gilding

The ancient craft of sticking gold (or other metallic) leaf on to a surface, usually wood. Gilding flourished in the Middle Ages in manuscript illumination and panel painting, and later for picture-frames and furniture.

Gilding is the art of spreading gold, either by mechanical or by chemical means, over the surface of a body for the purpose of ornament.

The art of gilding was known to the ancients. According to Herodotus, the Egyptians were accustomed to gild wood and metals; and gilding by means of gold plates is frequently mentioned in the Old Testament. Pliny the Elder informs us that the first gilding seen at Rome was after the destruction of Carthage, under the censorship of Lucius Mummius, when the Romans began to gild the ceilings of their temples and palaces, the Capitol being the first place on which this process was used. Owing to the comparative thickness of the gold leaf used in ancient gilding, the traces of it which yet remain are remarkably brilliant and solid.

Gilding has in all times occupied an important place in the ornamental arts of Oriental countries;

For the gilding of copper, employed in the decoration of temple domes and other large works, the following is an outline of the processes employed. Next the surface is rubbed over with mercury, which forms a superficial amalgam with the copper, after which it is left some hours in clean water, again washed with the acid solution, and dried. It is now ready for receiving the gold, which is laid on in leaf, and, on adhering, assumes a grey appearance from combining with the mercury, but on the application of heat the latter metal volatilizes, leaving the gold a dull greyish hue. The weight of mercury used in this process is double that of the gold laid on.

For the gilding of iron or steel, the surface is first scratched over with chequered lines, then washed in a hot solution of green apricots, dried and heated just short of red-heat.

For the traditional gilding of carved wood ("gilt gesso"), a thin layer of white gesso mixed with rabbit skin glue is applied to the carving, which may be touched up for crispness by recarving details. A reddish or ochre water-based bole is applied, the color of which will affect the color of the finished gilding.

Oil gilding is the traditional process for building exteriors. Oil gilded surfaces are not burnished any further.

Mercury gilding was the technique employed for European gilt bronze.

Modern gilding is applied to numerous and diverse surfaces and by various distinct processes, so that the art is prosecuted in many ways, and is part of widely different ornamental and useful arts. Further, gilding is much employed for coating baser metals, as in button-making, in the gilt toy trade, in electro-gilt reproductions and in electroplating; The various processes fall under one or other of two headings — mechanical gilding and chemical gilding.

Mechanical gilding embraces all the operations by which goldleaf is prepared (see goldbeating), and the several processes by which it is mechanically attached to the surfaces it is intended to cover. Polished iron, steel and other metals are gilt mechanically by applying gold-leaf to the metallic surface at a temperature just under red-heat, pressing the leaf on with a burnisher and reheating, when additional leaf may be laid on.

Chemical gilding embraces those processes in which the gold used is at some stage in a state of chemical combination. Of these the following are the principal:

Cold Gilding: In this process the gold is obtained in a state of extremely fine division, and applied by mechanical means. Cold gilding on silver is performed by a solution of gold in aqua regia, applied by dipping a linen rag into the solution, burning it, and rubbing the black and heavy ashes on the silver with the finger or a piece of leather or cork. The ether will be found to have taken up all the gold from the acid, and may be used for gilding iron or steel, for which purpose the metal is polished with the finest emery and spirits of wine. Fire-gilding or Wash-gilding is a process by which an amalgam of gold is applied to metallic surfaces, the mercury being subsequently volatilized, leaving a film of gold or an amalgam containing from 13 to 16 O/ of mercury. In the preparation of the amalgam the gold must first be reduced to thin plates or grains, which are heated red hot, and thrown into previously heated mercury, till it begins to smoke. the gold, with about twice its weight of mercury, remains behind, forming a yellowish silvery mass of the consistence of butter. When no such preparation is applied, the surface to be gilded is simply bitten and cleaned with nitric acid. A deposit of mercury is obtained on a metallic surface by means of quicksilver water, a solution of nitrate of mercury, the nitric acid attacking the metal to which it is applied, and thus leaving a film of free metallic mercury. The amalgam being equally spread over the prepared surface of the metal, the mercury is then sublimed by a heat just sufficient for that purpose; for, if it is too great, part of the gold may be driven off, or it may run together and leave some of the surface of the metal bare. When the mercury has evaporated, which is known by the surface having entirely become of a dull yellow color, the metal must undergo other operations, by which the fine gold color is given to it. First, the gilded surface is rubbed with a scratch brush of brass wire, until its surface be smooth; By this method its color is further improved and brought nearer to that of gold, probably by removing any particles of copper that may have been on the gilt surface. This process, when skilfully carried out, produces gilding of great solidity and beauty; Gilt brass buttons used for uniforms are gilt by this process, and there is an act of Parliament (1796), yet unrepealed, which prescribes 5 grains of gold as the smallest quantity that may be used for the gilding of 12 dozen of buttons 1 in. The gold tised is dissolved in aquaregia, and the acid is driven off by heat, or the gold may be precipitated by means of sulphate of iron. The mixture is applied to the articles with a camels hair pencil, and after passing through the fire the gold is of a dingy color, but the lustre is brought out by burnishing with agate and bloodstone, and afterwards cleaning with vinegar or white lead.

Mechanical and chemical gilding of metals has been largely superseded by electroplating.