96 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS but glass is an extremely durable material which starts melting at around 800º1200°C. So, the actual damage is typically caused by either the sudden change in temperature of the glass, as outlined earlier, or by the failure of the lead cames themselves. The melting point of lead is typically around 330°C, so in a church fire the cames melt very easily, resulting in the disintegration of the panels as the lead structure falls away. At the east end of St John’s, we found that it was the loss of lead that was the principal cause of damage. When we visited for the survey, the church was as it had been in the immediate aftermath of the fire following the fire team’s hard work to extinguish it. This afforded us an excellent opportunity to examine more thoroughly the process and damage caused by the intense heat of something like a fire. As was clear, the fire had occurred at the east end of the church, causing the east window to be completely destroyed and the surrounding windows extensively damaged. The remains of the east window, merely broken fragments on the ground, were all bearing severe thermal breaks and some pieces were crizzled and crumbly. We also found the results of molten drips of lead, accumulated around the floors, further evidencing the intensity of the temperature during the fire. This was a prolonged event, not just a short sharp blast. Only one panel remained in place in the glass groove of the masonry window structure, surrounded by the results of dripping lead and covered in crizzling breaks. The thermal stress was initially caused by one glass face becoming very hot due to the fire and the other face being exposed to the cooler outside environment and, as the temperature of the fire grew, so did the gradient. Moving down the nave, large amounts of crizzling and thermal breaks were evident throughout the windows, despite these windows being located nowhere near the severity of the fire at the east end of the church. What was clear, however, was that the damage became more sporadic the further away they were from the principal point of the fire. Thus, while the temperature gradient was clearly reduced at these points, thermal stress was still able to occur. A similar reduction in lead damage was also apparent, but given its low melting point, the leading still suffered. In the intermediate windows it was found that the solder and small portions of lead had melted and dripped down, showing that while there was a reduction in temperatures this far from the fire, it was still near the 330 °C mark. In those furthest from the fire there was also bowing in the lead net that had occurred due to the heat, but no discernible thermal damage to the glass itself. So while patterns of cracking were identified throughout the church’s windows, the important thing here was being able to discern the difference between a more typical glass fracture/crack already present at the point of the fire and a thermal break. This should be fundamental to allow our team to deliver a well-considered conservation report, but one that should be noted that only specialists in conservation would be able to impart. SO WHAT SHOULD WE TAKE FROM ALL THIS? Whilst obviously a much rarer occurrence than more typical glass fractures that one would find in historic stained glass, the results of thermal stress are still likely to be found in our built heritage, and will probably increase in the decades ahead. With our ever-changing climate, it is clear that where the windows are exposed to prolonged sunlight and changing temperature conditions, thermal stress can occur. This can be caused by a difference of just 40°C, so the temperature differential doesn’t have to be so extreme to cause the glass to expand or contract abnormally and thus break unevenly with thermal stress. With the climate changes we all face, the issue of thermal stress in the conservation of historic glazing has the potential to grow and become more prevalent. Thus, it is important that research like that of Dr Ivo Rauch is continued. For specialists working in glass conservation, we must continue to look to expand our knowledge and understanding of the issues faced and how we might go about conserving our rich heritage of stained glass. This includes investigating opportunities for removing and reducing any possible likelihood of thermal stress occurrences in the first place. Recommended reading Wang Q, Chen H, Wang Y and Sun J (2013), ‘Thermal shock effect on the glass thermal stress response and crack propagation’, Procedia Engineering 62, 717 Peter Gibson, (1988), ‘Our Heritage of Stained Glass and its Care in the Twentieth Century, RSA Journal 136, 167. Rauch, Ivo (2020) ‘Heat and Light: the Jerusalem Windows’, Vidimus 131, https://vidimus.org/issues/issue-131/heatand-light/ Fensch P and Wagner E, “Das vereinfachter periodische System der Glasbrüche”, Trends und innovationen, (1999) E. Wagner, Glasschäden: Oberflächenbeschädigungen, Glasbrüche in Theorie und Praxis, (Stuttgart: Fraunhofer IRB Verlag, 2012), CARLOTTA CAMMELLI is a stained glass conservator with Recclesia (see page 98). The article was prepared with the help of Lee Bilson, Conservation Manager, Recclesia. Fragments of glass recovered from St John the Evangelist. Even among these fragments one can notice the unusual shapes they have been broken into, proving Wagner’s observations to be true.
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