Impact of post-depositional processes on charcoal fragmentation and archaeobotanical implications: experimental approach combining charcoal analysis and biomechanics

Generally speaking, charcoal analysis is based on identifying and counting charcoal fragments in order to calculate the relative variations in taxa frequency. All post-depositional processes are likely to induce fragmentation of the anthracological material, raising the question of the representativeness of taxa. Based on an innovative experimental approach combining both charcoal analysis and biomechanics, this paper explores how the mechanical properties of charcoal can influence the fragmentation and the quantification of species in anthracological assemblages. We carried out standardized laboratory compression tests on 302 samples issued from 10 taxa, charred at three different temperatures, in order to characterize the mechanical properties of common species in temperate and Mediterranean Europe. Our results highlight the differential responses of the tested species in terms of resistance to compression and fragmentation, two processes which do not appear to be correlated. Charcoal is very resistant to pressure (up to 22.5 MPa). Our results show that significant fragmentation differences exist between taxa. The total number of fragments after compression is largely dependent on the species, regardless of the charring temperature. However, this interspecific variability is more significant for small fragments [1-2 mm], than for larger fragments [2-4 mm] and >4 mm, with the exception of Quercus, which displays differential reactions to compression. Finally, a multifactorial analysis brings to light the impact of the physical and anatomical characteristics of the different species on charcoal fragmentation. (C) 2014 Elsevier Ltd. All rights reserved.