Bone ash is calcium phosphate, traditionally made from calcined animal bone, and it is the material that makes bone china what it is. In glazes it has a quite different job: phosphate encourages opalescence and the milky blue blush of the chun and jun glaze families.
In bone china
Bone china is a body containing a large proportion of bone ash alongside china clay and feldspar, and the phosphate is what gives it its translucency and its particular whiteness and strength. It is a British invention and it remains one of the few bodies where a single ingredient defines the whole material.
In a glaze
Small additions of phosphate encourage a glaze to separate into two glassy phases as it cools, and that separation scatters light, producing opalescence and a blue-white blush over darker areas. That is the mechanism behind chun glazes, and it is one of the few optical effects in ceramics that comes from structure rather than from a colourant.
Synthetic calcium phosphate
Suppliers stock both natural bone ash and synthetic calcium phosphate, and the synthetic version is generally more consistent in composition. For anyone who would rather not use an animal-derived material, the substitution is straightforward, and most recipes work with either.
Handling and buying
A fine powder with the usual precautions and no special hazards beyond that. Because glaze additions are small, a modest quantity lasts a long time; body use is a different scale entirely and is essentially an industrial rather than a studio application.
Questions people ask about bone ash
What is bone ash used for?
In bodies it is the defining ingredient of bone china. In glazes small additions give opalescence and the blue blush of chun glazes.
Is there a non-animal alternative?
Yes, synthetic calcium phosphate, which is also more consistent in composition and substitutes directly in most recipes.
How much goes in a glaze?
A small percentage. The effect is optical rather than a colour, and too much dulls the glaze.