GILDING, is the art of ornamenting various articles, by covering them with a superficial coat of gold, to obtain the brilliant appearance of that valuable metal.
For some purposes the appearance is not the only object; for, in situations where wood or metal work is much ex posed to the weather, gilding forms a more protec tion from decay than any kind of paint or varnish, as gold, if well put on, is equally impenetrable to the wind, or frost.
Gilding is an art with which the ancients were acquaint ed, although they had not discovered the means of extend ing the leaves of gold to such a surprising degree as is done at present. From the information of Pliny we learn, that their thickest leaves were called bractex finviestinx, because a statue of Fortune at Prxnestina was gilded there with. An ounce of gold formed 750 of these leaves, each being 4 fingers or 3 inches square : this is 9 square inches in each leaf, or 47 square feet very nearly for the whole ounce. The Romans employed thinner leaves, which they called bractex questorix. From the great malleability of gold, the moderns have discovered means by which it can be reduced to such extremely thin leaves, that, notwith standing its high specific gravity, a given weight of gold can be made to cover a greater surface than any other me tal. An experiment by M. l(eaumur shewed the superfi cies of a grain troy weight of gold leaf to be 42 square inches and three-tenths. This is at the rate of 141 square feet per ounce, and the thickness of such a leaf has been estimated at the part of an inch. Mr Boyle found that a grain might he spread to 50 square incites and seven tenths. This gives 169 squire feet for the ounce ; and, by the same mode of estimation, the thickness would be only the part of an inch. Gold is not at all liable, like other metals, to tarnish or oxidate by the action of the air or damp ; and therefore this thin covering is very durable. is not susceptible of any other decay or waste, than from those causes which produce friction or abrasion of the gild ed surface.
Gilding is performed either upon metals, or upon wood, leather, parchment, or paper ; hut three distinct methods are employed : the first called wash or water gilding, in which the gold is spread whilst reduced to a fluid state by solution in mercury ; 2d, Leaf gilding, either burnished or in oil, is performed by cementing thin leaves of gold upon the xvork, either by size or by oil ; 3d, Japanners in which gold dust or powder is used instead of leaves.
Wash or water gilding.—The method of gilding by a solution of gold is only applied to metals. The gold is first dissolved, or reduced to the consistency of a semifluid, by amalgamation with mercury. In this state, by the as sistance of an acid, it can be spread or washed evenly upon every part of the surface of the article to be gilded ; the mercury is afterwards evaporated by heat, and leaves a co vering of pure gold, the thickness of which is inconceiva bly small, although forming a most perfect surface. By this method, brass ornaments, watch-cases, buttons, and jewellery of all descriptions, are made to represent gold. The latter branch of the art is carried to such a high de gree of pet fection, that the deception can only be detected by the weight of the article, by cutting into it, or by some chemical test.
A mixture of copper and brass is the metal most com monly employed for this kind of gilding : pure copper does not readily receive the mercury amalgamated with the gold ; and, being of a soft and rather porous nature, requires more gold to produce an equally brilliant cover ing. About one-seventh pant of brass, being mixed with the copper, renders it harder, and makes the gold spread very readily : brass itself will receive a very fine surface of gild ing. The endeavour of the workman is, to make such a mixture of brass and copper as will produce nearly the same colour as the gilding is intended to have ; for a great variety of shades in the colour of the gold can be produced by a subsequent process, after it has been laid on ; but it requires less trouble to give the gilding the same colour as •he metal which is beneath, than to make a different shade. In °ffect, the gold adheres to the metal by forming a very su pet ci.1 1 amalgamation or alloy therewit h; • hence the gold will parte.ke of the colour of the metal. A second coat of gilding laid on will have a greater appearance of pure gold, and Will have a more durable brilliancy than an equal quantity of gold spread upon the metn1 at one process.