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Ormolu is a process for gold-plating brass. The gold is first dissolved in Mercury to create a mercury-gold amalgam, and this is then used to paint the exterior of the object to be plated. The piece is then placed in a furnace to evaporate the mercury (which boils at 356.7 °C), leaving behind just the film of gold (which in its pure form melts at 1,064 °C).

The ormolu process was replaced in the Twentieth Century by gold-plating, which was easier, required less specialised equipment (once electrical power supplies became commonplace), and had the advantage of not creating breathable mercury vapour (mercury being a heavy-metal toxin and neurotoxin).

Mercury toxicity

Mercury was traditionally used in feltmaking and in the extraction of gold from gold-bearing rocks (since gold is very chemically unreactive, but dissolves in mercury), and in gold-plating before electrolysis became the preferred method. This meant that hatmakers (who worked with felt) and gilders (who worked with gold-plating) traditionally had a high exposure to mercury ... as a result, hat-makers had a reputation for going mad (hence the Mad Hatter character in Lewis Carroll's Alice in Wonderland), and the life expectancy of an Eighteenth-Century gilder could be as low as thirty years.

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