Someone told me that I could place this back into the kiln and fire it again to remove the black.
I did just that and found that it does not go away! I also tried pickleing it in Sparex, it still didn't fade away!
The only way to remove it is to sand!
So, fair warning to everyone. Do not use an investment plug to control the ring size. Wrapping it with fiber paper didn't work either. For the sterling silver metal clay, create the ring three sizes up from the wanted finished size allowing it to shrink down close to the size needed.
Until next time!