Editing Gilding (section)

== Chemical gilding ==

[[File:Johann Jacob Kirstein 001.JPG|thumb|Silver gilt ''toilette'' set by Johann Jacob Kirstein (1733–1816) in the [[Musée des Arts décoratifs, Strasbourg]]]]
Chemical gilding embraces those processes in which the gold is at some stage of chemical combination. These include cold gilding, wet gilding, fire gilding and depletion gilding.{{sfn|Chisholm|1911|p=13}}

=== Cold gilding ===

In cold gilding, the gold is obtained in a state of extremely fine division (i.e. atomized or finely powdered),{{explain|reason=What is a state of fine division?|date=October 2021}} 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.{{sfn|Chisholm|1911|p=13}}

=== Wet gilding ===

Wet gilding is effected by means of a dilute solution of [[gold(III) chloride]] in aqua regia with twice its quantity of [[diethyl ether|ether]]. The liquids are agitated and allowed to rest, to allow the ether to separate and float on the surface of the acid. The whole mixture is then poured into a separating funnel with a small aperture, and allowed to rest for some time, when the acid is run off from below and the gold dissolved in 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.{{sfn|Chisholm|1911|p=13}} 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 ===
[[Image:Vajrasattva Tibet.jpg|thumb|upright|A gilded Tibetan [[Vajrasattva]]]]

Fire-gilding or wash-gilding is a process by which an [[amalgam (chemistry)|amalgam]] of gold is applied to metallic surfaces, the [[mercury (element)|mercury]] being subsequently [[Volatility (chemistry)|volatilized]], leaving a film of gold or an amalgam containing 13 to 16% 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.{{sfn|Chisholm|1911|p=13}}

When the metal to be gilded is wrought or chased, the application of mercury before the amalgam is applied allows for it to be more easily spread. When the surface of the metal is plain, the amalgam can 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.{{sfn|Chisholm|1911|p=13}}

After the amalgam is equally spread over the prepared surface of the metal, the mercury is then carefully [[Volatility (chemistry)|volatilized]] with heat just sufficient to do so, as a temperature too high may cause part of the gold to be driven off, or otherwise run together, leaving some of the metal surface bare.{{sfn|Chisholm|1911|p=13}}

When the mercury has evaporated, indicated by the surface taking on a dull yellow color, the metal must undergo further steps to exhibit its fine gold color. 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, as a result of the perfect dissipation of some of the remaining mercury. The gilt surface is then covered over with [[potassium nitrate]], alum or other salts, ground together, and mixed into a [[paste (rheology)|paste]] with water or weak [[ammonia]]. The piece of metal is then exposed to heat, before being [[quenching|quenched]] in water.{{sfn|Chisholm|1911|pp=13–14}}

By this method, the color of the gilding 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.{{sfn|Chisholm|1911|p=14}} 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 [[neurology|neurological damage]] and [[endocrinology|endocrine disorders]], since inhalation is a very efficient route for mercuric compounds to enter the body; the mercury used in the process also evaporates into the atmosphere, thus polluting it. This process has generally been supplanted by the electroplating of gold over a [[nickel]] [[Substrate (materials science)|substrate]], which is more economical and less dangerous.

=== Depletion ===
{{Main|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 [[Spanish colonization of the Americas|Spaniards]]. The surface is etched with acids, resulting in a surface of porous gold. The porous surface is then [[Burnishing (metal)|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 [[archaeology|archaeologists]], because at first the pieces resemble electroplated articles. {{transliteration|ko|[[Keum-boo]]}} is a special [[Koreans|Korean]] technique of silver-gilding, using [[depletion gilding]].