Forming mechanisms of vitrified charcoals in archaeological firing-assemblages

Abstract

Anthracological research has not yet elucidated wood vitrification ranging from partial fusion of the cell structure to a dense vesicular glass which has been so far not experimentally reproduced. We have proposed previously that vitrification would trace wood flash-pyrolysis induced by aeroplasma and lightning due to the production of ionized aerosols in the atmosphere and their accumulation in the tissues during the tree growth.Here we further explore the factors involved in vitrification by flash-pyrolysis, in terms of firing processes, heating rates and wood composition. Our study promotes the challenging potential of vitreous chars and related by-products in archaeological firing assemblages to explore the link between atmospheric conditions, fuel properties and heating processes which has never been so far debated.Our study is based on the comparison of firing-assemblages from diverse archaeological contexts and the ones from solar reactor experiments as well as present-day firing situations. We focus on the characteristics of charcoals showing different stages of vitrification and on the tightly associated polymeric compounds. Their in situ structure characterization and composition are performed at meso to nanoscale with the SEM-EDS in backscattered mode and completed by TEM, Raman spectrometry and XRD analyses.Biomass thermochemical conversion for gas production provides a solid knowledge to understand that the extreme vitrification of wood fuel can only be induced by flash-pyrolysis. This process is shown to involve depolymerisation of wood tissues and reformation of diverse carbonaceous solid phases, along to degassing of the most volatile compounds. Wood vitrification due to flash-pyrolysis can thus be traced by a continuum of pyrolytic residues formed of lamellar to vesicular charcoals, black carbon aggregates, polymerised plant fragments, polymer films and filaments and densely recrystallized ash.We establish the key influence of non-condensable gas with metal and mineral impurities of high electrical conductivity that were incorporated within the plant tissues on the characteristics of the solid residues formed by flash-pyrolysis and on their properties. The pyrolytic debris share in common a nanostructuration, an imbrication at nanoscale of mineral and carbonaceous phases and nanostructured metal films.Using the record of present-day lightning-strikes, we describe the assemblage of pyrolytic debris and their characteristics to trace primary flash-pyrolysis that is instantaneously produced by the sudden release of incondensable gases and segregation of impurities. We show the difference of pyrolytic debris formed by a secondary flash-pyrolysis that can be locally achieved in long-maintained high temperature heating due to enhanced concentrations of non-condensable gases in confined pores.The characteristics of the strongly vitrified chars with the associated pyrolytic debris of the archaeological firing assemblages match the ones of primary flash-pyrolysis, but not the ones of lightning-strikes. Instead, we propose that they would have formed under periods marked by enhanced atmospheric electrification and production of electrically charged aerosols.Our observations emphasize the key-role of graphitized nanocarbons with their bound catalytic metals from possible volcanic sources on the initiation of flash-pyrolysis in air or at the ground and on the long-term protection of the produced pyrolytic residues.