The process of coating a base metal such as brass with a very fine layer of gold, much used in watch-movement finishing. For clocks it is largely confined to decorative external details such as spandrel mounts for dials. Because gold does not oxidise or tarnish it makes a good finish and gives an attractive appearance. The early gilding process was known as 'fire', or 'mercurial' gilding; in simple terms, the gold surface was obtained by rubbing it with a mercury-gold amalgam and heating until the mercury evaporated away, leaving a layer of pure gold behind. After this treatment the gilding was washed and finished by burnishing to bring up the bright quality of the metal. This method was highly injurious to workers because of the toxic effects of mercury, and in the 19th century it was replaced by electrogilding.