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Gold has captured the human imagination since ancient times, mainly due to its rarity and lustrous beauty. It does not corrode or rust and is extremely malleable, allowing it to be shaped into forms (including some that are so thin that light can pass through them). Its ability to resist corrosion, conduct electricity and heat, and sustain its shape are other reasons why it has been valued.
During the manufacture of jewellery, it is often necessary to add other metals to gold in order to improve its workability. These metals are referred to as alloys. Copper is one of the most common additions. When it is combined with gold, it helps to lower the melting point, reduce its surface tension and make it harder. It also has the unique property of dissolving in water, which allows it to be pounded into sheets and then treated with acid to remove the copper, leaving behind a hard, durable gold-copper sheet — a process known as depletion gilding.
In addition to the well-known yellow and white alloys of 10 and 14 karat gold, other colours can be obtained by varying the nature and content of base elements used in conjunction with gold. Colouring is achieved by forming coloured oxide layers during heat treatment in air, a process called surface oxidation. This can be controlled to a certain extent by modifying the composition of the alloy or by annealing after the article has been cold worked and then cooled.