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These two unglazed pieces are the same clay fired to the same temperature. But the one on the right has 1% talc added. Greying of the straw color is a characteristic visual change when this clay body goes from cone 5 to 6. It is accompanied by a 1% drop in porosity (according to our SHAB test). This tiny amount of talc has another detectable benefit: It is also acting as a grease, the body has a noticeably smoother feel and better workability on the wheel. Talc is refractory, this curious firing benefit only happens in small amounts, beyond about 3% it has no effect. Beyond that it makes the body more refractory.
Materials |
Talc
A source of MgO for ceramic glazes, a flux or thermal expansion additive in clay bodies, also used in the manufacture of cordierite. |
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Glossary |
Vitrification
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose. |
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