Differential Burning, Recrystallization, and Fragmentation of Archaeological Bone

Abstract

This paper presents research on the conditions under which progressive levels of burning may occur to archaeological bone, and how burning damage changes bones' crystal structure and susceptibility to fragmentation (a.k.a. friability). Experiments were conducted to simulate common patterns of high-temperature bone diagenesis and fragmentation previously documented in Paleolithic shelter sites. Bones buried up to 6 cm below the coal beds of the experimental fires were carbonized, but calcination occurred only with direct exposure to live coals. Analysis by infra-red spectroscopy reveals that marked changes in crystallinity accompany the macroscopic transformations in colour and friability of modern, fire-altered bone; specifically, a monotonic, non-linear decrease in mean fragment length across six colour categories was observed when samples were agitated or trampled, and a concordant decline in bone identifiability, first with respect to skeletal element and ultimately the recognizability of bone tissue itself. These findings help qualify the behavioural and taphonomic implications of fragmented, burned bones in archaeological sites, especially with regard to potential stratigraphic associations between artefacts and hearth features in sites and the intensity of space use by human occupants. The identification of burning damage on archaeological bone is a separate issue, however. It was found that the molecular signatures of recrystallization in modern burned bones partly overlap with recrystallization caused by weathering after only 1 to 2 years of exposure in an arid setting and by partial fossilization of archaeological bones over the long term. While infra-red and X-ray diffraction techniques effectively describe heat-induced changes in modern bone mineral and are an important aid for modelling diagenetic processes, these techniques did not reliably identify burning damage to archaeological bones. Cross-referencing readily visible colour phases with HCl-insoluble fraction data proves much more effective and economically feasible for the latter purpose.