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A copper gold alloy is a mixture of gold and copper that is used for jewelry. It is softer than pure gold and is more malleable. It is also a much more durable alloy than gold alone. It is also more resistant to corrosion than silver. It is also easy to work and does not oxidize as quickly as pure gold. The alloy was popular in ancient Greece and Egypt and has been used since that time. It was sometimes known as electrum. It can be enriched with up to four percent cadmium to give it the color of 18 karat green gold. Fired enamels adhere better to this alloy.
A gold-silver-copper-zinc alloy can be made with varying the proportion of each element to create different colors of gold and to modify other properties. In particular, the two-phase immiscibility gap is symmetrical around the copper content not the gold content as it is in the gold-silver-copper system. Zinc is often added to this alloy to lighten the metal, decrease hardening that may occur on air cooling and improve oxidation resistance. Small fractional percentages of iridium, rhodium or ruthenium can be added to further refine the alloy and reduce its melting point.
A copper gold alloy is also called tumbaga. Tumbaga is an alloy of gold and copper that was used in pre-Columbian Mesoamerica. It is not a standard alloy but rather a generic term given by Spanish conquistadors to any of the numerous copper gold alloys in use during that period. It is more hard than pure gold but still malleable and is a very durable alloy. When pounded thin it becomes very shiny and can be gilded. It is also able to be reshaped by hammering, a process known as depletion gilding.