|Monthly Tech-Tip |
Alternate Names: Calcium Phosphate
Bone ash is 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 calcining 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. Today the availability of super-white kaolins, low iron feldspars and processed bentonites, smectites and hectorites makes it possible for almost anyone to make very white, translucent and strong porcelains even at cone 6.
Bone ash is not common in glazes. When employed it can cause the slurry to flocculate and thicken (and produce a very thick layer on the ware surface which cracks during drying). People often react to this by adding more water, producing a glaze that shrinks even more on drying and eventually thickens again. A better way is to add a little deflocculant to the glaze slurry (like Darvan).
Up to 1-2% bone ash can be used in enamels for opacification (more will usually cause pinholes). In glazes, 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.
The whitest test bar here is a New-Zealand-kaolin-based cone 6 porcelain (NZK). NZK has low plasticity, so this body employs VeeGum to improve it. Immediately to the left of it are three North American-koalin-based bodies using standard bentonites. The bar to its right is a Grolleg-based body that uses a standard bentonite rather than a white burning one. All are plastic.
These are two cone 6 transparent-glazed porcelain mugs. On the left is the porcelainous Plainsman M370 (Laguna B-Mix 6 would have similar opacity), it is semi-vitreous and has no translucency. Right is a highly vitreous, New Zealand kaolin based porcelain, Polar Ice. The secret to making this porcelain super-white is the NZ kaolin. The secret of its impossibly-high plasticity is the very expensive plasticizer, VeeGum T. What about the translucency? Nepheline syenite is used as the feldspar, but it alone cannot deliver this kind of translucency at cone 6. Amazingly the 4% Veegum acts as a translucency catalyst, it is the secret. Commercial manufacturers could never use a sticky and difficult-to-dry porcelain like this, but a potter can do incredible things with it (e.g. throw thinner, lighter, bigger than any other clay he/she has ever used!).
Synthetic Bone Ash
Interesting history of Bone Ash
Bone Ash Calcined
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
Opacifiers are powders that turn transparent glazes opaque by various chemical and physical mechanisms (and combinations of mechanisms).
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
|Oxides||P2O5 - Phosphorus Pentoxide|
|Oxides||PO4 - Phosphorus Oxide|
|Frit Softening Point||1670C|
|Density (Specific Gravity)||3.10|
|Glaze Opacifier||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.|
|Glaze Surface Texture||Bone ash encourages strong glaze textures; however, too much can produce crazing or blistering.|
|By Tony Hansen|
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