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 like an arrow into the heart of the sacred space. Now, it was silent but for the flutter of house sparrows. The space, normally sweet with incense, was acrid with ash and stale smoke. Light beamed through voids in the vaulted stone ceiling, cutting through the gloom and illuminating piles of debris on 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," says Aline Magnien, director of the Historical Monuments Research Laboratory (LRMH). "The heart of Notre Dame had been saved."
On 15 April 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. President Emmanuel Macron has vowed to reopen Notre Dame by 2024, and he has appointed a military general to lead the operation, which involves many government agencies and has drawn philanthropic pledges of about €1 billion. But it is the LRMH scientists who lead the critical work of deciding how to salvage materials and stitch the cathedral back together. And even as they try to reclaim what was lost, they and others are also taking advantage of a rare scientific opportunity. The cathedral, laid bare to inspection by the fire, is yielding clues to the mysteries of its medieval past. "We've got 40 years of research coming out of this event," says LRMH Assistant Director Thierry Zimmer.
The LRMH researchers work in the former stables of a 17th century chateau in Champssur-Marne, in the eastern suburbs of Paris, that once housed a horse research center. Here, they have analyzed 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 neighborhood is quiet, with a quaint brasserie and a shop offering €10 haircuts. But on a day in January, the lab is anything but sleepy. "It's an ambiance of speed!" says Zimmer, sporting a brown wool beret and a bushy mustache.
Véronique Vergès-Belmin, a geologist and head of LRMH's stone division, was sorting cathedral stones until 10 p.m. last night. This morning, she's the first to unlock the laboratory's ancient oak door.
She slips a hazmat suit over her dress clothes and slides on a respirator mask-necessary when dealing with samples contaminated with lead. In the lab's high-roofed storage hangar-once a garage for the chateau's carriages-she presents several dozen stones that fell from the cathedral's vaulted ceiling. Fallen stones hint at the condition of those still in place, which are largely inaccessible. The scientists can't risk adding their weight to the top of the vault, and debris falling near the holes in the ceiling makes it dangerous to inspect the structure from below. Many of the samples in the lab were retrieved by robots.
Heat can weaken limestone, and knowing the temperatures endured by these fallen stones can help engineers decide whether they can be reused. Vergès-Belmin has found that the stones' color can provide clues. At 300°C to 400°C, she says, iron crystals that help knit the limestone together begin to break down, turning the surface red. At 600°C, the color 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. "It's an entire progressive process," she says, enunciating carefully through the muffle of the mask. "Any colored stones or parts should not be reused."
Color evaluation isn't an exact science, she says. Still, in lieu of mechanically testing each of the hundreds of thousands of stones that remain in the cathedral, color could be a useful guide to their strength.
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