I've never had a lid from an ^04 piece warp in firing before. I didn't have enough to do my own firing, so I figured it would be okay to let the grad students fire my work along with the other students' work. What I have learned from this is that it's time for me to get my own kiln.
^6 plate. This was a flat plate until it got fired and it warped. I've never had a problem with the glaze on the bottom of the plate. I suspect the kiln was fired fast, there was no hold at the top, and it was cooled way too fast.