Rebuilding the cathedral after the blaze

EMMANUEL MACRON was unequivocal. Speaking after a fire destroyed the roof and spire of the cathedral of Notre Dame in Paris, the French president vowed: “I solemnly say tonight: we will rebuild it together”. An international fund-raising drive has already received its first big pledges. François-Henri Pinault, boss of the Kering luxury-goods company, said he will donate €100m ($113m); the family of Bernard Arnault and his LVMH group, another luxury-goods firm, promised a further €200m.

Exactly how much money will be needed will depend on exactly what the police and fire service find as they rummage through the wreckage. But early signs are encouraging. The efforts of the firemen prevented the blaze from reaching the cathedral’s two belfries. And while pictures of the blaze looked catastrophic, says Lisa Reilly, an architectural historian at the University of Virginia, Notre Dame’s Gothic architecture was designed partly to resist such accidents.

The heart of the cathedral is an enclosed stone vault, supported at the sides by buttresses and with a timber-framed roof built atop the ceiling. The idea, says Dr Reilly, is that if the roof catches fire—which was a serious risk in an age when lighting came from candles or oil lamps—it can burn away without compromising the structural integrity of the vault itself. That theory seems to have (mostly) held up. The main body of the cathedral is still standing. But early pictures show that at least one section of the vault has fallen in, scattering charred debris across the cathedral floor.

Stone may be more resistant to fire than wood, but it is not immune to its effects. One critical question is just how hot the vault became. There was plenty of fuel at Notre Dame. The cathedral’s roof was held up by oak beams so densely packed that they were known as “the forest”.

Under the right conditions wood fires can reach temperatures of 1,000°C or more. Temperatures much lower than that can be enough to cause damage. After a fire at Windsor Castle outside London in 1992, the Building Research Establishment, then a part of the British government, commissioned studies into the effects of fires on stone buildings. Notre Dame is built from limestone, which the BRE found became discoloured at the relatively low temperature of 250°C, thanks to chemical reactions in iron compounds within the stone. Higher temperatures caused more serious problems. In laboratory tests, limestone began to weaken at around 600°C. At 700°C its compressive strength had fallen by about 40%. Rapid cooling—thanks to being deluged with water from firehoses, for instance—weakened the stone still further.

Stone is a poor conductor of heat, which means that the temperature on the side of a masonry block facing a fire can be far higher than on the side facing away. Such thermal stresses can cause big cracks. After a fire at York Minster cathedral in 1984 around 100 tons of cracked stonework needed replacing. Even the cooling process can cause problems. “The vault will have had burning timbers sitting on it for some hours,” says Robert Bowles, a British structural engineer who specialises in conserving old buildings. “As the masonry cools down and shrinks it may deform.” One of the priorities will be trying to determine just how sound the damaged portions of the vault’s ceiling remain.

History, though, offers reasons to be cheerful. Notre Dame has been restored before, after vandalism during the French revolution and neglect until the 19th century, when the spire was rebuilt. Dr Reilly points to the cathedral at Reims, whose roof was similarly burnt away after it was hit by shelling in the first world war. Later attacks caused even more damage, leaving the building in a much worse state than Notre Dame. A restoration project, partly funded by the Rockefeller Foundation, saw Reims reopened in 1938.

Today's restorers should have an easier time of it. Researchers led by the late Andrew Tallon, of Vassar College, in New York, conducted a detailed laser scan of the entire building. Their billion-plus data points constitute a virtual copy of the building, accurate to within five millimetres, that could prove vital in the reconstruction effort.

A resurrection of Notre Dame, in other words, seems eminently possible. Another priority task, says Mr Bowles, will be to erect a temporary roof so the drying-out process can begin, and to protect the remaining masonry from the elements. Here, at least, the gods seem to be smiling: the weather forecast in Paris is for a week of bright sunshine.