far left Notre Dame cathedral in Paris ablaze on 15 April 2019. left The cathedral’s 19th-century spire was completely destroyed in the fire. The French authorities have now stated that it will be rebuilt exactly as it was.

Saving Notre Dame

When flames ripped through the cathedral of Notre Dame de Paris in April 2019, the world feared for its survival. Now a small team of scientists is working towards its restoration and discovering secrets along the way. Christa Lesté-Lasserre spoke to some of them about their work and its challenges.


Fate.’ It is the word, etched into a stone of the North Tower, that inspired Victor Hugo to write The Hunchback of Notre Dame, the novel that purportedly changed the history of the most-famous cathedral in Paris. Pillaged and vandalised after France’s 1789 revolution, Notre Dame de Paris had become a symbol of monarchical power worthy of destruction. But when Hugo’s book bearing its name appeared in 1831, deep emotion stirred the nation. By 1843, the French government had commissioned a massive overhaul of the 65,000-square-foot monument, conserving some of the world’s richest medieval architectural designs – employing stone, wood, metal, paint, and glass – and adding what some observers might describe as ‘ambitious’ 19th-century touches.

far left Notre Dame cathedral in Paris ablaze on 15 April 2019. left The cathedral’s 19th-century spire was completely destroyed in the fire. The French authorities have now stated that it will be rebuilt exactly as it was.
Notre Dame cathedral in Paris ablaze on 15 April 2019.

Touches, for example, like the 315-foot-high spire that collapsed in a ball of smoke and flames last April – as, to the horror of the entire world, the 19th century came crashing down in the 21st. But, if the terrible events of that night represented the end of one chapter in Notre Dame’s history, the next will be written by a small group of French scientists who specialise in the restoration of historical monuments, and who have been collaborating with and advising the architectural team in charge of its rescue. As they strive to understand the stricken structure’s most-pressing scientific needs, they have been working days, nights, and weekends – in temperatures ranging from 50°C, in its the ray-reflecting white stone heights during France’s heatwave last summer, down to near-freezing in the damp winter months in the windswept and as-yet-uncovered nave.

The cathedral’s 19th-century spire was completely destroyed in the fire. The French authorities have now stated that it will be rebuilt exactly as it was.Image: Guillaume Levrier/Wikimedia Commons.

While other research teams are now starting to trickle in – seizing the rare opportunity to study the cathedral’s fascinating history and structural secrets – that work is secondary, the experts agree. First and foremost, the scientific efforts must focus on a single aspect of Notre Dame de Paris: the future survival of the building itself. In a word, its fate.

Just before 7pm on 15 April 2019, firefighters rushed to Notre Dame armed with protective gear, hoses – and a plan. Well-briefed and expertly trained in the specific protocols for fire-management at each of Paris’s historical monuments, they evacuated the cathedral’s priceless works of art in a strategic order. They also knew the right actions to take to preserve as much of the delicate structure as possible, thanks to evidence-based scientific recommendations – such as keeping water away from the stained-glass windows to prevent the thermic shock that would have shattered them. As Thierry Zimmer, associate director of the Regional Laboratory for Historical Monuments (LRMH), explains, such procedures are all based on vital knowledge gleaned from decades of scientific investigations into disasters at other historical monuments. Located in the eastern Parisian suburb of Champs-sur-Marne, the LRMH’s laboratory is commissioned by the country’s Ministry of Culture to manage the conservation science for all its historical monuments nationwide.

‘The firefighters have a sort of “instruction manual” that’s unique for each monument, and they practise regularly,’ Zimmer says, adding that their excellent execution of the plan, lasting nine hours, was a ‘heroic’ first step in saving Notre Dame. Immediately after the fire had been extinguished, architects and conservation scientists from the Regional Directorate of Cultural Affairs (DRAC) were able to begin their investigative work in the monument. Their mission: to evaluate the damage done and to assess the potential danger to anyone, including forensic teams and the 23 LRMH researchers, who needed to get into the building.

A beam of wood under archaeometric study (left) and debris in the choir stalls (right). Images: LRMH.

