What is Firescale?
Firescale Resembles a Tea Stain
Firescale is a reddish-brown mark with a similar appearance to a tea stain. It can appear on metals alloyed with copper such as sterling silver.
Firescale is caused when oxygen comes into contact with the copper alloy whilst the metal is being worked on at high temperatures. During this chemical reaction oxygen oxidises the copper which causes a copper oxide to come to the surface. That causes the mark known as firescale.
The best way to prevent firescale on silver is to eliminate oxygen from the surface of the metal whilst it's being heated. This can be done in a number of ways, some are more successful than others.
Firescale marks often appear when a piece of silver jewellery is being soldered. The heat from the solder iron reacts with oxygen in the air.
In fine sterling silver firescale marks are usually relatively unnoticeable. If the metal is allowed to tarnish, as it becomes darker the mark tends to become more visible.