Firescale also known as firestain, is a reddish purple or smokey brown stain which can appear on mixtures of silver and copper an example of which is sterling silver. It is caused at high temperatures as oxygen interacts with the copper. Attempts to reduce firescale usually involve replacing oxygen with another gas such as hydrogen or ammonia.
Firescale can form on the surface of sterling silver during the use of a burning hot tool. It 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 immediate polishing or removal of the affected metal.
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