The terrible fire at Notre-Dame de Paris cathedral, which broke out around 6:50 p.m. on April 15, 2019, caused the neo-Gothic style spire rebuilt by Eugène Viollet-Le-Duc to collapse an hour later in the mid-19th century.e century. The fall of the large stone and wood structure, weighing 250 tonnes, caused the destruction of part of the nave vaults, then those of the transept crossing.
The oak of the medieval framework was charred. The firefighters were able to water the debris from the arrow which was then recovered from the ground by robots. Elements of the stool still showed the last ring of radial growth (one ring per year) and even the cambium, the inner bark of the trees.
Dendrochronological analyzes determined that the felling of the wood used for the construction of the spire, 93 meters long, had been done in February 1857 and again in the winter of 1858. Eugène Viollet-Le-Duc’s diary indicates that the companions completed the beautiful work a year later.
The discovery confirmed the use of green wood (rather than dried and hardened). It was therefore necessary to proceed in the same way for the reconstruction. Other pieces of framework, these calcined, made it possible to understand that part of the “forest” of the roof had been transported by flotation raft, two centuries before the first mentions in the archives of this form of log drive .
This is the kind of information that was delivered last week at the interdisciplinary conference Birth and rebirth of a cathedral. Notre-Dame de Paris under the eye of scientists, organized by the National Center for Scientific Research (CNRS) and the Ministry of Culture, at the Cité de l’architecture et du patrimoine, place du Trocadéro, in Paris. A first conference held in 2022 made it possible to specify areas of study.
This time, it was time to take stock of the results, in front of the public, a few months before the reopening of the restored monument. The presentations at the conference were organized around sessions corresponding to as many interdisciplinary working groups, at work for five years to study built structures, wood, stone, glass, metal, acoustics, and even heritage emotion. A digital group interacts with all the others.
Nothing to throw away
Martine Regert remembers very well the shock she felt when she learned of the disaster. “I was on a train between Nice and Paris when I discovered that the cathedral was burning,” says the specialist in biomolecular archaeology, deputy scientific director in a CNRS institute. It was in this capacity that she was quickly appointed project manager to bring together scientific studies on Notre-Dame. “The next day, I was at the CNRS, and the emails and the phone calls asked us not to throw anything away and to make the material available for study. Wood, stones, iron, all that is research material and it was necessary to launch study programs in different fields. »
She got to work with materials chemist Philippe Dillmann. Nearly 200 specialists and some 50 laboratories have since participated in the autopsy of the damaged building.
The President of the Republic decided to restore the cathedral in five years. The creation of the public establishment responsible for the conservation and restoration of Notre-Dame gave scientists access to the extremely complex site where up to 500 specialists are active at the same time. Very strict security measures complicate all interventions, and agendas evolve quickly.
Search results too. Scholarly conclusions can even influence reconstruction work. The mechanical engineering group studying the stability of the building was contacted and interacted closely with the chief architects of historic monuments, Pascal Prunet, Philippe Villeneuve and Rémi Fromont. The stabilization of the vaults has benefited from modeling and new methods of calculating the acting forces.
The future of debris
Presentations at the conference focused on the evolution of construction techniques and the chronology of work spanning centuries. “When we think of Gothic cathedrals, we immediately see the stones and less the framework and the metal frames,” summarizes M.me Regert. The fire revealed a whole chain of staples, and specialists wondered whether the metal frames had been designed at the start of the medieval construction site or added afterwards to consolidate the structures. »
Staples were indeed systematically used from the first stages of construction in the second half of the 12th century.e century, a few decades before what historians thought. The analysis of ferrous alloys containing carbon makes it possible to date the fasteners and even to understand the origin of the metal and its very extensive recycling. A doctoral thesis in progress focuses precisely on this aspect.
The discoveries will be transposable to the restoration projects of other monuments in France and Europe. A fire destroyed the Copenhagen Stock Exchange and its iconic spire three weeks ago. The future is also taken into account differently by French researchers. Teams sorted and stored evacuated debris for later studies, when new cutting-edge techniques will make it possible to further refine the analyses.
The filmed presentations of the conference should be available online in the coming weeks. A City of Paris website allows you to follow the progress of the restoration work on the cathedral.
The reconstructed spire has regained its place in the Parisian landscape since last March. It took a 600-ton scaffolding with 48 levels to rebuild it. The cathedral with its fully restored interior, including 19th century wall paintingse century and the cleaned stained glass windows, miraculously spared by the infamous day of April 15, 2019, will reopen its doors to visitors at the end of the year.
