This crack is characteristic of a firing dunt: stresses created when different parts of the piece are changing temperature, and therefore dimensions, at different rates. When those stresses exceed the strength of the body, a crack releases them. The quartz inversion around 570C is especially important - a temperature difference across the piece at this point means one area can be undergoing the quartz volume change while another is not.
Large, heavy pieces and those having uneven cross sections develop temperature gradients more readily. This piece has another disadvantage: its wide flat bottom is coupled thermally to a heavy kiln shelf while the vertical walls are much more directly exposed to radiant heat. That can produce substantial temperature differences during both heating and cooling, especially if one side faces the elements more directly. Slowing the firing through quartz inversion, on both the way up and the way down, and avoiding uneven radiant exposure can greatly reduce the risk.
| Troubles |
Dunting and Cracking of Clay Bodies 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. |
| Glossary |
Quartz Inversion
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down a temperature window centering on 573C. |
| Glossary |
Dunting
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. |
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