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

International audienceMedieval 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 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 nu max, and the polymerisation 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 vs. nu max. For further differentiation, the Si-O stretching nu max against Si-O bending δmax plot was exploited. This plot allowed a separation of glass types which superimposed in the plot of Ip vs. nu max, 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 allows to obtain complimentary data on compositional differences and heterogeneities of the glass matrix, which are difficult to detect by common laboratory micro-X-ray fluorescence analysis