Combusted bone: Physical and chemical changes of bone during laboratory simulated heating under oxidising conditions and their relevance for the study of ancient fire use

Abstract

Heated bones are an important part of the material remains within the archaeological record and, among others, can reflect aspects of hominin diet, fuel management and funerary practices. A better understanding of the properties of heated bone potentially provides a better understanding of hominin behaviour and the timing and nature of fire use and control. When bone is heated in a fire, thermal alteration (charring) and oxidation (combustion) processes result in changes of the physical and chemical properties of the bone, depending on the heating intensity (temperature and duration) and oxygen availability. We conducted a series of controlled laboratory-based heating experiments (200–900 °C) under oxidising conditions (i.e. combustion) and analysed the samples using a variety of different analytical techniques. Although many studies on the heating of bone exist, these focus just on combusted bone. We, however, are able to directly compare the combustion of bone to the thermal degradation of bone under reducing conditions by applying the same experimental protocol with the same bone material, and similar analytical techniques, as in our previous work on charred bone. This allows us to not only distinguish between bone heated to different temperatures but also to distinguish between bone heated in the presence or absence of air, which might occur, for example, when bone is heated while buried. As bone responds differently to heat in the presence or absence of oxygen, being able to distinguish between both heating conditions is vital in reconstructing the temperature and other heating conditions. Together the reference data and analytical techniques presented in our current and previous study provide the archaeologist with a robust toolkit to determine the temperature and oxygen availability of heated archaeological bone, which will help to reconstruct aspects of hominin fire-related behaviours, including early food processing.