A devastating fire nearly destroyed Notre Dame de Paris two years ago. Now scientists are leading the effort to restore the beloved cathedral to its former glory
Eight restoration scientists put on hard hats and heavy-duty boots, and stepped inside the blackened shell of Notre Dame de Paris, the world’s most famous cathedral. Ten days earlier, a fire had swept through its attic, melted its roof, and sent its spire plunging into the sacred space.
Now, it was silent but for the flutter of house sparrows. The air, normally sweet with incense, was acrid with ash and stale smoke. Piles of debris covered the marble floor.
Yet the scientists, called in by France’s Ministry of Culture to inspect the damage and plan a rescue, mostly felt relief—and even hope. Rattan chairs sat in tidy rows, priceless paintings hung undamaged, and, above the altar, a great gold-plated cross loomed over the Pietà, a statue of the Virgin Mary cradling the body of Jesus.
“What matters isn’t the roof and vault so much as the sanctuary they protect,” said Aline Magnien, director of the Historical Monuments Research Laboratory (LRMH). “The heart of Notre Dame had been saved.”
On April 15, 2019, an electrical short was the likely spark for a blaze that threatened to burn the 850-year-old cathedral to the ground. Following a protocol developed for just such a disaster, firefighters knew which works of art to rescue and in which order. They knew to keep the water pressure low and to avoid spraying stained glass windows so the cold water wouldn’t shatter the hot glass.
But even though their efforts averted the worst, the emergency was far from over. More than 200 tons of toxic lead from the roof and spire was unaccounted for. And the damage threatened the delicate balance of forces between the vault and the cathedral’s flying buttresses: the entire building teetered on possible collapse.
At LRMH, the laboratory tasked with conserving all the nation’s monuments, Magnien and her 22 colleagues apply techniques from geology to metallurgy as they evaluate the condition of Notre Dame’s stone, mortar, glass, paint, and metal. They aim to prevent further damage to the cathedral and to guide engineers in the national effort to restore it.
"More than 200 tons of toxic lead from the roof and spire was unaccounted for"
A carpenter shapes wood into beams needed for the Notre Dame's roof
President Emmanuel Macron has vowed to reopen Notre Dame by 2024. The operation involves many government agencies and has drawn philanthropic pledges of about €1 billion. But it is the LRMH researchers who lead the critical work of deciding how to salvage materials and stitch the cathedral back together.
The LRMH team works in the former stables of a 17th-century chateau in Champs-sur-Marne, in the eastern suburbs of Paris, which once housed a horse research centre.
Here, they have analysed samples from France’s top monuments—the Eiffel Tower, the Arc de Triomphe—in the same rooms where some of the world’s first artificial insemination experiments in horses occurred 120 years ago. The neighbourhood is quiet, but on a day in January 2020, when I visited, the lab was anything but sleepy.
Véronique Vergès-Belmin, a geologist and head of LRMH’s stone division, slipped a hazmat suit over her dress clothes and slid on a respirator mask—necessary when dealing with samples contaminated with lead. In the lab’s storage hangar—once a garage for the chateau’s carriages—she presented several dozen stones that had fallen from the cathedral’s vaulted ceiling. Fallen stones hint at the condition of those still in place.
Heat can weaken limestone, and knowing the temperatures endured by these fallen stones can help engineers to decide whether they can be reused. Vergès-Belmin has found that the stones’ colour can provide clues.
At 300°C to 400°C, she said, iron crystals that help knit the limestone together begin to break down, turning the surface red. At 600°C, the colour changes again as the crystals are transformed into a black iron oxide. By 800°C, the limestone loses all its iron oxides and becomes powdery lime.
"At 300°C to 400°C, she said, iron crystals that help knit the limestone together begin to break down"
“Any coloured stones or parts should not be reused,” Vergès-Belmin said. Colour evaluation isn’t an exact science. Still, in lieu of mechanically testing each of the hundreds of thousands of stones that remain in the cathedral, colour could be a useful guide to their strength.
Water can also wreak havoc. When the firefighters drenched the stone vault, the porous limestone gained up to one-third of its weight in water. In the lab, LRMH researchers monitored a fallen stone, weighing it to track the drying process. The last of the waterlogged stones finished drying in May this year.
Meanwhile, rain continued to fall on the roofless vault. Engineers couldn’t install a temporary cover because of a mangled skeleton of scaffolding, set up in 2018 for long-term renovations. The cathedral walls supported the scaffolding, so it had to be dismantled carefully to prevent a potentially “catastrophic” collapse, Magnien said.
Until the stones finish drying on their own, their changing weights will likely continue to have “non-negligible” effects on the vault structure, according to Lise Leroux, a geologist in the LRMH stone division. Not only does the extra weight play with the precarious balance of forces, but when the water freezes in winter, individual stones expand or contract.
