Firescale | Subtle Mark Similar to a Tea Stain
Firescale is a reddish brown coloured mark similar to a tea stain which can appear on metals alloyed with copper such as sterling silver. The mark appears when the metal is being worked on at high temperatures in the presence of oxygen. An example could be when silver is being soldered.
The firescale mark is caused by a reaction between oxygen and the copper in the metal. The best way to prevent firescale from occurring is to eliminate oxygen from the surface of the metal whilst it's being heated.
In fine sterling silver firescale is usually relatively unnoticeable. If the silver is allowed to tarnish however as it becomes darker the mark can become more visible.