I have a problem with testing my silver i hope someone can help with im useing .9999 silver grain for my bars and its testing like it should but when i acid test the bars after i pour them the acid is showing up grayish im useing a graphite crucible and mold and a electric furnace im stumped i thought at first the silver testing acid was bad so i bought a new bottle with the same result ive never had this issue before any ideas on what could be causeing this would be greatly appreciated