95 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 METAL, WOOD & GLASS 3.3 HOW THERMAL BREAKAGE DIFFERS In a normal glass fracture caused, for example, by the impact of a fast-moving object, the pressure applied to the glass is mechanical. But when we look at thermal breakage, the pressure is caused by the high temperature differential which causes the glass to expand or contract abnormally, causing it to break unevenly. The morphology of these thermal breaks is vastly different from those of normal cracks as they are much more irregular and seemingly unpredictable in form. But as Wagner has shown in research (published in Glasbrüche in Theorie und Praxis, see recommended reading), the nature of these cracks, despite their seemingly random appearance, actually have a specific behaviour. These cracks always start from the border of the glass with a 90-degree angle, which then change direction multiple times in cold and warm areas, then end in sharp corners. What is more, the structure of thermal breaks changes in relation to the intensity of thermal stress involved, along with the intensity of the temperature gradient. This can change from a single break to several breaks that completely destabilise the entirety of the glass piece. Putting it simply, the higher and more sudden the change of temperature, the quicker and greater the damage. If we go back to York, we can find an excellent example of this phenomenon from the most recent fire at the Minster in 1984. This fire affected the Rose Window on the south end of the cathedral, where it has been suggested that the majority of the damage to the window occurred in the moment the roof of the south transept collapsed, causing a rapid cooling of the building and, of course, the glass. While fires might be extreme and thankfully rare, examples of heat impact show that the heat intensity does not have to be this extreme to cause thermal stress to occur. Dr Ivo Rauch has studied and presented the example of the windows of the Lutheran Church of the Redeemer in Jerusalem, where the windows had been exposed to prolonged sunlight suggesting that a difference of just 40°C is enough to cause thermal stress and breakage. CASE STUDY: ST JOHN THE EVANGELIST, STOKE-ON-TRENT More recently, Recclesia’s stained glass team had the somewhat sad opportunity to survey the church of St John the Evangelist in Stoke on Trent following its own devastating fire in April 2022. It was a rare opportunity to observe the extent of damage caused by thermal stress on the stained and leaded glass and how the various areas of heat intensity in different parts of the church building affected it. We were therefore presented with clear examples of the impacts of different levels of thermal stress and breakage. The east window and the adjacent windows (nII, sII) were all completely lost, while the rest of the windows had survived with varying but extensive damage. But before we dive into the examples found in the church, it is worth addressing a misconception we often encounter where stained glass is damaged by fire: some people assume the damage is due to the windows catching fire or burning. This is not the case. When looking at church fires, the majority of the fuel for the fire is timber, which often burns between 600–700°C, The exterior of St John the Evangelist’s, Stoke on Trent after the fire, and (right) the only surviving panel from its east window: the lead had completely melted and dripped to the ground. Glass in a window (n111) close to the fire at St John the Evangelist, Stoke on Trent: the fractures illustrate the morphology of thermal breakage described by Wagner, with cracks starting from the border at a 90-degree angle and then changing direction.
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