New chemical and mineralogical data on mosaics from the South aisle wall of Monreale Cathedral (Italy)

Due to the surprising lack of knowledge concerning raw materials and production technology employed to make mosaics after the twelfth century, an archaeometric investigation was carried out on the materials constituting the mosaic decoration of the South aisle wall of Monreale Cathedral (Italy), taking advantage of a conservation intervention. In this work, fallen coloured or gilded glass tesserae to be repositioned were studied by means of a combination of a molecular technique (Raman spectroscopy) and an elemental one (portable X-ray Fluorescence, pXRF); also, efflorescences affecting the general conservation state and samples of the respective bedding mortar were analysed with the former technique and with X-ray diffraction (XRD). The raw materials used and, consequently, the different compositions characterizing gilded vs. coloured glass tesserae were highlighted with the vibrational spectroscopy; chromophores and trace elements were detected by pXRF. This complementary approach allowed to disclose clues about glass-manufacturing technique and raw materials. The nature of the salts was also ascertained through Raman spectroscopy and XRD, for the benefit of the conservation procedure, and connected to the mortars’ composition.