Gilding, the covering of the surfaces of bodies with a thin coating of gold. This method of economizing the precious metal, and imparting to solid bodies the appearance of being wholly composed of it, was practised at very remote periods. The sacred books allude to it; in Exod. xxvi. 29 there is a command to overlay boards and bars with gold. That the early Egyptians understood it well is evident from the gilding of the coffins of The-ban mummies, in which the gold leaves resemble those now prepared. Homer makes mention of it, and the later Greeks thus decorated the exterior sculpture of their temples and statues. The Romans after the destruction of Carthage applied the process to ornamenting the ceilings of their public buildings, and at last of their private houses also. The thickness of the leaf is spoken of by Martial as like a vapor, and by Lucretius the substance is compared to a spider's web. According to Pliny, an ounce of gold was made into 750 leaves, each four fingers square. This is about three times the thickness of the leaf now in common use; but some qualities are so thin that 290,000 sheets make a pile only one inch in height; and specimens have been made only 1/367,500 of an inch thick, which is 1,200 times thinner than ordinary printing paper.

In modern times the use of gilding in architecture has been carried to the greatest extent by the nations of Further India. It is practised by them with great skill and in the most profuse manner.-Besides the method of gilding by covering objects with gold leaf, there are processes of modern invention, distinguished as chemical gilding, in which the gold is incorporated with the substance of the article it covers, and the same quantity is thus made not merely to spread over a much larger surface, but to be permanently attached to metallic bodies, so as to withstand the action of heat and of atmospheric agents without injury; an art incompatible with the attainments of the ancients in chemistry.-Gilding with gold leaf is distinguished as the mechanical branch of the art; and of this there are two distinct processes, one of which is called burnish gilding or gilding in distemper, and the other oil gilding. In the former the article to be ornamented, as the moulding of a picture frame, is received from the joiner before it is made up. A priming of hot size and whiting is first applied, and when dry all irregularities in the moulding are corrected with the same composition, made of the consistency of putty, which then receives four or five coats of the priming.

This, which is now 1/16 to 1/12 of an inch thick, is carefully trimmed around the edges and smoothed with pumice stone and glass paper. This is the foundation for the so-called gold size (the bed upon which the gold leaf is to be laid), a composition of clay, red chalk, plumbago, suet, and bullock's blood; or, as used by the French, of a pound of Armenian bole to two ounces of red hematite and as much galena, each ground by itself in water, then mixed and ground with a spoonful of olive oil, and at last tempered with a clear white glue carefully prepared from sheep skins. When used, it is first melted with thin size, and while warm is laid on with a brush. The leaf is then laid on by means of a brush called a tip, an operation which requires considerable dexterity. When the whole is covered and dried, the work, or any portion of it, is burnished with smooth agates or flints set in handles for this use.-Oil gilding is practised by several different methods. For large objects, especially those exposed to the weather and of metallic composition, the priming used in Paris is white lead mixed with linseed oil and a little oil of turpentine. For equipages and indoor work a varnish polish is much used over the gold.

For elaborately designed frames oil gilding and burnish gilding are often employed upon the same piece, care being taken that the applications for the former do not touch the spots intended to be burnished, which are treated in the manner already described. The frames intended for this process are furnished to the gilder made up. They are then thoroughly washed, and afterward receive two or three coatings of thin white, and more upon the parts to be burnished. A strong size called clear cole is then laid in several coats over those parts only intended for oil gilding, and upon this the oil gold size, a mixture of boiled linseed oil and ochre. By standing over night this becomes ready for the gilding, which is effected without using water. The gold leaf is pressed with cotton wool into all the depressed portions, and when all is laid the work is smoothed over with a brush, by which its irregularities are removed, and the gold is uniformly distributed.-Book covers are ornamented with gilt letters and figures in the following manner. If of cloth, the leaf is laid on over the parts to be ornamented, and the cover is then placed in a press, in which a heated metallic block, having the intended designs cut upon its face, is powerfully pressed against the cover as in printing.

