Example of a dunting crack in a flat deep cone 6 porcelain bowl. The bowl has a wide bottom that heat-sinks to the shelf, so during firing there is a temperature gradient between the walls and the base. That difference in temperature translates to stress because it means that different parts of the piece are experiencing different thermal contractions as it cools in the kiln.
Dunting generally refers to firing cracks that occur in ceramic ware as it is cooled in the kiln. The reason is generally uneven cross section or too rapid cooling.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.