By 24 April, entry had been secured for police teams and a small group of LRMH scientists who were scoping out the enormous task ahead of them. Despite finding the entire roof and a spire missing, three holes in the vaulted ceiling, and a mass of debris on the floor of the transept, the scientists generally expressed relief. ‘It was affecting, of course, to see the cathedral so damaged,’ recalls Aline Magnien, the LRMH’s director. ‘But this is a religious building where what matters isn’t the roof and vault so much as the sanctuary they protect.’

Still, the scientific emergencies were evident. There were more than 200 tons of lead – toxic under certain conditions – to account for. And there was an entire building at imminent risk of collapse.

Emmanuel Maurin’s diagram shows how square beams were cut from round logs, demonstrating the size of trees needed to replace the burned attic timbers.

Stone might seem solid, but the integrity of any cathedral’s walls is subject to a delicate balance of forces, says Magnien. That’s part of the ‘genius’ of medieval architecture, which allowed early builders to construct impressively high, yet relatively thin, walls that can hold up thousands of tons of vaulted ceiling. The arched shape of Notre Dame’s 100-foot-high vault had an aesthetic purpose, but also served a critical engineering objective: to trap heat from potential roof-fires – a job it did very well last April, Magnien adds. But these vaults create a powerful outward force on the cathedral walls, which is why medieval architects added exterior flying buttresses that push the walls inward.

The roof’s beam supports also play into the balance of forces, according to Emmanuel Maurin, head of the LRMH’s wood division. Tie beams and plate beams – part of the attic’s lattice of ancient oak woodwork known as the ‘forest’ – appear to have been intentionally installed in a specific order so as to balance forces during construction, and they might have played such a role after construction was complete, too.

Most of the original 3,500 attic beams – including those tie and plate beams – burned in the fire. At the same time, many of the stones in the vault experienced such high levels of heat they might have ‘turned to powder’, or at least lost significant structural properties. ‘The balance has been upset, with a different ratio of forces left and right, inwards and outwards,’ says Magnien. The question is: which of them will win?

It is the job of the architects and civil engineers to calculate those forces and provide support where necessary – including, in one of their first post-fire actions, by adding wooden counter- forces to neutralise the action of the flying buttresses. But it is down to the scientists to analyse the materials to see how they have been modified by the heat.

Lise Leroux, dressed in protective gear, examines a lead-covered stone in her laboratory. The stone fell from Notre Dame’s vaulted roof during the fire.

One thing that can help, according to Véronique Vergès-Belmin, head of LRMH’s stone division, is knowing the temperatures to which they were exposed. For example, limestone reaches the ‘powder’ stage that Magnien mentioned when it reaches 800°C. But even at lower temperartures, its properties are already starting to be modified. ‘What you want to see is beige,’ explains Vergès-Belmin. ‘Beige stone contains iron hydroxides, which are in a sturdy crystallised form. At 300 to 400°C, they spontaneously transform into iron oxides, turning the surface red. At 600°C, they reach another state of iron oxide, making the stone black. Then, by 800°C, the calcium loses all its iron oxides and becomes the white chalk we know as lime. It’s an entire progressive process.’

Colour evaluation is not an exact science but, rather, a good indicator, Vergès-Belmin adds. In the absence of precise mechanical testing and sampling of each of the hundreds of thousands of stones, colour can be a useful guide to which stones ‘should not be reused’.

Heat is not the only thing affecting the properties of these stones. While the firefighters carefully avoided the stained-glass windows with their water jets, they had no other choice but to ‘totally drench’ the stone vault, says Lise Leroux, a geologist in the LRMH stone division. With a porosity of 30%, the limestone suddenly gained a third of its weight in water as a result – and it is not set to lose it very quickly. In-lab weight studies of one of the fallen stones indicates that the drying process is particularly slow, taking at least a year for the water fully to evaporate.

clockwise from far left Microscopic images of dust inside Notre Dame’s organ pipes; ; Aurélia Azéma carefully handles two lead pipes from the organ; scanning electron microscope reveals a nanoparticle of lead from a sample of dust from the organ seat. right Claudine Loisel examines dust on a panel of stained glass under a binocular microscope. below Blackened soot and dust on a panel of stained glass.
Microscopic images of dust inside Notre Dame’s organ pipes.

Meanwhile, the gaping holes in the ceiling have left the stones exposed to a regular supply of rainwater. ‘They can’t instal a cover until the damaged scaffolding is removed,’ Leroux explains. Around 60,000 bars of scaffolding, which partially melted in the blaze, were set up in 2018 in preparation for 20 years’ worth of renovation work. Facing its own issues of unbalanced forces now, this vast tangled construction was subject to a dismantling process that began in January and should have ended in July – had COVID-19 not brought the removal efforts to a temporary halt.

