All about wood ash: Long term fire experiments reveal unknown aspects of the formation and preservation of ash with critical implications on the emergence and use of fire in the past

Abstract

Despite the growing number of experimental studies of fire, there is still a lack of critical information on the physical alteration of ash remains and its importance in the preservation and interpretation of combustion features in the archaeological record. This study presents an 8-year long experimental investigation of several hundreds of open fires and laboratory experiments and the analysis of their chemistry, mineralogy, morphology, and physical parameters. Important observations have been made regarding ash weathering, charcoal production, post-fire use of heated ashes, and the influence of kindling and soil organic matter on thermal alteration and reddening of the fire substrate. A wealth of information is provided on the formation of ash minerals, their physical and chemical alteration with temperature, and post-fire reactions with water. The resulting evidence can explain the recent geo-ethnoarchaeological observations that ephemeral open-air fires are likely to disappear if they are not quickly buried by sediment by a non-eroding process. Ash formed during a single or a few relighting events is extremely porous (more than 90% total porosity), light (less than 0.2 g/cm3 bulk density), and highly compressible (about 90%) under the weight of foot traffic or 1.5 m of overburden sediment. This makes ash remains extremely vulnerable to modification in an open environment and macroscopically invisible in an excavation, even in the absence of the slightest post-depositional chemical alteration. Ash remains become progressively more sintered as relighting events increase, thus producing a crusted less porous and more stable formation. Nevertheless, remains of a 5 cm-thick unaltered ash buried in a sedimentary sequence will most likely represent several tens of fires made in the same area whereas a flat 20 cm-thick ash sequence represents hundreds of fire events on top of one another. These results are of extreme importance in evaluating the emergence and use of fire in the Pleistocene and they also provide a better understanding and interpretation of ash remains in later periods. Finally, it is suggested that only a multimethod research strategy can reveal the necessary information to understand and correctly interpret archaeological ash remains.