Bone Ash (TriCalcium Phosphate) is source of calcium and phosphate traditionally used in bone china for translucency and lowering the maturing temperature. The calcium acts as a flux, while the phosphorous acts as a glass former. When added to a clay body such as bone china, bone ash lowers the maturing temperature and adds translucency.

Bone ash may also be used to give texture in low fire glazes. In low-fire glazes, bone ash acts as an opacifier, while in high-fire glazes it can create translucence. You may find that it is rarely used in glaze recipes; this may be because bone ash can have a tendency to cause the glaze matrix to thicken, or materials within it to flocculate, or clump together. If you mix a glaze containing bone ash, it is recommended that you use a little bit of deflocculant such as Darvan 7.