Gold is traditionally the most precious metal. Pure or 24 carat gold is generally too soft to be used for making objects, and so gold is usually alloyed with other metals to add strength and/or to change its colour. Gold is most commonly seen as yellow, white or rose (pink) coloured, although other colours such as green gold are sometimes found. Generally speaking, there are five hallmarks associated with gold: a crown mark; a number stamp for carats (before 1798 only 22 carat was marked, from 1798 18 carat was marked, and from 1854 15, 12 and 9 carat were marked); a date letter; a maker’s mark and an assay office mark. The carat number shows the amount of gold in the alloy. So if pure gold is 24 carat, 9 carat gold contains under half gold; the rest being other materials.
Some objects have a thin outer layer of gold. There are four types of gilded surfaces on metals:
1. Gold leaf applied with a mordant (adhesive), for example on exterior iron railings. This type is rarely seen on decorative art objects.
2. Mercury amalgam gilding, the most common form of gilding until the 1840s. A paste made of scraps of gold dissolved in mercury was applied to the object, which was then heated until most of the mercury evaporated, leaving a gold layer fused to the metal surface.
3. Electrogilding became the most common gilding method post-1840s (see Silver). Until 1940 only 24 carat gold, which was very susceptible to scratching and wear, could be plated on in this way. In 1940 a method of depositing 22 carat gold was developed.
4. Rolled gold. A ‘sandwich’, with gold or gold alloy as the ‘bread’ and gilding metal/silver/nickel in between, is rolled so that it fuses together and is then fabricated into objects.