The heat-up was not slow enough. This kiln has no air flow. This piece was very thick. What would it take to fire this? Possibly days of holding at 250F to drive off pore water. Then days of slow heat up.
Body Cracking and Dunting During Firing
Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.