Firescale is a term used to describe a smokey brown coloured mark which sometimes appears on sterling silver while its being worked on at high temperatures.  An example could be a piece of jewellery that's being soldered.  Firescale looks like a tea stain and is caused by a reaction between oxygen and the copper alloy in the silver.  The best way to prevent firescale from occurring is to eliminate oxygen from the surface of the metal whilst it's being heated.