In a previous blog post, we discussed Argentium silver alloys and how they use the addition of germanium to impart a number of beneficial properties to the silver. One particularly noteworthy property was the elimination of firescale.
Firescale, (or as they say in London, firestain) is a dark discoloration caused by heating. It’s a cosmetic problem that can a real pain-in-the-neck for jewelry craftsman. The only way to fix an unsightly firescale is to either buff it off with acids or abrasives, remove the affected silver, or plate over it with a fresh layer of silver.
The reason why silver alloys are vulnerable to firescale is because of their copper content. As the silver is heated, it allows oxygen molecules to pass through. This causes the copper to oxidize well below the surface – creating firescale that is difficult to remove. However, germanium can prevent firescale because it oxidizes preferentially to the copper. Furthermore, germanium oxide is transparent.
When Argentium silver is heated, the germanium oxidizes creating a transparent layer that protects the copper in the alloy from oxygen exposure. Without that exposure, copper oxide never forms and firescale never appears. Since germanium oxide is self-regenerating, the alloy is permanently protected from firescale – making it a superior coating compared to rhodium, which will wear away eventually.