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Gilding

Gilding is the art of applying a thin layer of gold, simulated gold, or other metal to a surface. Products employed may be real gold leaf ranging in karats from 9 (white gold) up to 24; imitation leaf--composition gold, Dutch metal leaf, aluminum leaf, copper leaf; variegated leaf, mica powders; etc.

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1 Ancient practice
2 Modern gilding processes
3 See also
4 References
5 Further reading

Ancient practice

The art was known to the ancients. According to Herodotus, the Egyptians were accustomed to gilding wood and metals; and gilding by means of gold plates is frequently mentioned in the Old Testament. Certain statues of great prestige were chryselephantine, i.e. made of gold-plated wood (for the clothing) and ivory (for the flesh); most famously those of Zeus in Olympia and Athena Parthenos in the Parthenon. Extensive ornamental gilding was also used in the ceiling coffers of the Propylaea. 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. But he adds that luxury advanced on them so rapidly that in very little time you might see all, even private and poor people, gild the walls, vaults, and other parts of their dwellings. 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; and the native processes pursued in India at the present day may be taken as typical of the arts as practised from the earliest periods.

Modern gilding processes

Modern gilding is applied to numerous and diverse surfaces and by various distinct processes, so that the art is practiced in many ways, and is part of widely different ornamental and useful arts. It forms an important and essential part of framemaking (see also article on wood carving); it is largely employed in connection with cabinet-work, decorative painting and house ornamentation; and it also features largely in bookbinding and ornamental leather work. 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; and it is also a characteristic feature in the decoration of pottery, porcelain, and glass. The various processes fall under one or other of two headings — mechanical gilding and chemical gilding.

Mechanical gilding

Mechanical gilding embraces all the operations by which gold leaf is prepared, and the several processes by which it is mechanically attached to the surfaces it is intended to cover. It thus embraces the burnish or water-gilding and the oil-gilding of the carver and gilder, and the gilding operations of the house decorator, the sign-painter, the bookbinder, the paperstainer and several others. 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. The process is completed by cold burnishing.

Chemical gilding

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.

Wet gilding

Wet gilding is effected by means of a dilute solution of chloride of gold with twice its quantity of ether. The liquids are agitated and allowed to rest, when the ether separates and floats on the surface of the acid. The whole mixture is then poured into a funnel with a small aperture, and allowed to rest for some time, when the acid is run off and the ether separated. 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 fine emery and spirits of wine. The ether is then applied with a small brush, and as it evaporates it deposits the gold, which can now be heated and polished. For small delicate figures, a pen or a fine brush may be used for laying on the ether solution.

Fire-gilding

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% 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, until it begins to smoke. Upon stirring the mercury with an iron rod, the gold totally disappears. The proportion of mercury to gold is generally as six or eight to one. When the amalgam is cold it is squeezed through chamois leather for the purpose of separating the superfluous mercury; the gold, with about twice its weight of mercury, remains behind, forming a yellowish silvery mass with the consistency of butter.

When the metal to be gilt is wrought or chased, it ought to be covered with mercury before the amalgam is applied, that this may be more easily spread; but when the surface of the metal is plain, the amalgam may be applied to it direct. 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; then it is covered over with a composition called gilding wax, and again exposed to the fire until the wax is burnt off.

This wax is composed of beeswax mixed with some of the following substances: red ochre, verdigris, copper scales, alum, vitriol, and borax. By this operation the color of the gilding is heightened; and the effect seems to be produced by a perfect dissipation of some mercury remaining after the former operation. The dissipation is well effected by this equable application of heat. The gilt surface is then covered over with nitre, alum or other salts, ground together, and mixed up into a paste with water or weak ammonia. The piece of metal thus covered is exposed to a certain degree of heat, and then quenched in water.

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; but owing to the exposure of the workmen to mercurial fumes, it is very unhealthy, and further there is much loss of mercury.

This method of gilding metallic objects was formerly widespread, but fell into disuse as the dangers of mercury toxicity became known. Since fire-gilding requires that the mercury be volatilized to drive off the mercury and leave the gold behind on the surface, it is an extremely dangerous thing to do. Breathing the fumes generated by this process can quickly result in serious health problems, such as neurological damage and endocrine disorders, since inhalation is a very efficient route for mercuric compounds to enter the body. See Mercury toxicity for more details about mercury and its effects on the body. Numerous contrivances have been introduced to obviate these serious evils, but the process has generally been supplanted by the electoplating of gold over a nickel substrate, which is more economical and less dangerous.

Depletion gilding

In depletion gilding, a unique subtractive process discovered in Pre-columbian Mesoamerica, articles are fabricated by various techniques from an alloy of copper and gold (named tumbaga by the Spaniards). The surface is etched with acids, resulting in a surface of porous gold. The porous surface is then burnished down, resulting in a shiny gold surface. The results fooled the conquistadors into thinking they had massive quantities of pure gold. The results startled modern archaeologists, because at first the pieces resemble electroplated articles.

Gilding of pottery and porcelain

The quantity of gold consumed for decoration of pottery and porcelain is very large. The gold leaf is dissolved in aquaregia, and the acid is driven off by heat; or the gold may be precipitated by means of sulphate of iron. In this pulverulent state the gold is mixed with ~1th of its weight of oxide of bismuth, together with a small quantity of borax arid gum water. The mixture is applied to the articles with a camel's hair pencil, and after passing through the fire the gold is of a dingy color, but the luster 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.

References

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.