BOAS
A calcium phosphate — a source of both calcium and phosphorus · A glaze and clay body material · Small glaze additions give a milky blue-white opalescence, the chun effect · In a clay body, promotes translucency · A flux at high temperatures; refractory on its own
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Bone ash is a calcium phosphate, and the material bone china is named for. A small addition to a glaze gives a milky blue-white opalescence.
Bone ash supplies two oxides, calcium and phosphorus, and the phosphorus is what makes it interesting. Phosphorus makes a melting glaze separate into two glassy phases at a very fine scale, and light scattering off that structure gives a milky blue-white opalescence — a blue that comes from the structure of the glass itself rather than from any blue colourant. That is the effect behind chun glazes, and very little bone ash is needed to get it. Too much of it in a glaze tends to crawl or blister instead. In a body rather than a glaze the phosphorus does something different — it helps the body vitrify, which is where bone china gets its translucency, and that is where bone ash goes in as a major ingredient rather than a trace addition. It fluxes as well, though only at high temperatures; on its own it is refractory.
