Scientific investigation of two medieval panels of decorated glass

Scanning electron microscopy coupled with an energy dispersive detector (SEM-EDS) and fibre–optics spectrometry were employed to investigate two panels formed by decorated glass pieces which possibly came from medieval ecclesiastical buildings in the Aosta Valley (North-Western Italy). The panels, kept in the Priorato di Sant’Orso in Aosta, were subjected to scientific investigation with the aim of identifying the raw materials and techniques employed for production and decoration of the glass pieces. These latter are of coloured and clear glass, decorated by flashing, yellow-staining and grisaille. The pieces were held together, without any decorative coherence, by strips of lead. The panels were first analysed by means of a portable fibre–optics spectrometer working in transmitted light mode; then, when the individual panes were separated for restoration, fragments form those selected for the analyses were detached, mounted in cross sections and subjected to SEM-EDS analyses. The overall analytical procedures revealed that Cu(II) and Co(II) were used for obtaining blue-green and blue glass, respectively; the thin red layer was coloured by plasmons of metallic copper and the yellow stain was obtained by nano-dispersed silver particles. Moreover, the analyses revealed that most of the pieces were wood-ash or wood-ash-lime glass, as expected for medieval window glass, while two of them were soda-lime glass. Concentration maps and profiles were employed to investigate multi–layered glass and decorations thoroughly. Of particular interest were the profiles of silver distribution in yellow-stained glass, that put into evidence different situations. The composition and the morphology of the grisaille did not show any evident relationship neither with the composition of the glass nor with the decorative motives; besides this, the analyses indicated that they were obtained by the use of a lead–silica glass mixed with iron and copper oxides.