|Monthly Tech-Tip |
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.
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.