ExTraS program: documenting the processes of fixation, recording, and preservation of combustion products in speleothems

In recent years, pyroarchaeology has expanded rapidly and included experimental approaches. However, analyses of the combustion products carried by smoke (mainly soot and microcharcoal) and the study of fixing, recording, and preserving processes of this material on parietal surfaces remain limited. We therefore launched the ExTraS (Experiments on Traces of Soot) program, which aims to document the processes of fixation, recording, and preservation of combustion products in speleothems. We focused on various parameters, including the influence of wall surfaces, meteorological and seasonal conditions, and the nature of the fuel. Here, we present results acquired over several years (2015–2023) that combine macroscopic, microscopic, and colorimetric observations, as well as Raman microspectroscopy characterization. Our findings improve our understanding of the formation process of sooty speleothems observed at archaeological sites in rock shelters and caves.