The stones’ increased weight – and hence the forces they exert – is likely to be having ‘non-negligible’ effects on the entire vault structure, adds Leroux. Not only is the water playing with the building’s equilibrium, it is also causing expansion and retraction of individual stones, exacerbated by exposure to weather conditions.

While the stone scientists have been busy with mechanical forces, another team has been concentrating on the whereabouts of the lead roof that, according to reports at the time of the fire, had just ‘vaporised into thin air’. Fortunately, says Aurélia Azéma, the head of LRMH’s metal division, those reports turned out not to be true. ‘The press keeps talking about how 400 metric tonnes of lead went up in smoke, and that’s just not the case,’ she explains.

Aurélia Azéma carefully handles two lead pipes from the organ.

The laboratory’s archives hold the original construction estimate for Eugène Viollet-le-Duc’s 1850 roof renovation, calling specifically for 227 metric tonnes of lead (including 160 tonnes recycled from the then-existing roof). The total amount of lead on the spire probably did not exceed 15 metric tons. But, more importantly, relatively little of all this lead left the building, Azéma says. The vast majority of it reached the metal’s 300°C melting point, pouring into ‘pools of lead in the gutters’ and dripping into stalactite shapes in the vaults. None of it, according to the information gathered by the team, entered a gaseous state. ‘We can’t say it vaporised,’ Azéma explains. ‘Temperatures have to reach 1,700°C for that.’ Scientists from various disciplines have concluded that the fire probably reached a maximum temperature of about 1,000°C.

the lead challenge

A small portion of the lead did reach temperatures high enough to become airborne, however. (Exactly how much remains to be determined when scientists can safely access what is left to measure.) When lead reaches 600°C, it oxidises into PbO and enters an aerosol state, creating microscopic nodules that get lifted into the air. The sphere shape is lead oxide’s way of dealing with the high temperatures, as it is a form that ‘requires the least energy’, says Azéma.

Scanning electron microscope reveals a nanoparticle of lead from a sample of dust from the organ seat.

Lead oxides have various forms, or polymorphs, that are associated with different colours, Azéma explains. The polymorph from Notre Dame’s roof happens to be yellow – as could clearly be seen from the yellow cloud billowing out from the cathedral during the 2019 fire. The dust it created provides further evidence. Light layers of yellow powder settled on to clean surfaces in the cathedral – such as the wooden organ bench where the organist had sat only a few hours earlier, or the purple drape that had just been placed over the statue of Christ in the St Eloi chapel. Now that they know what to look for, Azéma says, the LRMH team is finding lead oxide nodules everywhere within the cathedral: ‘They’re on the stones, the paint, the wooden structures, the stained-glass windows, the floor, the furniture…’.

Claudine Loisel examines dust on a panel of stained glass under a binocular microscope.

Lead powder is hydroscopic, meaning it retains water, which can cause corrosion on the various ancient materials in the cathedral. But, more importantly, when consumed at high-enough doses, it can be toxic for humans and animals (leading to brain damage, among other issues). Although the scientists themselves generally express little concern about their exposure to lead during their working hours in the cathedral (‘It’s not like we’re licking the walls,’ Azéma says), the national work inspection agency has enforced stringent safety requirements. Anyone entering the building must take a lead safety test, and each entrance starts with stripping down to bare skin and donning a disposable paper safety suit and matching underwear before passing through a shower corridor to the ‘dirty side’ (where the lead exposure is). The LRMH researchers and other frequent visitors can dress in reusable ‘dirty gear’ on the other side of the corridor – but only provided it never leaves the dirty side.

Past the showers, a map of areas that are particularly dirty is updated every day, warning workers of sections of the cathedral where lead exposure is more likely, as the movement of materials stirs up the yellow dust. Once in the dirty areas, everyone has to wear protective masks with breathing assistance. After a maximum of 150 minutes’ exposure, workers peel off the dirty gear and mask to go through the shower corridor – the only way out – scrubbing the entire body from hair to toes, as well as any equipment they brought with them, before finally being able to get a towel back on the ‘clean side’.

