Contaminated Silver

I recently had a large fine silver ingot taken to a machine shop to
be cut up. The resulting smaller ingots seemed to not have much, or
any foreign metal contamination when viewed by the naked eye.
However, when I went to melt the ingots I found that they had blue
specks on them. When I made some sheet with it it had lumps in it. I
put it in a beehive kiln for filligree work and it all melted into
balls, which is not supposed to happen with finesilver. It is
unusable.

I suspect that my silver is contaminated with foreign metals which
maybe happened at the machine shop (although there didn’t appear to
be any foreign material on the broken down ingots). Perhaps there was
something in the borax that I used in melting it? This seems
unlikely… Is there any way I can get the rest of my ingots to come
out clean when I melt them for fabrication?

Thanks,
Carmen