Composition of the base glass used to realize the stained glass windows by Duccio di Buoninsegna (Siena Cathedral, 1288–1289 AD): A geochemical approach

A set of 35 samples from the rose window of the Siena Cathedral (Tuscany, Italy) has been studied using EMPA to define the chemical composition (17 major and minor elements) of the base glass. This paper focuses on colourless and natural-coloured glasses, i.e. glass produced without the intentional addition of colouring agents. The presence of natural impurities of Fe and Mn, as well as the control of the furnace conditions plays an important role as well. Three compositional groups of natural-coloured glass have been identified taking into account the Al/Ti ratio and the K2O and MgO concentrations. The most represented and homogeneous group contains the original glasses and defines the base glass composition. It is a soda–lime glass based on quartz (pure quartz sand or quartz-bearing pebbles/rocks) as vitrifying component and a Na-rich plant ash as flux. Restoration interventions explain the other two compositional groups. The compositional comparison of the original glass with coeval glass from Tuscany (Italy) has allowed us to postulate local production for them.