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It's most likely from contamination somewhere in the process of conditioning and opaquing, as I'm sure you realize. Here's what I do to avoid the same thing:
First, I use specific finishing/grinding tools for each metal and I don't cross contaminate. When using stones, I only use aluminum oxide.
Second, I fiinsh my metal in one direction, never crossing over perpendicular to previous grindings , or reversing direction. This can fold metal over and cause bubbling later on.
When I'm finished...(more)