A new assessment on yhe chemical compositin of stained gless from the Certosa di Pavia, Italy.

The samples from three stained glass windows examined bere afe made of Si-Ca-K glass. The exception is the yellow glass sample, which has anomalous content in alkalis and calcium, yellow glass has the lowest sodium content, but it contains appreciable amounts of manganese. Zoned glass was produced by first making a multi-layered glass. In the twelfth and thirteenth centuries translucent glass was produced by multilayering , because certain colored glasses were so dense in tone that they did not allow sufficient light through. The composition of the Certosa glass is similar to that of Medieval glass. Medieval glass from Northern Europe, has been classified as: i) early wood-ash, ii) late wood-ash and iii) wood-ash-lime glass. In those times (1000-1400 A.D.), beech-wood ash is considered to have been the raw material used by glassmakers. Differences in ash composition afe expected for beech trees growing in different countries and climates, but the differences in the Certosa glass are certainly due to different batch composition. Close relationships between the compositions and ages of glass pieces cannot be inferred, owing to the small number of analysed samples. Therefore, replacements of glass pieces in the course of a rather long history of repair works, are far from being identified. In any case, knowledge of the chemical composition of glass is useful not only to speculate about ancient recipes or provenance attributes, but also to provide constraints when evaluating the chemical stability of glass exposed to atmospheric and polluting agents over several centuries.