Dating ancient mosaic glasses by luminescence: The case study of San Pietro in Vaticano

The preliminary results of a study on the dosimetric properties of a set of glass tesserae from the mosaics of the vaults of St. Peter Basilica in Rome (late XVI century) are reported. Main goal of the research was to assess the possibility of dating them by means of luminescence techniques. The samples had already been extensively studied and investigated from historical, artistic and compositional point of view. The period of the making of the mosaic was rather well known, and could be the basis to test the experimental procedures we used for dosimetry. The experiments also aimed at demonstrating the validity of the hypothesis we put forward, i.e. the positive link between presence of micro-crystals and luminescence sensitivity in mosaic glass: to this aim, the samples richer in crystalline inclusions were selected. The role of calcium antimonate and cassiterite was definitely demonstrated. For what concerns their Thermoluminescence (TL) characteristics, a preliminary investigation suggested that the available configuration of the detection systems did not properly fit the wavelength of the emitted TL. Much more promising results have been achieved by the use of Optically Stimulated Luminescence (OSL). For samples with high OSL sensitivity, values of absorbed dose lower that 0.1 Gy were systematically obtained, indicating a recent age, relative to one of the documented restorations that took place during the last century.