X-ray and gamma-ray based spectroscopic analysis of a millefiori Roman glass fragment: degradation of sunken glass from a shipwreck

Particle induced X-ray and gamma-ray techniques (PIXE and PIGE) carried out on an external beam line, combined with X-ray dose monitoring and a helium atmosphere, have been performed in this work. These atomic and nuclear techniques, simultaneously used for complementary chemical information, have been demonstrated to be of great value as they provide non-destructive compositional information avoiding the use of vacuum chambers or sample preparation. The analytical set-up implemented has been applied in an archaeometry study: a millefiori glass fragment recovered from the sea in 1976 from a merchant Roman shipwreck (1st century B.C.). The fragment is composed of a yellow and blue millefiori base structure decorated with large coloured pieces and presents high levels of degradation due to the aquatic medium in which it has stayed for several centuries. The interest of the study lies in that the degree of deterioration is higher in the millefiori structure than in the coloured pieces. The composition of both has been determined in a non-destructive way in order to study the manufacturing process, the nature of the coloured pieces and the reason for the different degradation processes. According to the results, the coloured pieces are glass fragments similar in composition to the millefiori structure, which is a natron-based sodium silica glass, similar to other analysed Roman glasses. The blue colour is due to cobalt and copper elements. The use of one source of silica, probably coastal sand characterised by high contents of sulfur and phosphorus, can be inferred from the results.