Thanks all-- I'm not sure if it's coffee or mold, but either way, I'm planning to refire it to see if that helps. I figure either it fixes it, or this mug goes in the bin like it would have anyway.
Amaco suggested decreasing the HF-9 glaze I used inside from 3 coats to 2, as they said it looks like the glaze pooled here, causing lots of little bubbles that eventually wore through. I plan to try that going forward, and also to run some tests with the orange underglaze. Everywhere I used that orange, it turned into a sort of orange-peel texture (no pun intended) after glaze firing.
boardbutproductive got a reaction from Roberta12 in What are these little black dots?