Since phosphoric acid content in the glaze causes emulsion turbidity, bone ash containing large amounts of phosphate lime is used as an emulsifying agent for low-firing glazes. However, when used in large quantities or in high-firing glazes, it can cause loss of luster, pinholes, and glaze skipping. Bone ash porcelain, which has long been made in England and other countries, is mixed with bone ash as a fluxing agent to obtain a white color after firing. Bone ash can be made from any animal bone, but nowadays all bone ash is made from cow bones. This is because they contain less iron oxide as an impurity. Beef bones are boiled in boiling water, dried, and burned until only a small amount of white remains. If the bones are fired until they are completely white, the plasticity of the clay is lost. Bone ash should contain at least 75% calcium phosphate. (Ceramics for Crafts)
