XAS study on copper red in ancient glass beads from Thailand

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.