David and all,
Firescale should not be a problem when working with sterling silver.
There are anti-firescale fluxes that prevent firescale when soldering
and there is a way to prevent it when vacuum casting.
Try home made Prips flux or one of the commercially available
firescale prevention fluxes when soldering. Check out Orchids list
for firescale prevention when vacuum casting.
Don't let firescale be a problem. Eliminate it. If you have any
questions contact me.