Bone ash

Bone ash is a white material produced by the calcination of bones. Typical bone ash consists of about 55.82% calcium oxide, 42.39% phosphorus pentoxide, and 1.79% water. 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 a density around 3.10 g/mL and a melting point of 1670 °C (3038 °F). Most bones retain their cellular structure through calcination.

Antiquity

Bone ash had many uses in Antiquity. Its use is mentioned in the Book of Amos (2:1): "I will not turn away the punishment thereof, because he burned the bones of the King of Edom into lime." It was used in ancient formulas for white paint and cosmetic pigments, and in the cupellation process to separate silver from lead.[1][2] Xenocrates of Aphrodisias reported its use as a medicinal ingredient, although cannibalism was, according to Galen, prohibited under the laws of the Roman Empire.[3]

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 to Artemis in Eretria 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 to chthonian rituals, but this is disputed.[4]

Uses

Bone china

The raw material for bone china is about 50% bone ash derived from animal bones. These bones undergo multiple processing stages during which all meat is removed and the bone is completely cleaned. Once cleaned, the bone is heated to about 1000 °C (1832 °F) so that all additional organic material is removed from the bone and the bone becomes sterilized. Lastly, the newly sterilized bone is ground with water into fine particles which can be used as a raw material for bone china. Bone ash plays an important role in the creation of bone china, as the phosphate of the bone generates beta tricalcium phosphate, and other compounds from the bone create the calcium mineral anorthite. Synthetic alternatives dicalcium phosphate and tricalcium phosphate are used as substitutes for bone ash. Most bone china are produced with synthetic alternatives rather than bone ash.[5]

Fertilizers

Bone ash can be used alone as an organic fertilizer or it can be treated with sulfuric acid to form a "single superphosphate" fertilizer which is more water soluble:

Ca3(PO4)2 + 2 H2SO4 + 5 H2O → 2 CaSO4·2H2O + Ca(H2PO4)2·H2O

Similarly, phosphoric acid can be used to form "triple superphosphate", a more concentrated phosphorus fertilizer which excludes the gypsum content found in single superphosphate:

Ca3(PO4)2 + 4 H3PO4 → 3 Ca(H2PO4)2

Machining

Bone ash is used in machine shops for various purposes. Examples include polishing compounds, protective powder coatings for metal tools, and as a sealant for seams and cracks. As a powder coating, bone ash has many unique characteristics. First of all, the powder has high thermal stability, so it maintains its form in extremely high temperatures. The powder coating itself adheres to metal well and does not drip, run, cause much corrosion, 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).

Metallurgy

Bone ash is a material often used in cupellation, a process by which precious metals (such as gold and silver) are removed from base metals.

In cupellation, base metals in an impure sample are oxidized with the help of lead and are vaporized and absorbed into a porous cupellation material, typically made of magnesium or calcium. 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]

See also

References
  1. Phosphate Minerals. Springer Berlin Heidelberg. 2010. p. 3.
  2. Charvat, Petr (2003). Mesopotamia Before History. Taylor & Francis.
  3. Dalby, Andrew. Food in the Ancient World from A to Z. Taylor & Francis. p. 73.
  4. Knust, Jennifer; Moser, Claudia (2017). Ritual Matters: Material Remains and Ancient Religion. University of Michigan Press. p. 39.
  5. Mussi, Susan. "BONE ASH - It also creates teapots. Manufacture". Ceramic Dictionary. Retrieved 22 July 2015.
  6. The Editors of Encyclopædia Britannica. "Cupellation." Encyclopædia Britannica, Encyclopædia Britannica, Inc., 29 May 2017, www.britannica.com/technology/cupellation.
  7. Bayley, Justine. "Precious Metal Refining." Archeological Datasheet, no. 2, Mar. 1995, pp. 1–1., doi:10.1016/s0026-0576(03)80479-8.
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