|Monthly Tech-Tip |
Low fire terra cotta mugs have cracked. Why? The white glaze is under compression, its thermal expansion is too low (that is why it is also shivering off the rim). As the piece is cooling the kiln the thick layer of white glaze first solidifies. As cooling proceeds the body shrinks (thermally) at a faster rate than the glaze. The puts the glaze under compression and stretches the body. As some point (e.g. last stages of kiln cooling, a thermal stress during use) the body cracks to relieve the stress (notice how the white glaze is pushing the cracks apart). Neither the body or glaze are at fault, in this case they are simply made by different manufacturers and are thermal expansion incompatible. One solution would be to mix it with a white glaze that is crazing (the opposite problem). Or you could add some nepheline syenite to the glaze to increase its thermal expansion (maybe 10% by dry weight).
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
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.