Monthly Tech-Tip | No tracking! No ads! |
Simply put: Glaze misfit. The glaze is under compression and it is pushing outward. That compression was created as these terra cotta pieces cooled in the kiln. After the glaze solidified, somewhere above red heat, it became a glass and began to contract. The body, to which that glaze is attached by a glass bond, had its own higher rate of contraction. The glaze has some advantages in this battle. Its thick application gives it extra power to assert its thermal expansion. The body is over-fired and has become brittle. The unglazed outsides, incised designs and varying thickness provide points of weakness where cracks can start. The body resists the relentless force from inside but the odds were stacked against it and the pieces do not even make it out of the kiln. Of course, the glaze could be applied thinner, ware could be fired lower, it could have a more even cross-section and the outsides could be glazed. All will help, but increasing the thermal expansion of the glaze (by increasing KNaO at the expense of other fluxes), is one change that would fix this issue.
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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