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Concept
Ormolu
Summary
Ormolu (ˈɔrməluː; from French or moulu, "ground/pounded gold") is the gilding technique of applying finely ground, high-carat gold–mercury amalgam to an object of bronze, and for objects finished in this way. The mercury is driven off in a kiln leaving behind a gold coating. The French refer to this technique as "bronze doré"; in English, it is known as "gilt bronze". Around 1830, legislation in France had outlawed the use of mercury for health reasons, though use continued to the 1900s.
Craftsmen principally used ormolu for the decorative mountings of furniture, clocks, lighting devices, and porcelain.
The manufacture of true ormolu employs a process known as mercury-gilding or fire-gilding, in which a solution of mercuric nitrate is applied to a piece of copper, brass, or bronze; followed by the application of an amalgam of gold and mercury. The item is then exposed to extreme heat until the mercury vaporizes and the gold remains, adhering to the metal object.
This process has generally been supplanted by the electroplating of gold over a nickel substrate, which is more economical and less dangerous.
In literature there is a 1612 reference from John Webster:
Hang him; a gilder that hath his brains perished with quicksilver is not more cold in the liver
After around 1830, legislation in France had outlawed the use of mercury, although it continued to be commonly employed until around 1900 and was still in use around 1960 in very few workshops. Other gilding techniques, like electroplating from the mid-19th century on, were utilized. Ormolu techniques are essentially the same as those used on silver, to produce silver-gilt (also known as vermeil).
A later substitute of a mixture of metals resembling ormolu was developed in France and called pomponne, though the mix of copper and zinc, sometimes with an addition of tin, is technically a type of brass. From the 19th century the term has been popularized to refer to gilt metal or imitation gold.
Official source
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