Blackened soot and dust on a panel of stained glass.

Though necessary, such stringent safety requirements inevitably make the scientists’ work more complicated. But, while lead-decontamination protocols already exist for people, they do not for historical monuments or works of art – so, at the behest of France’s Ministry of Culture, the LRMH is finding effective, easy-to-execute, cost-efficient methods for ridding the cathedral of its lead pollution without consequences for the original material. Claudine Loisel, head of the LRMH glass division, has been testing decontamination techniques that preserve the integrity of the cathedral’s 113 windows. Blackened and sticky with soot, dust, and the residue from millions of tourists’ candles, the windows lack the yellow-powder look. But, using a binocular microscope and X-ray fluorescence, Loisel easily detects the lead-oxide nodules on three panels she has brought back to her laboratory.

Most of the cathedral’s polychrome – or painted surfaces – was unaffected by the fire itself. But it may have suffered minor damage from the firefighters’ efforts, explains Witold Nowik, head of the LRMH paint division. ‘There’s peeling paint on the walls, but we don’t know if that’s from their water sprays or if the damage dates to before the fire,’ he says. The primary challenge, as for the rest of the cathedral, is getting the lead out without damaging the integrity of the original art. ‘For the polychrome – especially that from the 19th century – it’s especially difficult because so much of it included lead to start with,’ he adds. With the help of an X-ray fluorescence spectrometer and a diamond microscope, Nowik’s team has been studying the polychrome materials used – often in multiple layers, as surfaces were repainted – on the cathedral’s structures.

above This golden angel survived both the fire and a significant fall, which revealed the previously unseen signature of the artist who created it.
This golden angel survived both the fire and a significant fall, which revealed the previously unseen signature of the artist who created it.

In the course of his work, Nowik has ‘discovered’ the origins of Notre Dame’s four gold-painted angels, which came crashing down during the catastrophe. Remarkably, two survived the 25-metre drop, and one was so well preserved it even revealed its hidden secret – a previously unseen signature, confirming the identity of the artist. ‘It seems simple, but getting access to these pieces is resolving mysteries of the artistic history of this monument,’ he says.

For Maxime L’Héritier, the co-founder of the Association of Scientists Supporting the Restoration of Notre Dame (an organisation which now has nearly 300 members), Notre Dame’s fire has sparked an ‘unprecedented cross-discipline dynamic in historical monument research’. The catastrophe stirred the emotions of the scientific community, he says, driving the collaboration: ‘We’re all asking, what can we as scientists do to help?’

Such emotion-driven approaches have even become the subject of their own unique branch of ‘Notre Dame science’. The official ‘Heritage Emotion’ work group, headed by Claudie Voisenat of the Centre National de la Recherche Scientifique (CNRS), is one of eight groups designated by the Ministry of Culture and the CNRS to cover different aspects of the restoration.

The other groups represent each of the structure’s materials, as well as acoustics, civil engineering, and digital data. CNRS researcher Livio de Luca, for example, is collecting thousands of images and 3D ‘point clouds’ of the cathedral to create a ‘4D’ interactive model that will allow users to visualise the structure in space and across its 850-year history. Scientists will be able to click and zoom in on precise details of the structure – down to a staple in a beam, for example – and watch how that detail evolved over time. ‘They’ll also be able to access the full range of research and data related to that one detail, across scientific disciplines,’ de Luca explains.

But again and again, it is the emotion that the researchers keep referring to as they speak of their work. ‘When I’m feeling down or overwhelmed, I just look at an exceptional stained-glass window, and I think, “What has it transmitted to us?”,’ says Loisel. ‘And I feel what it’s transmitted: value, strength, and art… It reminds me that I have to rise to the occasion!’ While the task before her might seem daunting, she says she is reminded that she and her colleagues ‘didn’t get here by chance – it’s in our roots to do this job!’

Eventually, as they forge ahead with the myriad tasks in front of them, as they uncover layers of plaster, polychrome, and dust, it is easy to believe that these scientists might even come across a word etched into the stone of the North Tower. A word that means ‘fate’ – first documented by that unwitting ‘researcher’ Victor Hugo nearly 200 years ago. And perhaps, starting from that moment, a whole new chapter in the story of Notre Dame de Paris will begin.

All photos: Christa L-L, unless otherwise stated.