STUDY OF THE CORROSSION BEHAVIOUR OF A RED GLASS TESSERA

The present work presents a study performed on a glass tesserae of one of the stained glass windows of the Santi Giovanni e Paolo church of Venice, Italy (Figure 1). It is a very important Italian gothic Basilica of the 15th century and one of the largest of Venice. The church is famous for the funeral services of venetian doges, actually more than twenty doges are buried in. The sample was collected during restoration in the 1990s and stored until analysis. The sample is a dull red glass tessera contained in the original lead came. The glass deterioration is evident: it is fragmented in pieces, covered by different alteration crusts and it tends to break in more pieces even when it is manipulated with care. There is no record about the tessera provenance, production period and exact position in the original stained glass window. In the Santi Giovanni e Paolo church it was a normal procedure to substitute the broken glass panes for new ones during the conservation-restoration campaigns. The colour and the alteration conditions presented by the samples are particularly interesting. Red glass has always been highly appreciated, but difficult to produce. The venetian environment is particularly salty and with elevate humid percentage. The sample has been studied by means of different techniques in order to have a better understanding of the causes that have reduced this pane to the present delicate conservation state. X-ray fluorescence (XRF), Optical Microscopy (OM), UV-Vis-NIR spectroscopy and other techniques were used. The obtained results have individuated some of the degradation products. The collected information suggest different chemical mechanisms involved in the corrosion process that are a consequence of the Venetian environment but also of various components of the location. The study contributes to a more detailed knowledge of stained glass windows production technology, deterioration phenomena and conservation requirements.