μ-LIBS imaging of archaeological ceramics: Quantification of clay fraction in provenance study

The provenance and circulation of archaeological ceramics are key to understanding ancient societies' trade networks and cultural exchanges. However, comparing coarse and fine wares using bulk compositional analysis remains challenging due to the variable chemical contribution of temper (e.g., sand), which masks the signature of the raw clay. This study introduces a novel quantitative μ-LIBS (micro-Laser-Induced Breakdown Spectroscopy) imaging methodology to isolate and analyse the clay fraction composition of archaeological ceramics, enabling direct geochemical comparisons between tempered coarse wares and fine wares for provenance studies. By combining multi-elemental mapping (Ca, Fe, Ti, K, Si, Al, Mg, Mn, Na, P, Zr, Rb, Sr, Ba) with calibration based on bulk X-ray fluorescence spectrometry, the method provides accurate microscale quantification. Elemental maps are segmented to extract the clay fraction composition, overcoming the limitations of traditional bulk analysis. The protocol was validated using fired briquettes with controlled sand contents and firing temperatures, confirming its accuracy and robustness. Applied to four coarse ware specimens (mainly amphorae) from Lugdunum (Roman Lyon (France) during the Augustan-period, the reconstructed clay compositions were compared to a reference database of 235 contemporaneous ceramics, including coarse wares and three Lugdunum Terra sigillata production groups. Results reveal a shared raw clay source between calcareous amphorae and La Muette B sigillata, supporting earlier hypotheses, while other sigillata wares remain compositionally distinct. This approach significantly advances ceramic provenance studies, particularly for complex assemblages, by enabling direct comparisons between ware types previously hindered by temper variability, together with the common total bulk elemental composition measurements.