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Why acts of god hardly ever harm gothic cathedrals

24 January 2017

During the night of 12/13 January, a violent storm tore into the west rose window of the cathedral of Saint-Gervais-et-Saint-Protais at Soissons in northern France, leaving a gaping hole in the façade of the medieval church. The central oculus of the vast stone rose was blown in and of the 16 radiating arches, or petals, only some frail colonnettes remain suspended in the air, while the stained glass hangs in tatters. The destruction of this 800-year-old fabric is both unfortunate and puzzling.

Soissons’s mid 13th-century rose window belongs to a period of dramatic structural developments in Western architecture when architects, encouraged by patrons, pushed the structural possibilities of stone construction to its limits. Without being able to calculate loads but armed with geometry and knowledge amassed by observation, architects reduced masonry to a framework for stained glass thereby transforming walls into luminous surfaces. Rose windows, a key part of the two-tower church façade since the 12th century, were now enlarged to sizes of above 10 metres in diameter, as at Soissons. The radiating patterns of rose tracery made from increasingly thin bars of stone gave the period style its name: Rayonnant, French for radiating.

For all their audacity, medieval architects were not foolhardy. A number of strategies were employed to safeguard the structure. The thin bars of tracery, for example, were held together by invisible iron dowels inserted into the core of the elements. At Soissons, some of the iron dowels can now be seen projecting from the fractured colonnettes. Above the rose, a heavy relieving arch ensured that the slender tracery elements had to carry as little of the weight of the façade as possible.

Thanks to these precautions, the Soissons rose survived not only an explosion when, in 1815, a powder magazine blew up in front of the façade, but also the heavy bombardments during the First World War. Both times, the stained glass of the rose needed replacing, but the rose’s stonework survived. The fact that, on Thursday night, the rose could not withstand Storm ‘Egon’ is due to a combination of circumstances. Exceptionally strong gusts of gale force wind would have pushed the stained glass panels inwards and the large oculus, in particular, at the centre of the rose would have acted like a drumhead, pulling the stonework with its low tensile strength. Weakened by age and previous damage, the stone finally buckled, bursting into the church and damaging the 1950s organ.

While losses during the two World Wars were common, wind damage to rose windows is rare, but not without precedent. Usually, only the stained glass is broken, as in 1768 at Reims, when a storm required the dismounting and re-leading of the façade’s rose window. However, in 1580 at Reims an exceptionally strong wind blew away both the glass and the tracery of the transept rose. Meanwhile, at Soissons the scaffolding has gone up and thoughts turn to repairing what some have called an act of ‘holy vandalism’.