I agree with Jeff. It's likely contamination of the metal from finishing stones. We had problems when a stone used for removing oxides from implant cases before polishing was then mistakenly used to finish...See more copings. If any of the metal from the machined gold interface gets into the stone, it can no longer be used for finishing.
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Graduated from J. Sargeant Reynolds Community College Richmond VA in 1980. Worked at Toothworks Lab. for 4 years. Opened Dominion C & B Lab. in 1984 in Leesburg Va. Have been running it as a small lab. ever since. Married with 2 kids.