On Monday 15 April, a fire broke out in the Notre-Dame de Paris. Believers and tourists were invited to leave the cathedral immediately. A race against time was launched which would last more than fifteen hours, defying all human bravery to save the Notre-Dame. Thousands of people gathered around the cathedral this evening to reflect, pray, and witness this catastrophe which none had thought possible to an eight-century old lady that had accompanied people in their joys and sorrows and had survived wars, plagues, revolutions, and occupations. The emotion was felt worldwide and donations for the reconstruction arrived quickly. The fire destroyed the spire, the timber roof structure, and part of the vault, at the level of the transept crossing. Three years later, the investigation to find the cause of this fire is still ongoing.
This article does not claim to be an exhaustive account of the issues associated with the reconstruction of Notre-Dame, nor does it claim definitive answers to an ongoing investigation. It rather seeks to put forward a scientific hypothesis on the causes of this fire, not only for the pure satisfaction of knowledge, but also in the hope to raise awareness about the widespread but relatively unknown phenomenon of pyrolysis during works on monuments.[1] It is crucial that architects and companies working on listed buildings are aware of the phenomenon of pyrolysis so that they can adapt their working protocols, be more vigilant, and request more efficient fire detection instruments to avoid future similar disasters. In recent years, other major French monuments undergoing restoration works had been destroyed without the causes being identified: the seventeenth-century Hôtel Lambert in the centre of Paris in 2013 and the flamboyant gothic townhall of La Rochelle in 2013 are just two examples. In the 1990s, a fire outbreak was discovered just in time at the Beauvais Cathedral the day following some hot-spot work using a blowtorch. It is worth reminding the reader that the vast majority of fires happening during work are caused by hot-spot works.[2]
The phenomenon of pyrolysis and, more generally, of slow combustion are still little known to those working on historic monuments, whether they are architects or craftsmen. The case of Notre-Dame de Paris deserves to be studied in greater depth because this hypothesis remains the most likely, given the conditions that existed during the restoration of the spire in the days preceding the fire.
The current investigation has only shown that the fire started at the foot of the spire (fig. 1). Samples of charred wood have made it possible to locate the fire’s starting point in the area of ‘the wall plate of the choir at the south-east corner of the transept crossing’ (fig. 2).[3]
