Usuário:JMagalhães/traduções7
O termo douramento engloba uma série de técnicas decorativas de aplicação de folha de ouro ou pó de ouro na superfície de materiais sólidos como madeira, pedra ou metal, de modo a revesti-los com uma fina camada de ouro. A aplicação pode ser química, manual – recorrendo-se ao uso de colas – ou ainda através de galvanização.[1] A aplicação pode ser feita apenas em parte da superfície; por exemplo, no interior de cálices dourados ou em painéis de gravuras que combinem áreas douradas com áreas de outro material ou cor. Os metais mais comuns como suporte de douramento são a prata e o bronze.
História editar
Heródoto menciona a existência do douramento de madeira e metais no Antigo Egito, relato confirmado pela arqueologia e descoberta de imensos artefactos dourados. Durante Antiguidade grega, nas estátuas criselefantinas de maior prestígio, como a Estátua de Zeus em Olímpia ou a de Atenea Partenos, eram usados marfim para a pele e madeira dourada para as roupagens, cabelos e acessórios. O douramento era também por vezes usado na decoração dos tectos dos propileus. Plínio, o Velho relata que o douramento em Roma começou a ser feito no Monte Capitolino durante o consulado de Lúcio Múmio, e rapidamente difundido. Devido à espessura da folha de ouro usada no douramento durante a antiguidade, os exemplares que chegaram até nós ainda se conservam brilhantes e íntegros. O douramento a fogo data, pelo menos, do século IV a.C..
Na Europa, o douramento de prata foi sempre mais comum que o de bronze, situação inversa à que ocorreu na China. Aliás, é na China medieval que se desenvolve a técnica de douramento da porcelana, mais tarde aproveitada pelos ceramistas franceses.
Técnicas de douramento editar
O douramento contemporâneo pode ser aplicado a diferentes superfícies, recorrendo-se a várias técnicas. As técnicas industriais contemporâneas envolvem quase sempre um banho de ouro. As técnicas tradicionais são ainda hoje usadas, sobretudo em marcenaria, construção de molduras, decoração de mobiliário, pintura, decoração de interiores, encadernação, e na decoração de cerâmica, porcelana e vidro.
Mecânicas editar
As técnicas mecânicas de douramento englobam todas aquelas em que o processo de aplicação da folha de ouro na superfície é meramente mecânico. As técnicas mais usadas são o douramento a água, o douramento a óleo.
Polished iron, steel and other metals are gilded mechanically by applying gold leaf to the metallic surface at a temperature just under red-hot, pressing the leaf on with a burnisher, then reheating when additional leaf may be laid on. The process is completed by cold burnishing.
"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),[2] and the Old Testament. The Ram in a Thicket of about 2600-2400 BC from Ur 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 substance made of finely ground gypsum or chalk mixed with glue. Once the coating of gesso had been applied, allowed to dry and smoothed, it was re-wet with glue water or sizing and 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 binder. 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.
Chemical gilding editar
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Chemical gilding embraces those processes in which the gold is at some stage of chemical combination. These include:
Cold gilding editar
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 editar
Wet gilding is effected by means of a dilute solution of gold(III) 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. The gold(III) chloride can also be dissolved in water in electroless plating wherein the gold is slowly reduced out of solution onto the surface to be gilded. When this technique is used on the second surface of glass and backed with silver, it is known as "Angel gilding".
Fire-gilding editar
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. When the mixture is stirred with an iron rod, the gold is totally absorbed. 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(II) 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 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 editar
Ver artigo principal: Depletion gildingIn 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. Keum-boo is a special Korean technique of silver-gilding, using depletion gilding.
Ceramics editar
The gilding of decorative ceramics has been undertaken for centuries, with the permanence and brightness of gold appealing to designers. Both porcelain and earthenware are commonly decorated with gold, and in the late 1970s it was reported that 5 tonnes of gold were used annually for the decoration of these products.[3] Some wall tiles also have gold decoration.[4][5] Application techniques include spraying, brushing, banding machines and direct or indirect screen-printing.[6] After application the decorated ware is fired in kiln to fuse the gold to the glaze and hence ensure its permanence. The most important factors affecting coating quality are the composition of applied gold, the state of the surface before application, the thickness of the layer and the firing conditions.[7]
A number of different forms and compositions are available to apply gold to ceramic, and these include:[8][9]
- Acid Gold - developed in 1860s at Mintons, Stoke-on-Trent, England. The glazed surface is etched with dilute hydrofluoric acid prior to application of the gold; the process demands great skill and is used for the decoration only of ware of the highest class.
- Bright Gold or Liquid Gold - is a solution of gold sulphoresinate together with other metal resinates and a bismuth-based flux. It is particularly bright when drawn from the decorating kiln and so needs little further processing. This form of gilding was invented or at least improved by Heinrich Roessler. Rhodium compounds are used to improve the binding to the substrate.
- Burnish Gold or Best Gold - is applied to the ware as a suspension of gold powder in essential oils mixed with lead borosilicate or a bismuth-based flux. This type of gold decoration is dull as taken from the kiln and requires burnishing, usually with agate, to bring out the colour. As the name suggests it is considered the highest quality of gold decoration. One solvent-free burnish gold composition was reported to consist of 10 to 40% gold powder, 2 to 20% polyvinylpyrrolidone, 3 to 30% an aqueous acrylate resin and 5 to 50% water.[10]
See also editar
- Gilding metal - imitation gold
- Metal leaf
- Gilded Age
- Gilt-edged securities
- French Empire mantel clock
References editar
- ↑ Sloan, Annie, Decorative Gilding, Collins & Brown, ISBN 978-0-89577-879-6
- ↑ "And as when a man overlays silver with gold, a cunning workman whom Hephaestus and Pallas Athena have taught all manner of craft, and full of grace is the work he produces, even so the goddess shed grace upon his head and shoulders" from this translation
- ↑ "Gold In The Pottery Industry. The History And Technology Of Gilding Processes." L.B. Hunt. Gold Bulletin. 12, (3), 116, 1979.
- ↑ "Gold And Lustres For The Ceramic Tile Industry." S.F.Etris. Ceramic Industries 119, (5), 36. 1982.
- ↑ "Comeback Of Gold Decoration? Trends And New Materials For Tile Decoration." K.Abt. Keram.Z. 60, No.1, 2008.
- ↑ "Precious Metal Preparations: Composition, Applications And Special Decorative Effects." E.Groh. Ceramic Forum International. DKG 72,No.3.1995.
- ↑ "Improving Gold Decorating Techniques." L.V.Gerasimova, V.M.Ivanova, E. Yu.Peskova, E.V.Druzhinin. Glass and Ceramics. 48, No.11/12. 1991.
- ↑ "Dictionary Of Ceramics." A.Dodd, D.Murfin. 3rd edition. The Institute Of Minerals. 1994.
- ↑ "Liquid Gold And Other Components Used In Decoration Of Glazed Porcelain And Glass Articles." N.V.Rovinskaya, E.V.Lapitskaya. Glass and Ceramics. 55, No.3/4. 1998.
- ↑ "Burnish Gold Decorating Composition." UK Pat.Appl.GB2216536 A, for Heraeus W.C.,Gmbh.
Este artigo incorpora texto (em inglês) da Encyclopædia Britannica (11.ª edição), publicação em domínio público.
Further reading editar
- Shretha, Sukra Sagar. "Gold Gilding (A Traditional Craft in Kathmandu Valley)." Ancient Nepal - Journal of the Department of Archeology, Number 128–129, February–May 1992, pp. 5–9. [A fascinating and detailed account of the complex traditional techniques of fire-gilding in Nepal.]
