Fire Scale or Fire Stain Problem is as a reddish purple, brownish or blackish tones and will see in high polished which appears by Sterling Silver Casting mixed with Copper or Non De-Oxidizer Alloy which is root caused from combining of oxygen with copper presents in Sterling Silver at high temperature.
Please check:

1. Too High Casting and/or Flask Temperatures, could be used lower parameters.
2. Improper mixer, selected using the Silver Alloy with De-Oxidizers.
3. Soldering by Torch, Should apply anti-oxidation protective agent before heating.
4. Annealing the metal should be used with hydrogen, nitrogen or ammonia will be useful to protect oxidation during heating for annealing.
5. Over re-used the metal, refine scrap and start the new metals.