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Gilding is the decorative technique of applying fine metallic-leaf or powder to solid surfaces such as wood, stone, or metal. Methods of gilding include hand application, chemical gilding, and electroplating.Sloan, Annie, Decorative Gilding, Collins & Brown, ISBN 978-0895778796

Ancient techniques

"Overlaying" or folding or hammering on gold foil or gold leaf is the simplest and most ancient method, and is mentioned in Homer's Odyssey (Bk vi, 232), and the Old Testament. The Ram in a Thicket of about 2600-2400 BC from Urmarker uses this technique on wood, with a thin layer of bitumen underneath to help adhesion.

The next advances involved two simple processes. The first involves gold leaf, which is gold that is hammered or cut into very thin sheets. Gold leaf is often thinner than standard paper today, and when held to the light is semi-transparent; in ancient times it was typically about 10 times thicker than today, and perhaps half that in the Middle Ages. The object being gilded was coated with adhesive, usually gesso. "Gesso" is a tacky substance made of finely ground gypsum or chalk mixed with glue. Once the coating of gesso had been applied, the gold leaf was layered on and left to dry. A second gilding process was using the gold as pigment in paint. The artist ground the gold into a fine powder and mixed it with a bindery. Then the gold was applied as with any paint. Sometimes, after either gold-leafing or gold-painting, the artist would heat the piece enough to melt the gold slightly, ensuring an even coat. These techniques remained the only alternative for materials like wood, leather, and the vellum pages of illuminated manuscripts.

Keum-boo is a special Korean technique of silver-gilding, using depletion gilding. Another depletion gilding process was developed by the Incas in Pre-Columbian South America.

Spread of gilding

Herodotus mentions that the Egyptian gilded wood and metals, and many such objects have been excavated. Certain Ancient Greek 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 Olympiamarker and Athena Parthenos in the Parthenonmarker. Extensive ornamental gilding was also used in the ceiling coffers of the Propylaeamarker. Pliny the Elder informs us that the first gilding seen at Romemarker was after the destruction of Carthagemarker, under the censorship of Lucius Mummius, when the Romans began to gild the ceilings of their temples and palaces, the Capitolmarker 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. Fire-gilding of metal goes back at least to the 4th century BC, and was known to Pliny (33,20,64–5) and Vitruvius (8,8,4).

In Europe silver-gilt has always been more common than gilt-bronze, but in China the opposite has been the case. The medieval Chinese also developed the gilding of porcelain, taken up by the French as ormolu.

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 part of framemaking and it is employed in general woodworking, cabinet-work, decorative painting and house ornamentation, bookbinding, and ornamental leather work, coating baser metals, in button-making, in the gilt toy trade, in electro-gilt reproductions, in electroplating, and in the decoration of pottery, porcelain, and glass.

Mechanical gilding

Mechanical gilding includes all the operations in which gold leaf is prepared, and the processes mechanically attach the gold to surfaces. It includes 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 gilded 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 is at some stage of chemical combination. These include:

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 gold chloride 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 six or eight to one. When the amalgam is cold it is squeezed through chamois leather to separate 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 gilded 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 directly. 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 mercury nitrate, 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 is smooth. It is then covered with gilding wax, and again exposed to fire until the wax is burnt off.

Gilding 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 potassium nitrate, alum or other salts, ground together, and mixed into a paste with water or weak ammonia. The piece of metal thus covered is exposed to 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 skillfully carried out, produces gilding of great solidity and beauty, but owing to the exposure of the workmen to mercurial fumes, it is very unhealthy. There is also much loss of mercury to the atmosphere, which brings extremely serious environmental concerns as well.

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 extremely dangerous. 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. This process has generally been supplanted by the electroplating of gold over a nickel substrate, which is more economical and less dangerous.

Depletion gilding
In depletion gilding, a 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

Gold can be used for the decoration of pottery and porcelain. Gold leaf is dissolved in aqua regia, and the acid is driven off by heat; or the gold may be precipitated by means of iron sulfate. In this pulverulent state the gold is mixed with ~1th of its weight of bismuth oxide, together with a small quantity of borax arid gum water. The mixture is applied to the articles with a fine 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.

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

See also



References

Further reading

  • Shretha, Sukra Sagar. "Gold Gilding (A Traditional Craft in Kathmandu Valley)." Ancient Nepal - Journal of the Department of Archeology, Number 128–129, February–May 192, pp. 5–9. [A fascinating and detailed account of the complex traditional techniques of fire-gilding in Nepal.]


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