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Why after applying heat onto my gemset silver ring, have they both changed colour?

If you are using sterling silver, this is an alloy that contains 92.5% silver and copper. If heated to a temperature of 650ºC, in air; the copper forms a deep black oxide. This is known as firestain.

To prevent this, we recommend using argotect which is mixed to a paste with water or methylated spirits and painted on the article prior to being heated.

With the gemstone, many semi-precious stones change their chemical composition when heated. We recommend that the stones are set after any heat application.