Time-of-flight neutron diffraction applied to the identification of crystalline phases in historical mosaic glasses

We present a completely non-destructive study of the crystalline phases present in the vitreous matrix of ancient mosaic glass tesserae, based on the application of the time-of-flight neutron diffraction (ToF-ND) technique. In contrast, standard phase analysis techniques may involve a sample preparation step, which is not compatible with the requirement of damage prevention of archaeological artefacts. Moreover, superficial or spot investigation by standard methods may not provide a complete description of the samples, especially in the case of inhomogeneous materials such as mosaic glasses, whereas ToF-ND can investigate the whole glass composition at the centimetre scale. The present study aims to obtain a bulk characterization of the glasses in terms of their mineralogical phase composition. Six glass specimens of ancient mosaic glass tesserae, that had been previously analysed with other non-destructive techniques to obtain their chemical and phase composition, were investigated by ToF-ND. These tesserae come from archaeological excavations or mosaics under restoration from different archaeological sites and geographical areas (Italy, Greece, and Syria) and date from the 1st to the 11th century AD. Several crystalline phases based on Ca, Sb, Sn, Cu and Pb minerals were identified, confirming the mineralogical phases identified with Raman spectroscopy in previous analyses. In addition, several other phases were identified for five of the samples. It is concluded that ToF-ND is an extremely important complementary tool for non-destructive bulk investigations of mosaic glass tesserae, which can overcome the heterogeneity of this kind of archaeological material.