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Pushing hard enough, apparently, to spontaneously split the wall vertically and separate it from the base. The body is a high-manganese, black-burning cone 6 oxidation stoneware. The open vertical crack is evidence of enormous residual hoop stress in the wall. When a glaze has a lower thermal expansion than the body, the body contracts more during cooling and squeezes the glaze into compression. In resisting that contraction and can, in effect, push outward on the wall, putting the ceramic body into circumferential tension. A vertical crack releases that tension by allowing the circumference to open slightly.
Thickness matters. If the same glaze was applied thinly, or at least equally to both surfaces, the piece might have survived.
| Glossary |
Glaze Compression
In ceramics, glazes are under compression when they have a lower thermal expansion than the body. A little compression strengthens ware, too much can weaken and even fracture it. |
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