Chemical composition and colouring agents of Roman mosaic and millefiori glass, studied by electron microprobe analysis and Raman microspectroscopy

About 100 fragments of Roman mosaic and millefiori glass were stylistically attributed to a Hellenistic type, a Ptolemaic and Romano-Egyptian period type and an early imperial period type. Twelve representative fragments were studied by electron microprobe analysis and Raman microspectroscopy. Eleven of them display a Na-pronounced recipe with low K, Mg and P contents, typical for the Roman period. Minor differences in composition are unsystematic, not reflecting the stylistic classification. Ionic colouring agents are Mn³⁺ for violet, Cu²⁺ for light blue, Co²⁺ for deep blue and Fe³⁺ for brown translucent colours. Calcium antimonates, lead antimonate and cuprite are the colourants responsible for white, yellow and red colours, respectively, and additionally serve as opacifiers. Mixing of ionic colouring agents and opacifying colourants led to a more differentiated palette of colours. Pb was used as yellow colouring agent, as a flux material and as a stabiliser for the colourant crystals. The remaining fragment consisting of a K-pronounced but still Na-bearing glass matrix was most likely produced during the Middle Ages or later.