Not entirely sure what happened to this mug! I had adjusted this glaze - Magic White - with the addition of a small amount of silica, because it was inclined to crawl, and a bit of CMC gum, because it was very fragile in the dry state. Only this mug and one other exhibited this very weird curling and flaking, though I used the glaze on many pots in this load, which is scheduled to fire on Monday. The other possible contributing factor is that this bisque got a little hotter than usual - probably ^04 - 03, where I prefer not to bisque above 05, because the ware gets too vitrified to accept glaze well. I guess that could account for it: the body is not absorbing moisture, so the glaze is only drying from one direction.
That doesn't explain why it only happened on two pieces. Not that I'm complaining! Just a weird vagary of the ceramic process, I guess; or the action of one of those five demons I was telling you about.
No matter! I'll wash it and glaze it again - in something else, thankyouverymuch.