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Bone Ash
Ca3(PO4)2, 3CaO.P2O5
Chemistry
Chemistry available on new site, see link above
Links to Other Materials
- Tricalcium Phosphate - Related
- Cache
Miscellaneous
- Family: Flux Source
- Region: North America
- Mined At: Unspecified
- Raw Mineral: No
- Generic: Yes
Notes
Bone ash is often theoretically Ca3(PO4)2 (triCalcium Phosphate) in the form of Hydroxyapatite Ca5(OH)(PO4)3. This reacts when making bone china to give Anorthite (CaAl2Si2O8) and Ca3(PO4)2.
2*Ca5(OH)(PO4)3 --> 3*Ca3(PO4)2 + Ca(OH)2
Real bone ash is obtained by calcinating degelatinized bone up to approximately 1100 °C and then cooling and milling. This material is still manufactured today since some of its important properties are due to the unique cellular structure of bones that is preserved through calcination. Real bone ash has excellent non-wetting properties, it is chemically inert and free of organic matters and has very high heat transfer resistance.
Bone ash has traditionally been added to porcelain to achieve a high degree of translucency (thus the name 'bone china'). The manufacture of bone china is difficult to master because the clays are non-plastic, ware is unstable in the kiln, and it is difficult to burn consistently to the body's narrow firing range.
Up to 1-2% bone ash can be used in enamels for opacification (more will usually cause pinholes). As with enamels, too much or too high a temperature will cause blistering. In this use the phosphorus' influence toward a stiff melt generally checks the fluxing action of the calcia.
Bone ash or calcium phosphate are used to opacify opal glass (1-3%) because the P2O5 content forms colorless compounds with iron impurities.
(Richard Willis)
The carbonized (calcinated) matter of animal bones. Chemically, a calcium phosphate of the form Ca3(PO4)2, with a typical empirical analysis of CaO 54.20%, P2O5 45.80%.
Bone ash is theoretically a tri-calcium phosphate. For practical purposes it is a hydroxy-apatite Ca5(OH)(PO4)3. During firing, a combined chemical and thermal reaction gives anorthite (CaAl2Si2O8) and calcium phosphate [Ca3(PO4)2].
[2Ca5(OH)(PO4)3 becomes 3Ca3(PO4)2 + Ca(OH)2]
Real bone ash composition is variable but usually contains around 67-85% calcium phosphate, 3-10% calcium carbonate, 2-3% magnesium carbonate, and traces of caustic lime and calcium fluoride [4Ca3(PO4)2 CaCO3]. see apatite
“Bone China” is normally made of 40-50% calcined bone (as finely milled bone ash) together with kaolin and Cornwall stone. The calcium fluxes the silica while the phosphorus stiffens and controls the glass melt during firing.
Bone ash additions to glaze batches rarely exceed 1-2% since larger amounts induce pinholes. Rapid or excessive temperatures lead to blisters.
For glazes a sodium phosphate is sometimes used instead of this calcium phosphate owing to a more consistent behaviour in holding colors during firing.
Use in low temperature opacity tends to a milky effect. In supplement to tin, glossy glazes can be sheened down. Pre-fire “dusting” of bone ash over any glaze can induce crazing.
Properties
Glaze Opacifier - White
Low temperature glazes sometimes employ bone ash for opacity because of the milky quality it produces. It can also be used to assist tin oxide where a less shiny surface can be tolerated.Surface Modifier - Texture
Bone ash encourages strong glaze textures; however, too much can produce crazing.
Data
- Melting Point (MP): 1670C
- Specific Gravity: 3.10
- Specific Gravity: 3.10
- Melting Point (MP): 1670C
URLs
- Synthetic Bone Ash - http://www.murlinchemical.com/syn1.html
- Ebonex Corp. - http://www.ebonex.com/ba_pro.htm
- Interesting history of Bone Ash - http://www.ebonex.com/hist.htm
Suppliers
- Generic
Authors
- Tony Hansen (Owner)
XML
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