In situ LIBS-XRF analysis as a combined approach to disclose the production technology of unique wall mirrors from Pompeii

Abstract

AbstractIt is assumed that the unique wall mirrors found at the Archaeological Park of Pompeii (PAP) are made of obsidian. To contribute to the knowledge of those archaeological artefacts, this work proposes in situ elemental analyses in collaboration with PAP seeking to determine, in a total no-destructive way, the composition and provenance of the main mirror preserved at the House of Gilded Cupids. Comparing the geochemical composition of this black glass with that of obsidian samples collected from the main Mediterranean sources, both X-ray fluorescence (XRF) and laser-induced breakdown spectroscopy (LIBS) confirmed an incompatible content of many key elements. LIBS in-depth analysis excluded the potential relation between the higher concentration of Ca and Mg and the presence of alteration products. In addition, XRF analysis missed the detection of Rb, Y, Zr and Nb, which are widely recognized as the elemental fingerprints of obsidian sources. Combined with the detection of a high content of strontium (500–700 ppm), the in situ elemental data proved that, rather than made of obsidian, the analysed mirror was handcrafted by the fusion of coastal sand. Waiting to extend this study to the other mirrors found at Pompeii, the results here presented indicate the history of these unique artefacts needs to be rewritten.