What is fire stain on silver?
By Jared James · Last updated 21 May 2026
Definition
Fire stain, also called fire scale, is the grey or purplish discolouration that appears on sterling silver when it is heated during soldering or annealing. It forms because sterling contains copper, which oxidises when exposed to heat and oxygen and migrates to the surface as a cuprous oxide layer. The frustrating thing is that fire stain sits just below the surface and can reappear through light polishing, requiring more thorough abrasion or a surface treatment such as rhodium plating to fully conceal it.
Frequently asked questions
- Why does fire stain appear on sterling silver but not fine silver?
- Fine silver is 99.9% pure with no copper, so there is nothing to oxidise and form the stain. Sterling silver is 92.5% silver with 7.5% copper, and it is that copper content that causes fire stain when heated.
- How do jewellers remove fire stain?
- It can be removed by pickling the silver in a mild acid solution after heating, and then polishing or abrasive finishing to take away the oxidised layer. For stubborn stain, more aggressive abrasion or rhodium plating can be used as a final remedy.
- Can fire stain be prevented?
- Using a boric acid and alcohol firecoat before heating helps significantly by creating a barrier that slows oxidation. Working in a reducing flame rather than an oxidising one also reduces staining, though it is difficult to eliminate entirely.
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