The heat of the block causes the glue on the back side of the cloth to melt and come through, and thus fasten the figure to the fabric. Leather covers receive an application of gelatine or of the white of egg dissolved in water, upon which when dry an almost imperceptible application of oil is made with a rag, and the gold leaf is then laid to be pressed as in the case of the cloth. The edges of the leaves are gilded in the bookbinder's press, the same gelatine solution and oil being applied, upon which the gold leaf is laid and afterward burnished. Chemical gilding is particularly applicable to metallic surfaces, but other surfaces, as of wood, leather, or paper, may be coated with some preparation by which they are rendered fit for it.-Wash or water gilding is the branch of this art in which the gold is applied by means of an amalgam of gold and mercury. In other processes it is deposited from its solution. Copper, or an alloy of copper with a little zinc, or zinc and nickel, or brass, is well suited for the amalgamation process; the nearer the color is to that of gold the better.

The surface to be gilded must first be thoroughly cleaned and brightened and freed from oil; and it is then advisable to wash it over with a solution made of 100 parts by weight of mercury in 110 of nitric acid, of specific gravity 1.33, diluted with 25 times the weight of the whole of pure water. This application leaves a coating of mercury upon the metal, which is more ready to take the amalgam than is the metal itself. Both the mercurial solution and amalgam are sometimes applied together by means of the gilder's scratch brush, which is dipped into the former and immediately rubbed over the latter, and then applied to the metal; the process being repeated as often as necessary. After the application the article is washed in water and exposed to glowing charcoal to expel the mercury. The amalgam is prepared by heating small particles of gold to redness and throwing them into a quantity of mercury heated so as to emit fumes, and stirring with an iron rod till the gold is dissolved. There should be about eight times as much mercury as gold, the excess of the former being removed by squeezing the amalgam through buckskin. The composition then contains about 67 parts of mercury to 33 of gold.

The gold thus deposited is of a dull yellow, and still retains some mercury, which is removed after washing and scrubbing the article with a scratch brush acidulated with vinegar, by applying gilding wax, and again heating. The wax is a mixture of beeswax with some of the following substances, viz.: red ochre, verdigris, copper scales, alum, vitriol, borax.When the wax is burned off, the color of the gilding is found to be improved, and it is still further heightened by burnishing. The amalgamation process is not well adapted for gilding articles of iron and steel, an oxide of iron being produced by the acid applications, which prevents the adhesion of the amalgam. For these the best method is to cover them w ith gold leaf. Copper may be treated in the same way. The metal is heated till it begins to assume a blue color; a sheet of gold leaf is then laid on and gently pressed with a burnisher, and the article is again heated. Other sheets are laid over the first to the desired thickness and heated, and the last is burnished down cold. The surface of iron, whether wrought or cast, or of steel, is sometimes covered with a coat of copper.-To gild silver, a very good process is that called cold gilding. Sixty grains of fine gold and 12 of rose copper are dissolved in two ounces of aqua regia.

The whole of the solution is absorbed by linen rags, which are then dried and burned to ashes. The black powder thus obtained is applied upon the silver, which has been annealed and polished, and is rubbed with a piece of moistened cork or washed leather. Burnishing completes the process.-A method of gilding buttons and other articles by immersing them in solutions of gold was introduced into the establishment of the Messrs. Elkington, in Birmingham, in 1836, by which the injurious effects of the amalgamating process on the health of the workmen were avoided. To a solution of chloride of gold prepared from one part of gold, 30 parts of bicarbonate of potassa are gradually added, and then 30 parts more of bicarbonate dissolved in 200 parts of water. The whole is then boiled two hours, and the color of the liquid changes from yellow to green. The articles, being perfectly well cleaned and annealed, are immersed for an instant in a mixture of equal parts of nitric and sulphuric acids, to which, if the gold is intended to have a dead appearance, a little chloride of sodium is added. The articles, washed in water, are plunged in the gold solution, and left half a minute, when they are removed, again washed, and dried in hot sawdust.

Articles of German silver, of platinum, or of silver, may be gilded by suspending them by copper or zinc wires for a time in the liquid.-For gilding porcelain or glass, gold precipitated by sulphate of iron is mixed with 1/12 its weight of oxide of bismuth and a small quantity of borax and gum water, and the mixture is then applied with a camel's hair pencil. The article is heated in a muffle, and when taken out the gold is burnished, and finally cleansed with vinegar or white lead. Vases and articles not exposed to wear may be gilded by fixing gold leaf upon them with copal varnish. Silks or other woven fabrics may be gilded by immersing them in a neutral solution of terchloride of gold, or moistening them with it in design, and then exposing them to the action of hydrogen, which reduces the gold to a metallic state.