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Ormolu (French). An alloy of copper, zinc, and tin, having the colour throughout of gold. It is similar in appearance to gilded brass and gilt bronze. Today the term is loosely applied in England and the United States to brass or bronze gilded by the process of fire gilding or mercury gilding, or even to copper so gilded as a cheap imitation. Ormolu has been used in England as a mount for objects of porcelain or glass, and only to a limited extent in jewelry.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson
Ormolu (from French or moulu, signifying gold ground or pounded) is an 18th-century English term for applying finely ground, high-karat gold to an object in bronze. The French refer to this technique as bronze doré, which is used to this day though the item may be merely painted with a gold-tone paint. The modern term in English is gilt bronze, which bears no relation to ormolu in process or materials. A later substitute of a mixture of metals resembling ormolu was developed in France and called pomponne, though, confusingly, the mix of copper and zinc, sometimes with an addition of tin, is technically a type of brass. Actual ormolu pieces by strict definition are rare, as they were no longer produced after the 1830s because of their impractical cost and health risks.
The manufacture of true ormolu employs a process known as mercury gilding or fire gilding, in which a solution of nitrate of mercury is applied to a piece of copper, brass, or bronze, followed by the application of an amalgam of gold and mercury. The item was then exposed to extreme heat until the mercury burned off and the gold remained, adhered to the metal object. Most mercury gilders died by the age of 40 due to exposure to the harmful mercury fumes. This gilding technique is similar to that also used on silver, which produced silver-gilt objects known as vermeil.