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These two unglazed pieces are made from the same clay, M340. They are fired at the same temperature. But the one on the right has 1% talc added. Greying of the color is a characteristic visual change as this clay body transitions into the vitreous state we target. That transition happens over a narrow temperature range. Because the raw materials naturally vary in the temperature at which they vitrify, we have to tune the recipe so that the transition happens from cone 5 to cone 6. It is accompanied by a drop in porosity of 2% or more (according to our SHAB test). Talc acts as a catalyst for this change; in this case, only 1% is needed. By itself, talc is refractory. Yet it acts as a flux here! The fact that it can effect this big of a change with only 1% is amazing. Interestingly, this phenomenon only occurs with tiny talc additions.
| 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. |
| Glossary |
Vitrification
A process that happens in a kiln, the heat and atmosphere mature and develop the clay body until it reaches a density sufficient to impart the level of strength and durability required for the intended purpose. Most often this state is reached near zero p |
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