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The top bar is fired at cone 4. It contains 10% black stain (Mason 6666 cobalt-free). It is firing to about 2% porosity, thus a stoneware. The bottom two bars are the same clay but with only 3.75% of the same black stain. They are fired at cone 6 (bottom) and cone 7 (center). Cone 6 vitrifies this body to porcelain density. But at cone 7 it is over firing, beginning to expand. How is it possible that the bottom bar has only one third the amount of black stain as the top one yet fires much blacker? The glass development that happens as a product of being totally vitrified. The lesson: If you need a porcelain to fire as black as possible at a specific temperature, first make sure there is enough feldspar in the body to fully vitrify it.
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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