The calculated thermal expansion of this glaze is 8.8, very high because of its high KNaO and low SiO2. These plates are unglazed on the underside. The upper one has a well-fitting glaze and its base remains flat. On the lower one, the high-expansion glaze on the inside contracts much more on cooling and, because it is bonded to the body, ends up in tension. The resulting stress is enough to pull the plate out of shape, bowing the base upward. This issue is common with ceramic tile, where an otherwise flat tile bows as the two layers contract differently on cooling.
A glaze this far into tension will also be highly prone to crazing. And its combination of high alkali and low silica raises another red flag: chemical durability. The solution is to reformulate, reduce KNaO in favour of lower-expansion fluxes (among which MgO is most effective), and increase SiO2 as much as the desired melting behaviour and surface will permit.
| Glossary |
Thermal Expansion in Ceramic Bodies and Glazes
The co-efficient of thermal expansion of ceramic bodies and glazes determines how well they fit each other and their ability to survive sudden heating and cooling without cracking. |
| Glossary |
Calculated Thermal Expansion
Calculation of the thermal expansion of glazes is a very useful comparative glaze-fit tool. But it’s not a laboratory measurement. |
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