A crack has released the stresses produced by uneven thermal contraction during cool-down in the kiln. This usually happens by cooling too quickly through quartz inversion.
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.
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down through 573C. Fired cracks are often related to this.
Dunting generally refers to cracking that occurs in ceramic ware as it is cooled in the kiln. The reasons for that cracking can be many.