Bone ash supplies CaO and P2O5.
Bone ash is a highly variable material produced by calcining animal (cattle) bones. Similar materials include the mineral Apatite (CaF Ca4(PO4)3) and calcium phosphate (Ca3(PO4)2). Bone ash generally acts as an opacifier in glazes. Can cause crawling and blistering. In the c/9-10 range and higher, bone ash decomposes and releases tricalcium phosphate, a highly reactive substance that helps flux the rest of the glaze [Obstler].
(courtesy of glazy.org)
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