Pushing enough to break the walls off the base. The body is a high-manganese black-burning stoneware. It is not just the higher thermal expansion of the glaze that is putting it under compression (as evidenced by the vertical crack that is being pushed apart), but also the fact that the glaze is applied thickly.
In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.
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