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These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!
Materials |
Calcined Alumina
|
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Glossary |
Co-efficient of Thermal Expansion
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. |
Minerals |
Quartz
Quartz is the most abundant mineral on earth, it is the main crystalline mineral form of silica (SiO |
Troubles |
Glaze Crazing
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body. |
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