The use of reflectance values for the interpretation of natural and anthropogenic charcoal assemblages

Abstract

Charcoal assemblages occur in both natural and archaeological settings. Cell wall reflectance values of charcoal in polished blocks under oil provide a proxy for temperature of formation. This paper aims to (1) determine whether wildfire charcoals and anthropological charcoals from various pyrotechnical activities can be distinguished using reflectance data and (2) establish if re-charring (i.e. use of charcoal fuel) can be recognised in the archaeological record through analysis of laboratory-produced re-charred charcoals and charcoals from an experimental iron smelt and traditional bronze casting which utilised charcoal fuel. Reflectance frequency data from assemblages representing burning of charcoal, in this case of iron smelting and bronze casting, indicates temperatures from above the mean value of charcoal production (>475°C) up to the maximum temperature reached in the subsequent process (i.e. >475 to >1,100°C). In contrast, wildfire charcoals showed a range of values including material with barely measurable reflectance (minimum values from 0.06% to 0.56%Ro) to maximum reflectance values varying from 1.65%Ro (Tilford) to 3.8%Ro (Zacca). The mean wildfire reflectance indicated temperatures in the range 325-400°C, which can therefore clearly be distinguished from that of the charcoal burning processes. The laboratory-produced re-charred charcoals take on the reflectance value of the highest temperatures experienced; reflectance values were not constrained by the original temperature of formation. High temperatures are most easily achievable by the burning of charcoal fuel, and hence high reflectance charcoals are likely to represent re-charred charcoal. Therefore, this quantitative reflectance method can be used in archaeology to determine the minimum temperature of formation of charcoals in anthropological processes which involve fire, can indicate the likelihood of use of charcoal or wood as fuel and can distinguish between an assemblage of high temperature anthropogenic charcoals and charcoals formed from natural wildfire.

Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød-Younger Dryas boundary

“…A wildfire temperature of approximately 420°C, as inferred from our reflectance analysis at Geldrop-Aalsterhut, also indicates that temperatures were slightly too low for known diamond forming mechanisms. It must, however, be noted that the final reflectance value depends not only on temperature, but also on the duration of charring (22,23). If heating occurred during a period of <1 h, the temperature would have been slightly higher then what we inferred from our reflectance values.…”

“…Here we report on more precise accelerator mass spectrometry (AMS) dates for 14 individual charcoal particles, 12 from the Usselo horizon, and 2 from just above the horizon. In addition, the reflectance of several charcoal particles was measured using light microscopy to estimate the wildfire temperature (22,23).…”

The formation of charcoal reflectance and its potential use in post-fire assessments

“…McParland et al 2009;Ascough et al 2010;Scott 2010;Hudspith et al 2015). However, charcoal reflectance may still have utility in bringing semiquantitative measurements to fire severity surveys, particularly where surface fires are an important part of the fire regime.…”

“…McParland et al 2009;Ascough et al 2010;Hudspith et al 2015), from which reflectance values of charcoal produced at unknown temperatures can be extrapolated (see Fig. S1 in the online supplementary material).…”

Charcoalified logs as evidence of hypautochthonous/autochthonous wildfire events in a peat-forming environment from the Permian of southern Paraná Basin (Brazil)

“…McParland et al, 2009), petrographic analysis was carried out in polished blocks under oil (Immersol 518 F) using microscope Axioskop 2 Plus Zeiss equipped with spectrophotometer J&M (MSP 200) through a ×50 objective. The microscope was calibrated with a Sapphire standard (0.595%Ro).…”