DIFFERENTIAL BURNING, RECRYSTALLIZATION, AND FRAGMENTATION OF ARCHAEOLOGICAL BONE

This paper presents research on the conditions under which progressive levels of burning may occur to archaeological bone, and how burning d amage changes bones' crystal structure and susceptibility to fragmenta tion (a.k.a. friability). Experiments were conducted to simulate commo n patterns of high-temperature bone diagenesis and fragmentation previ ously documented in Paleolithic shelter sites. Bones buried up to 6 cm below the coal beds of the experimental fires were carbonized, but ca lcination occurred only with direct exposure to live coals. Analysis b y 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 decre ase in mean fragment length across six colour categories was observed when samples were agitated or trampled, and a concordant decline in bo ne identifiability, first with respect to skeletal element and ultimat ely the recognizability of bone tissue itself. These findings help qua lify the behavioural and taphonomic implications of fragmented, burned bones in archaeological sites, especially with regard to potential st ratigraphic associations between artefacts and hearth features in site s and the intensity of space use by human occupants. The identificatio n of burning damage on archaeological bone is a separate issue, howeve r. It was found that the molecular signatures of recrystallization in modern burned bones partly overlap with recrystallization caused by we athering after only 1 to 2 years of exposure in an arid setting and by partial fossilization of archaeological bones over the long term. Whi le infra-red and X-ray diffraction techniques effectively describe hea t-induced changes in modern bone mineral and are an important aid for modelling diagenetic processes, these techniques did not reliably iden tify burning damage to archaeological bones. Cross-referencing readily visible colour phases with HCl-insoluble fraction data proves much mo re effective and economically feasible for the latter purpose.