What is "firescale"

Firescale, also known as firestain, is a layer of oxides that forms on the surface of metals when heated, as by a blacksmith heating a tool. On mixtures of silver and copper, such as sterling silver, it is a red or purple stain. 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. It may also appear on gold mixtures with a very high copper content.

Firescale commonly develops during the soldering process in jewellery manufacture, for example soldering a bale onto a finished pendant. The stain can only be removed by further polishing, or removal of affected metal; or it can be plated over.