94 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS GLASS AND HEAT Thermal stress and thermal breakage of historic stained glass CARLOTTA CAMMELLI WE HAVE all probably seen glass breaking due to a sudden change in temperature at least once in our lives. This might have been while taking a very hot glass tray out of the oven or pouring a hot drink into a glass not designed for the task. The quick action is caused by the rapid heating or cooling of the glass when intense extremes of temperature are applied to its surface. This process is what we refer to as thermal stress. The consequent expansion of different parts of the glass by different amounts can then cause thermal breakage. But what is probably less well known is that this phenomenon can also happen to window glass, and pertinent to this article, to stained glass windows, whether historic or new. THERMAL BREAKS TO STAINED GLASS The damage and decay of a stained glass window is often related to its composition and its environment. Often the damage we find ranges from glass corrosion to paint loss, lead breaks and cracks. As conservators and consultants we must consider and understand all types of damage and decay, some of which might seem unusual, but thermal stress and thermal breakage in stained glass are actually quite common in some circumstances. Thermal breakage usually occurs between two zones of the glass as a result of a large temperature difference between them. This is not only related to the nature of the glass, but the condition of its edges and its resistance to thermal movement. Thermal breakage in this context can be caused by a range of factors, from a window’s geographical or physical location, such as being on the southern side of the building or near a heating outlet, to specific incidents such as building fires. Wagner, Fensch and others have identified that if a window is partly in shade and partly in direct sunlight, this can create a high difference in temperatures on the surface which could lead to breakage. Moreover, climate change is undoubtedly becoming more of a factor, exacerbating this and other conservation issues. It is for all of these reasons that thermal stress and breakage must be recognised as a specific type of decay which needs to be more deeply understood to ensure appropriate conservation treatments are designed. When researching thermal damage caused by high temperature changes, as would be the case in a building fire, it is clear that there is a gap in the literature, with often vague and superficial information on both failure process and subsequent conservation. The majority of references refer specifically to single glazing or more generally glass sheets, but the reaction of stained glass to thermal stress will probably differ due to its unique nature and the varied glass types working together. The most basic behaviour of glass subject to temperature change has long been known. Medieval glaziers were aware of thermal breakage, evidenced by their use of it to cut glass. Theophilus, who wrote instructions in On Divers Arts for the preparation and use of glass in the 12th century, noted that the use of a red-hot iron on a wet trail on the glass surface would break the glass on that specific area. Medieval ingenuity in action – using controlled thermal breakage in their production methods! If we look more specifically at the problem of thermal breakage in the event of accidental fires, there are a range of example situations that have been very well documented, good examples being the Canterbury Cathedral fire in 1872 and the fires at York Minster in 1829, 1840 and 1984. In the latter there is a range of glass fragments bearing fire cracks and crizzling, evidence of which remains in some of the windows today. More recent examples include the infamous fire of Notre Dame in Paris in 2019, which occurred despite stringent fire precautions being in place. Even with the best will in the world, fires still happen. An example of thermal breakage in glass and its crizzled nature at York Minster (All photos: Carlotta Cammelli)
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