. This, after being
bruised in a mortar, or shaken in a strong phial, with repeated portions of salt and water, till the water ceases to be fouled by it, is fit for use, and may be kept for any length of time without injury in a corked phial. It is of essential impor tance that the materials of this amalgam, and especially the mercury, should be perfectly pure, as the least portion of lead or bismuth would very materially injure the beauty of the gilding, by de teriorating the colour of the gold, and filling it with black specks ; on this ac count no mercury ought to be employed, that has not been procured by distillation from the red precipitate (nitrous red ox ide of mercury) either alone or mixed with a little charcoal powder.
Gold is prepared in three different ways. The first and simplest is, to put into a glass or earthen mortar some gold leaf, with a little honey or thick gum-wa ter, and grind the mixture for a consider able time, till the gold is reduced to ex tremely minute fragments ; when this is done, a little warm water will wash out the honey or gum, leaving the gold be hind in a flaky pulverulent state. A less tedious and more effectual way of corn. minuting the gold is, to dissolve it in nitro-rnuriate acid, and then precipitate it with a piece of copper: the precipitate, after being digested in distilled vinegar, and then washed in water and dried, is in the form of a very fine powder, and both works better, and is easier to,burnisb, than the ground leaf-gold. The finest ground gold is however produced by heating very gradually the gold amalgam in an open earthen vessel, and continu ing the fire till the whole of the mercury is evaporated, taking care that the amal gam shall be constantly stirred with a piece of glass, rod, or tobacco-pipe, in order to prevent the particles of gold from adhering as the mercury flies off. When the mercury is completely evapo rated,the residual gold being then ground in a 1Vedgewood-ware mortar, with a little water, and afterwards dried, it is fit for use.