From Siena to Barcelona: Deciphering colour recipes of Na-rich Mediterranean stained glass windows at the XIII–XIV century transition

This paper offers new insights on the evolution of colour recipes for Na-rich Mediterranean stained glass, taking as a guideline the results of the study of the Duccio di Buoninsegna's rose window (1288–89 AD) at the Duomo (Cathedral) of Siena (Italy) and the Santa Maria de Pedralbes presbytery windows of the church at the royal monastery in Barcelona (1326–27 AD). In order to decipher the colour recipes in an original set of glass pieces, a number of chemical analyses have been performed, namely quantitative EPMA on thin sections cutting orthogonally the glass surface. This comparative approach is useful since the studied glass is well preserved in terms of good chemical conservation against corrosion and biological attack. Also, chemical composition of glass provides evidence of preservation of homogeneous original glass sets at each site. The Siena samples are representative of traditional colour production, as explained by the monk Theophilus two centuries before (regarding yellow, pink and colourless glass) or as is well known before Theophilus for Co-blue glass. Also, some chemical data on Cu–Fe-rich green glass provide evidence tentatively related to the use of metal-rich slag as colouring component. The Barcelona samples offer evidence of new recipes (i.e. yellow glass) and hence look more evolved from the glassmaker's point of view, in spite of the more pristine artistic features. Both glass windows can be regarded milestones in the interpretation of technological evolution with the introduction of new colour recipes at the XIII–XIV century's transition.