Thermal stress as a possible source of the paintwork loss in medieval stained glass windows

One of the main causes of the loss of paintwork from medieval stained glass windows is the propagation of cracks in the grisaille and in the underlying glass, which cannot be considered the result of a stabilization process, but the evidence of an ongoing stress. The thermal behaviour of different-coloured painted glass pieces exposed to direct solar radiation was investigated in order to study the effect of thermal stresses on the conservation of the paintwork. Experimental results have been compared with laboratory tests as well as with measurements carried out on original stained glass windows. The grisaille showed a thermal behaviour similar to that of the glass on which it is fired. A clear correlation was found between the glass thermal cycles, the increase in stress at the glass-grisaille interface and the growth of cracks in the grisaille. According to these results, protective glazing aids the preservation of the paintwork from the risk of damage and loss, reducing the thermal stresses on the glass of the stained glass windows in terms of amplitude and frequency.