I use silver wire in hollow wax designs as reinforcing rods. Some
times I have the same problem you described. The silver wire is
coated with a hard coppery colored investment layer.
I break off the coating and use the wire as scrap silver in my next
pour. The wire is just like you described. It is rough. I have
noticed that the wire does not melt as rapidly as sterling silver.
It may be just a stupid guess but I believe copper is precipitating
out of the sterling and imbedding itself into the investment around
the silver. That is what causes the hard copper colored investment.
Is it possible the silver has been somewhat refined into fine silver
and melts at a higher temp?
I find the coating problem occurs in flasks that are in the back of
the burn out surrounded by other flasks. My guess is that these
flasks see a higher temp than the ones in the front of the burnout
You might try lowering the burnout temp.