A bowl (about 9 in diameter) and abut 7 in tall really warped quite a bit during glaze firing. It looked perfectly round after bisque firing but is quite irregularly shaped after glaze firing. What did I do wrong? (speckled clay body)
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Also just piggybacking off of what was already said, the reason is that it appears in the glaze firing and not the bisque is simply because the piece is getting hotter and reaching its full temperature. Any subtle irregularities that might not have been evident in the bisque can arise once the clay is brought up to peak temp (bloating, warping, etc)
I have been told that variation in thickness in the piece combined with drying speed can cause warping. So if your bowl has a thick base and thin walls you should dry it slowly so that it is drying as uniformly as possible. You can expose the thicker areas to more air and keep the thinner areas more covered. If you think about the bowl as a spiral or spring you can imagine how it might "untwist" itself in certain conditions. (Drying rate is why handles can pop off mugs too, the handle dried faster than the cup). I don't know why it is more noticeable after glaze firing thoughn but I've had the same thing happen.
There is a podcast called “For Flux Sake” and there’s an episode called “38. does clay really have memory?”
(I haven’t listened to this episode yet, but I think this may have some good answers on your question)
Hope it helps!