Bone Ash

Bone ash supplies calcium and phosphorus, the phosphorus helps form glass, encouraging opacity, micro-crystalline textures, or broken/variegated effects in low- and high-fire glazes. A small amount in a glaze can give a milky blue white opalescence, a blue that comes from the structure of the glass and not from any colourant. Very little is needed, and too much makes a glaze crawl or blister. In a clay body it helps the body vitrify and become translucent, and that is where bone china gets its name, with bone ash as a major ingredient. It acts as a flux only at high temperatures. On its own it is refractory.