7th Experimental Archaeology Conference Abstract – Mike Charles et al

Some like it hot: the effects of charring on crop remains

Mike Charles, Glynis Jones, Emily Forster, Michael Wallace, Nick Fieller and Eleanor Stillman

The preservation of plant remains by charring is one of the major sources of information regarding the economy and environment of the past.  However, despite the importance of charring to archaeobotanical research, the conditions that led to preservation of plant material recovered from archaeological sites, and the effects of charring processes on the remains themselves are poorly understood. For example seed size may relate to growing conditions and/or genetic diversity between or within plant populations, but the measurement of charred seeds is of limited utility if the biases and morphological changes caused by charring are not taken into account. To address these issues a series of experiments was designed to test the impact of different charring regimes on modern grains of emmer (Triticum dicoccum).  Grains were charred in anaerobic conditions, with the variables altered being temperature and duration of heating.  Simple measurements (e.g. length and breadth) were complemented with more sophisticated computer-aided morphometric analyses. Grains were photographed before and after charring, facilitating one-to-one comparison of grain appearance, size, mass and morphology.  Grains from each experiment were also cross-sectioned and examined under both a high-powered light microscope and a scanning electron microscope, to examine the effect of different heating regimes on the internal structure of the grain.

Previous attempts to establish the minimum temperature at which crop plant remains would be charred sufficiently to survive within the archaeological record (e.g. Braadbaart and van Bergen 2005; Braadbaart 2008) concentrated on the apparent degree of carbonisation during relatively short periods of heating (with a maximum duration of two hours), but failed to account for drastic differences between the appearance of grain charred experimentally and that typically recovered from archaeological sites.  Using the techniques outlined above, we believe we have identified a fairly narrow range of temperatures and durations of charring that produce emmer grain comparable to that which archaeobotanists would consider to be ‘well-preserved’. Having established the impact of different charring regimes on plant material, it is possible to estimate the degree of distortion to which archaeobotanical remains were subjected during the charring process.

References

Braadbaart, F. 2008. Carbonisation and morphological changes in modern dehusked and husked Triticum dicoccum and Triticum aestivum grains.  Vegetation History and Archaeobotany 17: 155-166.

Braadbaart, F. and van Bergen, P.F.  2005. Digital imaging analysis of size and shape of wheat and pea upon heating under anoxic conditions as a function of the temperature.  Vegetation History and Archaeobotany 14: 67–75.

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