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Bone ash makes up about 50% by weight of the final Bone China body recipe. It is produced from animal bone, which is first processed to remove any adhering meat which is generally sold as pet food. The bone is then treated to remove glue, which is processed and upgraded for use in applications where glue is used, and also for the sizing of expensive paper. The remaining raw bone is then heated to over 1000 C to burn off any residual organic material and change the structure of the bone to a form suitable for the manufacture of bone china. The high temperature used also sterilises the bone. The bone is ground with water to a fine particle size before inclusion in the bone china body.
Real bone ash chemistry
The typical analysis of calcined bone is 67-85% calcium phosphate, 3-10% calcium carbonate, 2-3% magnesium phosphate, and small amounts of CaO and CaF2. However this can vary significantly depending on the source of bones and the process used. The approximate formula is 4Ca3 (P04)2.CaO.
Although raw bone is chemically Ca3 (OH) (PO4)3 hydroxyapatite many authors ignore the hydroxyl group as they believed it is removed on calcination?
Real bone ash chemistry in bone china
On firing as a constituent of a bone china body the bone ash works in a complex manner. The free lime reacts with the stone (type of feldspar mineral) to produce Anorthite (Ca feldspar crystal), whilst the phosphate produces a complex glass and leaving a high level of residual crystal Beta tri-calcium phosphate Ca3 (PO4)2. It is considered that English bone china contains 70% crystals (mix of phosphate and silicate crystals) and 30% glassy material.
Bone ash substitutes
Bone ash and calcium phosphate are both used almost interchangeably in the market although bone ash is a relatively crude product and calcium phosphate relatively pure. The properties of Ca3(PO4)2 and bone ash are however quite different. Bone ash retains its cellular structure even after calcination. Direct substitution of one grade for another is not always easy. For example the replacement of real bone ash by the synthetic bone 1 creates several problems. These include colour, chemistry reformulation and processing issues.
A number of potential substitutes exist for bone ash used in ceramic bodies as follows:-
Tricalcium phosphate-mineral route
Phosphate rock in the form of apatite (Ca5 (PO4)3(OH,F) can be calcined to produce principally tricalcium phosphate Ca3(PO4)2 synthetic bone (1)
Tricalcium phosphate-chemical route
Tri calcium phosphate Ca3(PO4)2 can be produced by a costly chemical route by reaction of phosphoric acid with lime solution.-synthetic bone (2)
Dicalcium phosphate-mineral route
Phosphate rock in the form of apatite (Ca5 (PO4)3(OH, F) can be used to produce Dicalcium phosphates in both the anhydrous and hydrated form. Although chemically similar these are not the same as bone ash but are often marketing as synthetic bone. –synthetic bone 3 and 4
Dicalciumphosphate (CaHPO4.2H2O) and CaHPO4 (anhydrous) are chemicals produced through precipitation from phosphate rock which are demineralised first by
a)sulphuric acid dissolution and then precipitated by CaCO3
b) hydrochloric acid treatment and then precipitated by a saturated lime solution Ca(OH)2.
Dicalcium phosphate-bone route
Processing of animal bone to dicalcium phosphate CaHPO4 (anhydrous) by chemical processing can produce a synthetic bone ash 5 as follows:-
Dicalciumphosphate (CaHPO4.) (anhydrous) is produced through precipitation from degreased bones which are demineralised by hydrochloric acid treatment and precipitated by a saturated lime solution Ca(OH)2.
Author – Ivan Wozniak
Nowadays the product used is synthetic and it is part of the tricalcium phosphate in glazes. It is considered a material for high melting temperature (calcium content) due to its phosphorus content which is glass-forming.