Preservation of burned bones: An investigation of the effects of temperature and pH on hardness

Burned or cremated sub-fossil bone material is normally handled and treated as unburned bones, although knowledge of physical and chemical properties of bones heated to different temperature levels is limited. Therefore, bones of modern roe deer (Capreolus capreolus) were heated at temperatures ranging from 100 to 1000 degrees C and then exposed to pH 3, 5 and 10. Bone samples from each temperature level were studied, and deterioration was evaluated using weight and hardness measurements. A new method for hardness testing of inhomogeneous materials was used. Significant high weight losses were observed for bone samples heated at 200 degrees C at all three pH values investigated. These were related to the denaturation temperature of bone collagen at 155 degrees C. Regardless of acidity treatment, very low hardnesses (much lower than for unheated bone) were observed for bone samples heated at 400-900 degrees C. At 1000 degrees C increased hardnesses were observed. This has implications for the handling of burned bones. Finally, it is suggested that reported discrepancies in the composition of calcined bone at different burning temperatures may be related to the Ca:P ratio of the initial bone hydroxyapatite before burning.