XAS study on copper red in ancient glass beads from Thailand

Abstract

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.

Study of an archeological opaque red glass bead from China by XRD, XRF, and XANES

“…On the basis of the simulated XANES spectra of the relaxed structures of different amounts of atoms of copper Face‐Centered‐Cubic (FCC), the diminished amount of the native edge of copper metal and the peaks on positions B and C become more spread out and shift to a lower photon energy. The Cu K ‐edge spectrum of the sample just fits the characteristics of nanoscale copper clusters as previously reported . Thus, it can be deduced that the size of the copper metal cluster in the sample is close to the nanoscale.…”

“…It was recognized that different shades of red color depend on the size of the copper nanoparticles. This relationship has been confirmed in many ancient objects, including beads …”

XAFS study of copper(II) complexes with square planar and square pyramidal coordination geometries

“…Absorption screens were in the form of pellets prepared from calculated amount of finely powdered samples. XANES and EXAFS have been recorded separately at the Cu K-edge at beamline BL-8 of the Synchrotron Light Research Institute, Thailand [15]. Ge(220) double crystal monochromator was used to scan the photon energy of the synchrotron X-rays in the transmission mode.…”

X-ray and optical spectroscopic study of the coloration of red glass used in 19th century decorative mosaics at the Temple of the Emerald Buddha

“…Red glass can also be colored by copper [24,25] or by selenium [26], both of which are common red colorants in contemporary glass. While the glasses studies here have copper in much higher concentration than gold, selenium is not present within the ∼1 mg/kg detection limits of the LA-ICP-MS and X-ray fluorescence spectroscopy presented.…”

“…We also considered the possibility that some of the dispersed metallic nanoparticles are copper, which also results [10,24,33] in red coloration in glass and ceramics, although the resonance absorption for Cu is red-shifted compared to Au to 2.18 eV (570 nm). The volume fraction of Cu nanoparticles is a fourth adjustable parameter in the fits using the Garnett theory.…”