On April 15, 2019, the world watched in horror as the roof of the famed Notre Dame Cathedral in Paris caught fire. The blaze spread rapidly, and for several nail-biting hours, it seemed this 850-year-old Gothic masterpiece might be destroyed entirely. Firefighters finally gained the upper hand in the wee hours of the following morning. Almost immediately after the fire had been extinguished, French President Emmanuel Macron vowed to rebuild Notre Dame.
But first, the badly damaged structure had to be shored up and stabilized and interdisciplinary teams of scientists, engineers, architects, and master craftspeople assembled to determine the best way to proceed with the restoration. That year-long process—headed up by Chief Architects Philippe Villeneuve and Remi Fromont— is the focus of a new NOVA documentary premiering tonight on PBS. Saving Notre Dame follows various experts as they study the components of the cathedral’s iconic structure to puzzle out how best to repair it.
Director Joby Lubman was among those transfixed in horror when the fire broke out, staying up much of the night as the cathedral burned, until it became clear that the structure would ultimately survive, albeit badly damaged. In the office the next morning, “Everyone was a bit shell-shocked talking about it,” he told Ars. “And it might sound opportunistic, but I thought, ‘The restoration of this icon is going to be quite something to document.'”
Lubman and his development team reached out to the appropriate authorities, and within a few weeks they were sitting in on meetings with scientists, architects, engineers, and others combining their expertise to restore Notre Dame to its former glory. “It’s the ultimate restoration and a perfect synergy between science and history,” he said.
While Lubman never felt he and his crew were in any real danger, filming on-site was a challenge because of the strict safety protocols in place. Nobody was allowed to work under the vaulting, which was cordoned off. And one scene in the documentary depicts an alarm going off on the very first day of filming because the 550 tons of badly mangled scaffolding towering over the restoration site had shifted precariously in the wind.
“We all had to leave the site,” he said, scientists included. “That set the tone for our filming, which was highly unpredictable. It was very much, we get what we get [on film] when we go on site.”
The original engineers
Notre Dame is an architectural masterpiece, a testament to the collective expertise of centuries of craftspeople. “They were the original engineers, before engineering as a term existed,” said Lubman. “You couldn’t go to school to learn engineering, it was passed down from father to son over many, many generations. It is rather lovely to think that these buildings are the product of thousands of years of experience. I think so much of that knowledge is imprinted in the materials themselves that have been used. People today can look at the craftsmanship and at the tool marks, and understand exactly how they did it.”
Among the experts featured in Saving Notre Dame is glass scientist Claudine Loisel, who was relieved to find that the cathedral’s famed stained glass windows were intact and not too badly damaged. There were microcracks in some panels from the thermal shock of the fire, and most windows were coated with the toxic lead dust that was emitted when the lead roof burned. She figured out an effective decontamination plan involving a tiny precision vacuum cleaner, followed by removal of any further residue with cotton balls soaked in distilled water. X-ray spectroscopy analysis helped her determine just how many wipes were needed to remove the lead without damaging the paint underneath.
For that segment, Lubman also took his film crew to York, where conservationists have adopted a new preservation approach to the stained glass windows of York Minster Cathedral. The cathedral caught fire in 1984, shattering the glass of the South Transept rose window, although the lead held it together, allowing it to be taken down and painstakingly reassembled. The new approach involves installing protective clear glass external frames before replacing the original stained glass windows. The gap between them improves ventilation and prevents condensation from building up on the original stained glass, as well as protecting it from damaging UV rays.
The lessons learned from the York Minster fire proved helpful to Parisian firefighters. “They learned that they must not spray water onto the glass, they have to cool the stone around the glass,” said Lubman. “If you spray water onto hot glass, it’s going to explode, which is what happened in York. It took them decades to put that glass back together and reinstate it, and it’s now very fragile because of the thermal shock.” Without that hard-earned knowledge, there could have been catastrophic damage to Notre Dame’s stained glass windows. “There could have been acres of windows that needed to be glued back together,” he said. “Instead, it was just some smoke damage, a bit of lead, and a few microcracks here and there.”
The cathedral’s massive limestone vaults, designed to fireproof the building’s interior, largely held up under the collapse of the burning roof. But there were significant punctures from the burning spire as it toppled that will need to be repaired. The task of determining the origin of the limestone used to construct the vault fell to geologist Lise Leroux. She was able to match microfossils found in samples from the original vaulting stones to make that determination.
It turns out that the limestone came from the catacombs under modern-day Paris—mining quarries that were repurposed in the late 18th century as a repository for the remains of some six million people relocated from Parisian overflowing cemeteries. “Of course, that’s intuitive, they would take the stone from the closest source,” said Lubman, who relished the chance to film in the catacombs. “The quarry itself posed constraints on the size of the stones for Notre Dame. That’s why you don’t see massive stones [in the cathedral]. They’re all small because they had to be, because the sedimentary layers that made up the limestone deposits were that size.” Larger blocks would have introduced structural weaknesses into the cathedral.
Finally, there was the challenge of rebuilding the intricate timber roof supporting the outer lead roof, which was completely destroyed. It was known that the oak trees used for the original roof had been cut down around 1160, amounting to about 13,000 trees. Timber scientist Catherine Lavier was able to use markings on burned beams and tree ring analysis to more precisely determine the age and origin of the oak trees, to ensure that similar modern-day trees could be found for the reconstruction. As for the roof’s intricate geometry, Fromont had serendipitously completed a 3D scan of the roofing structure in 2014, containing all the necessary information to rebuild it.
A “digital twin”
That 3D scan—as well as scans performed by a scientist named Andrew Tallon to create precisely detailed maps of the interior and exterior of the cathedral—will be incorporated into a planned, elaborate 3D “digital twin” of Notre Dame, encompassing all the data currently being gleaned with regard to materials, structure, geometry, and many other aspects of the cathedral. It’s an ongoing project meant to “protect against ever again risking the loss of the secrets and memory of what some call ‘the heart of France,'” per the documentary’s production notes.
“The people who built Notre Dame didn’t think of it as something they would see realized within their lifetime,” said Lubman. “In a similar way, this digital twin is all about using the technology of the day to capture whatever you can in terms of knowledge about the structure. The people making this digital model will never see it realized in full because all the technologies required are not there yet. They’re going to keep adding layers to this resource for future generations. It’s a rather lovely altruistic project that’s there for people who are not even born yet.”
One of those additional layers of information will likely concern the acoustics, which is also part of Notre Dame’s unique cultural heritage. As we’ve reported previously, given the ephemeral nature of sound, acoustic characteristics can be far trickier to preserve or reproduce. Fortunately, a group of French acousticians made detailed measurements of Notre Dame’s “soundscape” over the last few years, along with two other cathedrals. That data will be instrumental in helping architects factor acoustics into their reconstruction plans.
To capture the fine details of the cathedral’s unique soundscape, acoustician Brian Katz of CNRS at the Sorbonne set up a collection of omnidirectional 3D microphones, along with a dummy head microphone and strategically placed loudspeakers at the sound sources. That allowed his team to create an aural map of the cathedral’s acoustics using numerical simulation software. In early July, Katz’s acoustic task force made new measurements to map the acoustics of the cathedral since the fire. Katz et al. recently published its initial analysis of the before and after soundscapes in the journal Acoustics.
Saving Notre Dame only covers the preliminary work leading up to the actual restoration, which will be ongoing for the next several years—if not decades. Lubman is definitely keen to get his crew back on site to document that process as well, hopefully all the way through the ultimate reopening of the cathedral. The pandemic has precluded that for now, but we can probably look forward to additional episodes in the future.