Bone ash is a white material produced by thecalcination ofbones. Typical bone ash consists of about 55.82%calcium oxide, 42.39%phosphorus pentoxide, and 1.79%water.[1] The exact composition of these compounds varies depending upon the type of bones being used, but generally the formula for bone ash is Ca5(OH)(PO4)3. Bone ash usually has adensity around 3.10 g/mL and amelting point of 1670 °C (3038 °F). Most bones retain their cellular structure through calcination.
Burnt bones have been recovered from numerous Ancient Greek sanctuaries dating from the Late Bronze Age up to the Hellenistic period. The burnt bones are often calcined with a white or blueish color, allowing archaeologists to identify them as sacrificial remains. At the sanctuary toArtemis inEretria a round altar of fieldstones filled with soil was found, dating to the 8th century BC. The upper surface was covered with clay and animal bones were burned on top, then apparently swept off the surface with terracotta, metal objects, and pottery and trampled until the altar was eventually subsumed by the ritual debris. Some scholars have attributed these altars tochthonian rituals, but this is disputed.[2]Xenocrates of Aphrodisias reported its use as a medicinal ingredient, althoughcannibalism was, according toGalen, prohibited under the laws of theRoman Empire.[3]
Bone ash is a keyraw material forbone china. Constituting around 50% of the body, it reacts with other raw materials in the body during firing to form, amongst otherphases,anorthite.
In preparation for use in bone china, bones undergo multiple processing stages, including:
Since the 1990s, the use of synthetic alternatives to bone ash, which are based ondicalcium phosphate and tricalcium phosphate, has increased. Significant amounts of bone china is produced using these synthetic alternatives rather than bone ash.[4]
Bone ash can be used alone as anorganic fertilizer or it can be treated withsulfuric acid to form a "singlesuperphosphate" fertilizer which is morewater soluble:[citation needed]
Similarly,phosphoric acid can be used to formtriple superphosphate, a more concentrated phosphorus fertilizer which excludes thegypsum content found in single superphosphate:[5]
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Bone ash is used infoundries for various purposes. Examples includerelease agents and protective barriers for tools exposed to molten metal, and as asealant for seams and cracks.[citation needed] Applied as a powder or water slurry,[vague] bone ash has many unique characteristics. First of all, the powder has highthermal stability, so it maintains its form in extremely high temperatures. The powder coating itself adheres to metal well and does not drip, run, cause muchcorrosion, or create noticeable streaks. Using the bone ash is easy as well, as it comes in a powder form, is easy to clean up, and does not separate into smaller parts (therefore requiring no extra mixing).[citation needed]
Bone ash is a material often used incupellation, a process by whichprecious metals (such asgold andsilver) are removed frombase metals.
In cupellation, base metals in an impure sample are oxidized with the help oflead and are vaporized and absorbed into a porous cupellation material, typically made ofmagnesium orcalcium. This leaves the precious metals which do not oxidize behind. Bone ash's extremely porous and calcareous structure as well as its high melting point makes it an ideal candidate for cupellation.[6][7]
The chemical analyses, determined byX-ray fluorescence and reported as %, of three samples of ceramic grade bone ash:
| Reference | [8] | [9] | [10] |
|---|---|---|---|
| SiO2 | 0.02 | 0.90 | 1.08 |
| Al2O3 | 0.01 | 1.90 | 0.34 |
| Fe2O3 | <0.10 | 0.30 | 0.19 |
| TiO2 | <0.10 | - | 0.01 |
| CaO | 55.3 | 50.5 | 52.2 |
| MgO | 1.11 | 1.40 | 1.35 |
| K2O | 0.40 | 1.35 | 0.12 |
| Na2O | 1.33 | 2.15 | 0.51 |
| P2O5 | 41.9 | 37.2 | 44.2 |
| MnO | <0.10 | - | - |
| Loss on ignition | 0.0 | 4.3 | - |
FromIsaiah: "And the people shall be as the burnings of lime: as thorns cut up shall they be burned in the fire"
Its use is mentioned in theBook of Amos (2:1): "I will not turn away the punishment thereof, because he burned the bones of the King of Edom intolime."
It was used in ancient formulas for white paint and cosmetic pigments, and in thecupellation process to separatesilver fromlead.[11][12]