Green color chemical recipes in stained glass windows of NE Spain and N Italy (XIIIth to XVth centuries)

A great number of medieval religious buildings in Europe contain stained glass windows. A number of original glass fragments (i.e. with original medieval lead framework) coming from restoration projects (Girona, Tarragona, Pedralbes Monastery at Barcelona (Spain); the Siena Cathedral, the San Petronio Basilica and the San Giacomo Maggiore church in Bologna (Italy)) have been analyzed in term of major, minor and trace elements. In contrast to the coeval glass windows of central Europe, most of the studied glass fragments are soda-glass of Mediterranean origin, whereas the imported K-glass were used only to realize some colors in the windows (in most of cases red ruby plaque glass). Concerning the technological aspects, these glass were realized starting from the colorless base glass, to which metaltransition elements were added to obtain the different colors. A first conclusion is that some elements, singularly REE associated to Nb and other trace elements characterize the source of silica, that is quartz sand or quartz cobbles. This allows to the identification of homogeneous sets of colored and colorless glass (i.e., original medieval glass) in the same windows and sites, even in the cases of subsequent widespread reposition of glass fragments over the centuries. Total content in Sr provides information on the calcium carbonate source (limestone or aragonitic shells).The different shades of the green color were obtained by using a Fe-Cu association. The study of green glass fragments by ICP-MS and laser ablation HR-ICP-MS allows to characterize different mineral salts taking into account the associated trace element metal associations. We acknowledge the technical support of the CCiT of the Universitat de Barcelona and the ICTJA-CSIC labGEOTOP, infrastructure co-funded by ERDF-EU (Ref. CSIC08-4E-001).