Studying glass on metal: Raman analysis of medieval champlevé enamelled objects

Abstract

Medieval champlevé enamelled objects are from copper sheets decorated with glass. The enamels were analysed by Raman spectroscopy to study the compositional differences in the glass matrix. Additional analyses were carried out by element analytical methods such as μ‐X‐ray fluorescence analysis and environmental scanning electron microscopy with energy dispersive X‐ray analysis. The two most frequently used glass types are mineral soda lime glass opacified with antimony compounds and vegetal soda lime glass with a more significant addition of PbO, opacified with SnO2. The enamel composition can succour the classification of the object regarding production time and place.The wavenumber of the bending and stretching regions, δmax and υmax, and the polymerization index Ip were extracted from Raman spectra, and results were interpreted with respect to the so‐called glass families from the literature. The two most frequently used glass types can be distinguished in the plot of Ip versus υmax. For further differentiation, the Si‐O stretching υmax against Si‐O bending δmax plot was exploited. This plot allowed a separation of glass types which superimposed in the plot of Ip versus υmax, such as mixed alkaline glass and soda lime glass with added PbO. The small spot size and sensitivity of Raman analysis for lead and alkaline components allow obtaining complimentary data on compositional differences and heterogeneities of the glass matrix, which are difficult to detect by common laboratory micro‐X‐ray fluorescence analysis.

Applications of Raman spectroscopy in art and archaeology

“…The Kunstgewerbemuseum, Staatliche Museen zu Berlin‐Stiftung Preußischer Kulturbesitz (SMB‐SPK) holds a collection of medieval champlevé enamelled objects from the main production centres in Limoges and the Rhenish/Mosan region. Röhrs et al [ 12 ] characterized enamels of 6 of such objects by Raman spectroscopy (using a 532‐nm laser) in combination with additional element analytical methods such as μ‐X‐ray fluorescence (μ‐XRF) analysis and environmental scanning electron microscopy with energy‐dispersive X‐ray analysis. Wavenumber parameters of the SiO bending and SiO stretching regions, δ max and υ max , and the polymerization index I p derived from Raman spectra analyses give information on compositional differences in the glass matrix.…”