Application of Raman thermometry to charcoals from archaeological sites: assessing the effect of wood chemistry and post-depositional effects on apparent carbonization temperature

• The initial chemical composition has an effect on Raman spectra in charcoals. • This composition effect disappears in charcoals formed above 600 °C. • Raman spectra may reveal differences in the intensities of archaeological fires. Charcoal is a key archaeological artefact, providing evidence of human presence, enabling site dating and delivering information about ancient practices notably through Raman thermometry. This study explores how it can be used to investigate past anthropogenic practices, considering carbonization temperature, wood chemistry, and post-depositional processes. Two complementary approaches were applied. Firstly, the experimental approach quantified the impact of initial wood chemistry on Raman parameters. 41 wood species were chemically characterized by FTIR and carbonized at 400, 600, and 800 °C. Raman spectra of charcoal showed that the D/G band height ratio (H D /H G ) is influenced by wood chemistry, although this effect decreases at higher temperatures. In contrast, the area ratio (A D /A G ) was less sensitive. Parameters associated with aliphatic compounds (α 1200-1265 ) and hydrocarbons trapped within aromatic structures (α 1415-1445 ), also varied with wood chemistry. Secondly, archaeological charcoals from Matadi and Ndouni (Gabon) were analysed to see if Raman spectra could reveal past uses. Results showed that post-depositional alterations affect H D /H G and A D /A G ratios, but, despite this, archaeological information was retrieved. At Matadi, radiocarbon dating distinguishes two fire episodes, while Raman analysis indicates higher fire intensity during the Early Iron Age, consistent with magnetic susceptibility measurements. At Ndouni, pottery-embedded charcoals displayed stronger aliphatic and hydrocarbon signals than those from soils, likely due to contact with fats from culinary use. Raman spectroscopy can be a complementary tool for enriching archaeological interpretations such as fire intensities or potential uses, despite the combination of multiple factors.