Firescale

Firescale, also known as firestain, is a red or purple stain that appears on mixtures of silver and copper, such as sterling silver. At high temperatures, oxygen mixes with the copper to form cuprous oxide and then cupric oxide. Attempts to reduce this problem usually involve replacing oxygen with another gas such as hydrogen or ammonia.

With a very high copper content, this may appear on gold mixtures.

A fire scale is a layer of oxides that forms on the surface of metal when a blacksmith heats a tool.

Fire scale commonly occurs during the soldering process in jewellery manufacture, for example soldering a bale onto a finished pendant. The stain can only be removed with further polishing, or removal of affected metal.