Non–Destructive Raman Analysis of Ancient Glasses and Glazes

Abstract

Glass has been produced and used since ancient times because of its malleability at moderate temperatures and its more or less pronounced optical clarity. The latter characteristic makes optical spectroscopic methods ideally suited for its study. Glass has not only been produced to make glass artefacts, in fact the largest range of glass compositions was prepared by potters in order to coat pottery with glazes and to apply complex enamelled coatings to make porous ceramic bodies impermeable or for decorative purposes. Enamels have also been deposited on metals, mainly copper and its alloys, since the earliest times. All of these uses require the preparation of glass compositions ranging from a high silica content (typically 11 SiO 2 , ∼1 K 2 O for porcelain glazes melting at ∼1400 • C) to a low-melting lead-rich glass (PbO−SiO 2 ) with a melting temperature below 600 • C [1].All applications in the science, art and technology of glasses, glazes and enamels consist of a controlled modification of the three-dimensional Si−O network by replacement of Si 4+ covalently bonded atoms by noncovalently bonded atoms, hence decreasing the number of Si−O bridges and the connectivity of the network, that is, modifying the glass nanostructure. Consequently, the melting temperature decreases and many other physical/chemical properties related to the density and network connectivity, such as thermal expansion, chemical resistance, and so on are modified accordingly. This was performed more or less empirically by glass-makers by mixing a glass former, namely silica, with low-melting temperature compounds, such as soda-or potash-rich materials. Grinding the materials is the most difficult aspect in preparing ceramics and glass batches, and therefore ancient glassmakers and potters used natural fine silica powders (fine sand or roasted flint pebbles crumbled by dipping in water) as glass formers and some fine-grained minerals, shells Modern Methods for Analysing Archaeological and Historical Glass, First Edition. Edited by Koen Janssens.

The Reliquary Bust of Saint Lambert from the Liège Cathedral, Belgium: Gemstones and Glass Beads Analysis by pXRF and Raman Spectroscopy

“…Orange beads are close to the PbO field, corresponding to the high PbO contents (46 to 55 wt.%) observed for these samples. The only turquoise bead analysed shows a glass composition between "PbO + Na 2 O" and "Na 2 O" fields, in good agreement with the pXRF analyses (Table 1) (Colomban et al 2006, Tournié 2009, Colomban 2013.…”

“…The only turquoise bead analysed shows a glass composition between “PbO + Na 2 O” and “Na 2 O” fields, in good agreement with the pXRF analyses (Table 1) (Colomban et al . , Tournié , Colomban ).…”

Non-Invasive On-Site Raman Study of Pigments and Glassy Matrix of 17th–18th Century Painted Enamelled Chinese Metal Wares: Comparison with French Enamelling Technology

“…Raman spectra of the cloisonné enamels show a broad band between~900 and 1100 cm −1 with various shoulders/components, characteristic of the Si-O stretching mode of the silicate matrix of the glassy enamels (Table 3, Figures 5 and 6, part of Figure 7). Comparison of the specific Si-O stretching band components and Si-O bending to stretching band area ratio helps to distinguish between different enamel compositions, as previously demonstrated empirically [65][66][67][68] and by Density Functional Theory (DFT) modelling [69]. Due to the complex spectral background observed when using mobile edge filtered Raman spectrometers, comparison of the Si-O bending band is very much dependent on the signal intensity, and thus comparison is only valid if recording procedures are identical, such as in this study.…”

“…Table 5 compares the glassy silicate matrix signature of the painted enamels in the SiO 4 stretching range. Three types of glass composition were observed, according to references [65][66][67][68][69]: lead-rich composition with the strongest component at~980 cm −1 (Type I), the lead-(earth)alkali composition with the strongest component at ca. 1020 to 1040 cm −1 (Type IIa) and intermediate composition with strong peaks both at ca.…”

“…Here, only the Si-O bending band obtained on some of the blue or green coloured areas can be observed with good reliability. Low-wavenumber components (900 cm −1 shoulder, strong component at~970 to 990 cm −1 ) indicate a low-polymerized (lead-based) glass network made of more or less connected SiO 4 tetrahedron fired at lower temperatures [65][66][67][68] and this type of glass exhibits a stretching band stronger than the bending one. According to previous studies of Chinese cloisonné enamels [12] and representative glass types [65][66][67][68], four types of glasses either with lead-rich or lead-(earth)alkali compositions were identified for the artefacts analyzed (Table 3): lead-rich glass with the strongest component at ca.…”