They bind silver filings with saltpeter which refines it & does not make it brittle. But gold filings are bound with borax or, to save the borax, with lead, which refines the gold & softens it, for the saltpeter would make it brittle, which it does not do to silver. This is why, in order to save, goldsmiths use it to bind it in order to save borax, which costs viii sols per ounce & saltpeter costs x sols per lb. When goldsmiths have thus bound their silver filings with salpeter, a red enamel vitrifies at the bottom of the crucible. I do not know if the copper mixed in with the silver is the cause for this. Try for enamel.