Characterization of the coloring techniques of Roman mosaic glass beads using μ-XRF and μ-Raman

A combination of microscopic X-ray fluorescence spectroscopy (μ-XRF) and microscopic Raman spectroscopy (μ-Raman) was used to non-destructively analyze four Roman-period mosaic glass beads collected in the Iridescen Art Museum, Thailand. Specifically, μ-XRF was used for elemental mapping and quantitative analysis of the surface patterns and matrix of the glass, while μ-Raman was applied to identify the microcrystals of compounds dispersed in the opaque regions of the glass beads. This study revealed the elemental and phase composition, distribution characteristics, colorants, and opacifiers in the different colored regions of the glass beads. All the beads were made from natron soda-lime-silica glass. The opaque yellow regions were primarily associated with Pb and Sb, with lead antimonate (Naples yellow, Pb₂Sb₂O₇) serving as the main colorant/opacifier. The opaque white regions were mainly related to Ca and Sb, with calcium antimonate compounds (orthorhombic Ca₂Sb₂O₇ and hexagonal CaSb₂O₆) acting as the primary colorants/opacifiers. The black regions were associated with Mn, while the transparent to semi-transparent blue and green regions were linked to Cu. The red regions were related to Pb and Cu, without distinct crystals detected, indicating that metal ions functioned as the main colorants. The combination of the two non-destructive micro-techniques enabled comprehensive identification and characterization of the content and distribution features of key elements related to color in the glass phase and crystal phase of these glass beads. This study contributes to the understanding of Roman glass bead coloration techniques and, furthermore, offers valuable insights for the non-destructive analysis and scientific conservation of cultural heritage.