GILDING, the art of applying and permanently attaching gold leaf or gold dust to surfaces of wood, stone, metals, etc. The Egyptian monuments present numerous traces of the existence of the art in ancient Egypt. The process seems to have been the same with that now used. The Persians also were acquainted with this art as appears from the ruins of Persepolis. The Greeks and Romans employed gilding for many purposes. The Greeks used to gild the hoofs and horns of victims. The practice of gilding statues prevailed in the infancy of the art of sculpture and was never entirely dropped by the ancients. The Romans used to gild sweetmeats and many articles of furniture and utensils which have come down to us are gilt. There are also specimens of gilt glass and metals. The gilding which still remains on some ancient bronze monuments is remarkable for its brilliancy. The ancients carried the practice of gilding to a greater extent than the moderns; they gilded almost all their statues of bronze, wood or plaster and frequently those of marble, the ceilings of rooms and even marble columns. The most remarkable examples of gilding employed with taste and effect in architecture are the ceiling of Saint Peter's and that of Santa Maria Maggiore.
The art of gilding at the present day is performed on metals, or on wood, plaster, leather, parchment, paper, glass, etc. Chemical processes are those which are usually employed for metals. Gilding on copper is performed by the process called wash or water gilding, with an amalgam of gold and mercury. The surface of the copper, being freed from oxide, is covered with the amalgam and afterward exposed to heat till the mercury is driven off, leaving a thin coat of gold. Copper, however, is rather too soft and dark-colored a metal to be treated in this way with advantage. Brass is a very suitable metal for this mode of gilding, but the best of all is a mixture of copper with one-seventh of brass. Copper, brass, etc., are gilded by being attached to wires and plunged into a mixture, where they are allowed to remain as long as the workman thinks necessary, from a few seconds to a minute when the mixture is newly prepared, but longer if it has been used for some time. Gilding is also performed by dipping a linen rag in a saturated solution of gold, and burning it to tinder. The black powder thus obtained is rubbed on the metal to be gilded with a cork dipped in salt water till the gilding appears. Iron or steel is gilded by applying gold leaf to the metal, after the surface has been well cleaned and heated till it has acquired the blue color which at a certain temperature it assumes. Several leaves of gold are thus applied in succession, and the last is burnished down cold. The same process may be applied to copper. The operation of gilding may also be performed on iron and steel by diluting the solution of gold in nitro-hydrochloric acid with alcohol and applying it to a clean surface. A saturated solution of gold in nitro-hydrochloric acid, being mixed with three times its weight of sulphuric ether, dissolves the chloride of gold and the solution is separated from the acid beneath. To gild the steel it is merely necessary, the surface being previously well polished and cleaned, to dip it in the ethereal solution for an instant, and on withdrawing it to wash it instantly by agitation in water. Before being gilded, masonry must be rendered waterproof by means of a solution of shellac and gutta percha in naphtha or some other coating.
Gilding on wood, plaster, leather, parchment or paper is performed by different processes of mechanical gilding. The first of these is oil gilding, in which gold leaf is cemented to the work by means of oil size. In the case of paper or vellum the parts to be gilded receive a coat of gum water or fine size to render them non-absorbent, and the gold leaf is applied before the parts are dry. They are afterward burnished with agate. Lettering and other gilding on bound books are applied without size. The gold leaf is laid on the leather and imprinted with hot brass types. Brass rollers with thin edges are employed in the same way for lines, and similar tools for other ornaments. When the edges of the leaves of books are to be gilded they are first cut smooth in the press, after which a solution of isinglass in spirits is laid on, and the gold leaf is applied when the edges are in a proper state of dryness. Japanner's gilding is another kind of mechanical gilding which is performed in the same way as oil gilding, except that instead of gold leaf a gold dust or powder is employed.
Porcelain and other kinds of earthenware as well as glass may be gilded by mixing a layer of gold in a powdered state by the action of fire. The gold dust or powder required in this operation may be obtained by precipitating it from a solution in aqua regia, either by means of sulphate of iron or protonitrate of mercury. In order that the gold powder may be applied to the surface of the article to be gilded it must be well mixed with some viscous vehicle such as spirits of turpentine mixed with some fatty matter, or strongly gummed water. It is then laid on with a fine camel's hair brush. When the article to be gilded is made of soft porcelain, delft-ware, or any kind of earthenware with a plumbiferous glazing, nothing else is required than to apply the gold in this manner, and then subject the piece of earthenware to a heat sufficient to soften the glazing, and thus fix the gilding. But in the case of hard porcelain, some kinds of stoneware and other varieties of pottery, in which the glazing does not soften at a suitable temperature, the gold powder, before being mixed with the viscous vehicle by which it is applied, must have a flux added to it, which serves as a means of attachment between the metal and the earthenware. The best flux is oxide of bismuth precipitated by water from a solution of nitric acid, with the addition of one-twelfth part of melted borax. One-tenth or one-fifteenth part of this flux is added for every part of gold contained in the mixture, which is applied to the surface of the earthenware. Heat is applied in the same way as in the previous case to melt the flux, and thus fix the layer of gold to the article. The gilding must finally be burnished in order to bring up the gold color. Another method of gilding these substances is to mix neutral chloride of platinum with rectified spirits of turpentine in such a manner that the chloride is held in suspension in a finely divided state in the turpentine, to apply this liquid to the article to be gilded by means of a brush, and then to subject the article to heat so as to volatilize the spirits of turpentine and leave a uniform layer of platinum affixed to the glass or earthenware. The article, after being cooled, cleaned with aqua fortis and washed with water, is next dipped in a gilding liquid; the gilding is then completed by rubbing the gilt parts with chamois leather. This method of gilding has the advantage of enabling the gilder to dispense with the burnishing, which is a very hazardous operation for fragile articles, and in the case of those which are of a very intricate form or very deeply cut out often impracticable.
It was announced in 1912 that the brothers Marino, Italian chemists, had invented a process whereby metals and metallic alloys can be deposited on other metals, ceramic ware, wood, celluloid and other substances by electric agency. In the case of glass, for example, the surface to be metallically mounted is first subjected to sand blasting, to remove the polish and give the metal a grip. This abrased part is then chemically treated, so that when the article is suspended in the electro-plating bath, the metal, whether it be of gold, silver or any other metal or its alloy, is attached and forms an integral part of the foundation. See Metallurgy.