A few weeks after the fire, engineers installed steel beams above the vault so technicians could rappel with ropes as they removed scaffolding and stabilised the structure. Leroux earned a rappelling certification so she could take a closer look. When she inspected the top of the vault for the first time in February 2020, she found that its plaster coating was still mostly intact and had shielded many stones from fire and rain. “It seems to have done its job,” she said.
The Notre Dame after the fire
The COVID-19 lockdowns slowed the removal of the scaffolding, which was finally completed in November 2020. Work could now begin on the cathedral’s interior. In December, the Grand Organ was dismantled and removed, and the pipes taken for repair and cleaning to remove lead dust from the fire. Next, a 89ft-high scaffold was built to give access to the vaults. Reconstruction of the interior was due to begin in the second half of 2021.
Among Parisians, the fire stirred both grief and fear that vaporised lead from the roof and spire had drifted into nearby neighbourhoods. In fact, Aurélia Azéma, a metallurgist who leads LRMH’s metal division, and other scientists have concluded that the fire maxed out well below lead’s vaporisation temperature of 1700°C. Most of the lead simply melted at 300°C, pouring into the gutters and dripping into stalactites hanging from the vaults.
In places, however, temperatures did exceed 600°C, at which point lead oxidises into microscopic nodules. “It’s like hair spray,” Azéma said. A yellow cloud that billowed from the cathedral during the fire showed that at least some of the lead did become airborne.
Some nearby schools were decontaminated after samples showed worryingly high lead levels. But it’s not clear whether the lead came from the Notre Dame fire or from some other source, such as lead paint, car batteries, or leaded gasoline.
"Much of the lead mobilised by the fire remains in the Notre Dame"
Much of the lead mobilised by the fire remains in the Notre Dame. In June 2019, when Azéma and her colleagues brought their first samples from the cathedral back to the lab, tightly sealed in plastic bags, yellow lead dust appeared to be everywhere. She unrolled small organ pipes from layers of bubble wrap and pointed her gloved finger at their holes. “Even down in here,” she said.
Because of lead’s toxicity, France’s national health agency imposes a legal limit of 0.1 micrograms per square centimetre on the surfaces of any building, including historical monuments. “My first sample was 70 times that,” said Emmanuel Maurin, head of LRMH’s wood division, who tested surfaces like the oak confessional and choir seats.
The national work inspection agency has enforced stringent safety requirements. People entering the cathedral must strip naked and put on disposable paper underwear and safety suits and wear protective masks with breathing assistance before passing through to contaminated areas. After a maximum of 150 minutes’ exposure, they hit the showers, scrubbing their bodies from head to toe. “We’re taking five showers a day,” Zimmer says.
The Ministry of Culture has charged LRMH’s researchers with finding a way to cleanse the cathedral of lead without harming it. For most smooth surfaces—glass, metal, waxed wood, and even paint—they’ve found that a shop vac and cotton pads, moistened with distilled water, safely remove the lead. Raw wood surfaces require fine sanding first, Maurin said. The best method for porous stones turned out to be cleaning with compresses and latex, supplemented with laser cleaning for the joints.
Glass researcher Claudine Loisel tests techniques for cleaning lead from Notre Dame's 113 stained glass windows
As the first “emergency” phase of scientific work advanced, Notre Dame started slowly opening to “second phase” scientists interested in studying its history and architecture, now exposed by the fire and available to study without intruding crowds of tourists.
The Ministry of Culture and CNRS created a dedicated science team of about 100 researchers from multiple institutions. “We’re sorting all these thousands of fragments—some from our world, some from another and more ancient world—and it’s like we’re communicating with the Middle Ages,” Dillmann said.
Yves Gallet, an art historian at Bordeaux Montaigne University, oversees a group that aims to study stones that are still in place. Through detailed photographic analysis, researchers want to understand
how 13th-century stonecutters designed and assembled the encasements that cradle the four-story-diameter rose windows.
The charred remnants of attic timbers have stories of their own to tell. “Wood registers absolutely everything while it’s growing,” said Alexa Dufraisse, a CNRS researcher heading the wood group. Notre Dame’s oak beams grew in the 12th and 13th centuries, a warm period. By connecting the growth ring record with what’s known about economic conditions at the time, researchers hope to see how climate variations affected medieval society, she said.
Across centuries marked by war and disease, Notre Dame has witnessed cycles of decline and renewal before. The LRMH scientists hope that when the vaults and buttresses are again dry and sound, the lead accounted for, and the great cathedral’s history and resilience understood more deeply than before, the sense of grief and loss surrounding the fire will once again turn to joy and gratitude.
“There’s an extraordinary unity of people coming together to not only save this monument, but to learn from it,” Magnien said. “Notre Dame will be restored! Its artwork, stone, and stained glass will be cleaned; it will be more luminous and beautiful than before.
“Notre Dame will come out of this experience enriched. And so will we.”
Science (March 13, 2020 Vol 267, Issue 6483), Copyright © 2020 by Christa Lesté-Lasserre. This article has been updated since its original publication.
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