Glass has been used in ornaments and decorations in Thailand for thousands of years, being discovered in several archeological sites and preserved in museums throughout the country. To date only a few of them have been examined by conventional methods for their compositions and colorations. In this work we report for the first time an advanced structural analysis of Thai ancient glass beads using synchrotron X-ray absorption spectroscopy (XAS) and energy-dispersive X-ray (EDX) spectrometry. Four samples of ancient glass beads were selected from four different archeological sites in three southern provinces (Ranong, Krabi and Pang-nga) of Thailand. Archaeological dating indicated that they were made more than 1,300 years ago. A historically known method for obtaining a red color is to add compounds containing transition elements such as gold, copper, and chromium. For our samples, EDX spectrometry data revealed existing fractions of iron, copper, zinc, and chromium in ascending order. Thus, copper was selectively studied by XAS as being potentially responsible for the red color in the glass beads. K-shell X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of copper were recorded in fluorescence mode using an advanced 13-element germanium detector. Comparisons with XANES spectra of reference compounds identified two major forms of copper, monovalent copper and a metallic cluster, dispersed in the glass matrix. The cluster dimension was approximated on the basis of structural modeling and a theoretical XANES calculation. As a complement, EXAFS spectra were analyzed to determine the first-shell coordination around copper. XAS was proven to be an outstanding, advanced technique that can be applied to study nondestructively archaeological objects to understand their characteristics and how they were produced in ancient times.

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were used from the beginnings of glass production through into the Roman period for the opacification of green, blue and white samples. Subsequently, they started to be replaced by tin-based opacifiers during the fourth century AD in glass produced from the eastern Mediterranean through into northern Europe. For the production of red and orange samples, it seems that across the centuries only Cu-based opacifiers were used. In this work, the opacifiers identified in a series of mosaic tesserae coming from Mediterraneran region and dated from the 6 th to the 11 th century are compared and employed to sketch the glass technolgy evolution on a chronological and geographical base. The coupling of the chemical data with the SEM-EDS and XRD investigations allows the identification of the opacifiers employed in the glass production. In particular, the data relative to San Severo Basilica (VI century AD, Classe-Ravenna-Italy), represent an useful tool to understand the shift from Sb to Sn bearing opacifiers. The sample set coming from this site presents a peculiar characteristic: all the blue samples, colored mainly with copper, are lead and tin free and are opacified with calcium antimoniates, on the contrary all the green samples are antimony-free and contain high levels of lead and rather high levels of tin. On the whole, San Severo samples fit with the chemical composition of the coeval mosaic tesserae. The co-presence of Sb-based opacifiers and of tin bearing samples in the sample set, represent a rather peculiar trait , but, it is well known that in Italy, the tin based opacifiers have been found from the fifth century AD onwards, but at the same time, antimony-based opacifiers continued to be used, or reused, until the 13th century AD. The simplified pattern of Sb and Sn bearing opacifiers is then complicated in some areas of the Mediterranean glass production by the occurrence of quartz crystals dispersed in the glass matrix. The study of the opacifiers could be an indirect method for dating and provenancing the mosaic glass.

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