What is Firescale?
Firescale | Mark Similar to a Tea Stain
Firescale is a reddish brown mark similar to a tea stain. It 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.
Firescale is caused by a reaction between oxygen and the copper in the metal. The best way to prevent these marks 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 metal is allowed to tarnish, as it becomes darker the mark can become more visible.