Fuel origin and firing product preservation in archaeological occupation contexts

Abstract

Abstract We intend here to examine the range of factors involved in the preservation at short to long-term of firing byproducts with respect to the nature of combustion processes and the fuel origin. Our objective is to refresh the cultural and palaeoenvironmental relevance of combustion deposits. The study is based on the comparison of two recent situations of natural fires and a selection of well preserved firing records from rockshelters and caves in diverse geographical and cultural contexts. The recent analogues comprise a two year investigation of the 2012 July large forest fire at Alt Emporda (Northern Cataluna, Spain) and a study of the 2014 lightning struck pine tree (Allanche, Cantal, France). In situ characterization at micro to nanoscale of firing products has been performed using the SEM–EDS in backscatter mode on individual grains and composite aggregates which were extracted by gentle water-sieving from combustion microstrata and firing products meticulously sampled in the field. The survey of the Alt Emporda firing record confirms the rapid loss of intact ashes and heating scars, leaving after two years a regenerated soil without any obvious evidence of this fire, except for scattered charcoal. The study of Allanche lightning struck pine tree shows that lightning produces a suite of robust components diagnostic of flash-heating and plasma-assisted combustion: polymer filaments, vitreous char, metal coatings and singular minerals. Their durability expresses the refractory and hydrophobic properties of materials formed by transient heating and plasma processes. The identification of similar robust components in all the firing products of the archaeological contexts tested here shows that plasma-assisted combustion and flash heating exert a major role on the durability of firing records. The archaeological combustion contexts are separated into two types of intact firing records: (1) the ones with abundant mineralised charcoal, vitreous char and metal coatings would trace the use of fuel acquired from wood affected by a lightning strike; (2) the thick white ash with polymer films, refractory minerals and metal coatings would indicate wood fuel collected during periods of active aeroplasma processes in the atmosphere due to increased dust loading and intense ionization. The study is suggested to offer a challenging perspective to elucidate further the role that lightning and plasma processes exerted on ancestral pyrotechnology by occasionally providing wood fuel resources of long durability, low degradability and high calorific power.