Impact of prolonged heating on the color and crystallinity of bone

Duration of an anthropogenic fire event is one aspect of fire use and maintenance that is linked to combustion feature function but has low archaeological visibility. In this study, we describe the transformations to fresh, modern cortical bone with prolonged exposure to heat in order to evaluate the utility of archaeological bone for the recognition of long duration thermal alteration. Cores of bovid cortical bone were heated exposed to air at 300, 550, and 750 °C in a sequence of experimental trials in a Nabertherm muffle furnace for periods of 10 minutes, 9 hours, and 48 hours, plus an extensive cooling period on heat retaining sediments (gravel or gravel compacted with fine quartz sand) to mirror the smoldering and extinguishing of actualistic fires. After heating, bone cores were analyzed with a color tool, Fourier-transform infrared spectroscopy, and X-ray diffraction to evaluate changes in structure, composition, and crystallinity of bioapatite as a function of different temperature thresholds and time. Results indicate that prolonged heating in air induces specific structural and chemical changes in bone compared to shorter duration burned counterparts. Coloration changes also demonstrate that white coloration, a primary characteristic utilized by zooarchaeologists to record information about burning intensity, is not an exclusive indicator of calcination at moderate to high temperatures but may also result from long duration exposures at low temperatures.