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The mug on the left, made by Holly McKeen, is a typical cone 10 Grolleg kaolin mullite porcelain (highly vitrified, low in residual quartz). Its glaze is crazed. Crystalline glazes are high in Na2O, making crazing virtually certain. Since most pieces are decorative, crystal glazers just accept this as part of the process. But these are functional mugs, the glaze needs to fit (if only for ware strength).
But what if the thermal expansion of the body could be significantly raised? The body on the right is Crystal Ice, it contains 40% silica (vs 20-25% in a typical porcelain). The percentage of Nepheline has been reduced, lowering vitrification to about 1.5% porosity. As a result, more quartz survives undissolved and less mullite develops, raising the body’s thermal expansion. The result is a body with a much higher thermal expansion, so it can not only relieve the glaze tension but actually put a squeeze on it. There is a downside: These are less resistant to dunting and thermal shock failure during use.
Could the glaze be adjusted instead? Yes. Some of the Na2O could be substituted for Li2O, the latter is also a strong melter but has a much lower thermal expansion. Glaze chemistry could be used to source it from Spodumene (to avoid solubility issues with lithium carbonate). However, zinc-silicate crystalline glazes are very sensitive systems, so the more lithia is introduced the more likely the effect on the firing window, crystal size/density and background clarity.
| Glossary |
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated. |
| Glossary |
Crystalline glazes
A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling |
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