Reconstruction of Notre-Dame de Paris: from cellars to tower, how the site helped reveal the cathedral’s secrets and advance scientific knowledge


Since the fire in 2019, almost 200 scientists from 50 laboratories have participated in the restoration of the monument. The result is several discoveries rich in lessons.

The Notre-Dame de Paris cathedral is still recovering, five years after a fire consumed its frame in April 2019. During the excavations that followed the disaster, scientists had a rare opportunity to explore the bowels of the important medieval cathedral. While the scars are gradually healing, hundreds of artifacts are still preserved in Saint-Witz (Val-d’Oise), a specialized study and storage center. The titanic reconstruction project has brought to light treasures and improved understanding of medieval architecture, and the collected objects have far from revealed all their secrets.

Notre-Dame Cathedral, unlike other monuments, “was ultimately under-explored due to tourist trafficexplains franceinfo Philippe Dillmann, one of the site’s scientific coordinators for the CNRS. On the other hand, it is the first time that all the specialists are at the same monument at the same time and can compare their results. Material specialists, art historians, acoustics, geologists, sociologists… In total, almost 200 experts from around fifty laboratories tried to unravel the mystery of the famous monument. By the beginning of 2024, around thirty research articles had been published in the special issue Journal of Cultural Heritage.

Two lead sarcophagi

Archaeological finds in particular were widely publicized. At the end of 2021, the state prescribed a preventive archaeological survey before the future installation of the 100-meter-high and 600-ton scaffolding necessary for the reconstruction of the tower. At the beginning of February 2022, archaeologists from the National Institute for Preventive Archaeological Research (Inrap) set to work, with only five weeks left to explore the 120 square meters of the transept crossing. atA stone base or raft was quickly uncovered with a system of brick pipes to heat the ground.

A surprise is hidden under one of the pipes: a team of archaeologists led by Christophe Besnier discovers a sarcophagus sealed in embankments from the 14th century. In itself, the presence of tombs in these places is not a surprise. In addition, numerous stone or plaster vaults were found during the research. On the other hand, the use of lead was something rather rare at the time. A little later, another sarcophagus made of the same metal was discovered in a separate archaeological layer.

The two coffins were sent to the forensic institute of the Toulouse University Hospital. The identity of one of the dead could not be ascertained, but analyzes made it possible to identify the rider, whose age was between 25 and 40 years, probably a rich, prominent man. He was actually embalmed, as his severed skull suggests. The identity of the latter is known because the name of Canon Antoine de La Porte, who died in 1710, appears on the epitaph. His profile appears on three medals bearing his effigy.

Polychrome fragments of the screen

Archaeologists eventually gained four more weeks of excavation, until April 8, 2022, as they also discovered carved elements of an ancient rood screen. This term refers to the richly decorated wall that separated the nave and the choir to isolate the faithful from the clergy during liturgical ceremonies. These fragments represent scenes from the Old and New Testaments. Red, blue and sometimes gilded with gold leaf… Their colors struck researchers. Until now, only eight fragments have been preserved in the Louvre after the work carried out by Eugène Viollet-le-Duc in the mid-19th century. They were all black and white.

“There are several other examples of buried screens, but this one is truly exceptional because of the quality of the fragments.”

Philippe Dillmann, co-coordinator of research work at Notre-Dame de Paris

on franceinfo

“This debris could have been evacuated, placed elsewhere or reusedthe researcher continues. This therefore raises questions about the funerary practices of ancient statues.” This screen, a forgotten treasure, is part of a special program. In the future, it may be possible to digitally reconstruct this carved wall if the fragments agree to dialogue with each other.



After the time allowed for excavations, the entire area was covered with sand and clay to facilitate possible excavations in the future. “Most of the fragments have been collected, although there may be other ancient architectural parts of the cathedral that could not be excavated.” With the utmost discretion, another excavation took place at the end of 2022, France Télévisions then revealed, but without any communication between the various institutional actors about the place and the result.

Iron staples

Scientists also used the scaffolding that was set up to reconstruct the tower to take a closer look at the treasures of the building’s heights and construction methods. “This access to the top of the cathedral allowed us to discover a network of staples”, explains Philippe Dillmann. And identify “the earliest use of iron elements in Gothic architecture”. These ties, which can weigh several kilograms, are used to fasten the stone blocks together.



The staples have been dated by carbon 14. The earliest were installed in the 1260s and those at the top of the walls in the early 13th century. So they are all contemporary with the medieval construction site. Notre-Dame Cathedral was an innovative project for its time one of the first cathedrals to start this height race”, comments Philippe Dillmann. And years before Beauvais or Cologne in particular. These conclusions were the subject of an article published in the journal Plos Onein March 2023.

Green wood in frames

Analysis of the woods of the “forest of Notre-Dame”, this dense network of frames, also revealed that the woods were installed while still green and then dried in place. “We have been able to date the woods to 1185, while the first ceremony in the cathedral took place in 1183”explains Philippe Dillmann. This discovery raises new questions about the construction of the building. “Young wood is certainly easier to work with than dry wood, but then you have to consider how it dries and how it’s going to be processed afterwards.” The cracks appear along the length, but without affecting the stiffness and resistance of the beams, emphasized the specialists from the “wood” group, one of the eight thematic teams set up by the CNRS and the Ministry of Culture.

A delicate arc like lace

When the tower collapsed, the vaults were pierced in three places, which made it possible to measure their thickness in addition to radar measurements. Their thickness was measured between 12 and 15 centimeters in the chancel and between 19 and 25 centimeters in the nave. Moreover, this did not prevent the vaults from playing their protective role, as they largely survived the fire. “For comparison, Saint-Etienne de Sens Cathedral, built in 1135 and therefore older, has vaults 35 centimeters thick, explains Philippe Dillmann. This inspires researchers in the “structure” group, who are responsible for studying the equilibrium system and construction processes of the cathedral.

Resurrected decorations and new details on the stained glass windows

Future visitors will discover a cathedral that is lighter than before the fire. As the lead melted, it turned into an aerosol in the form of microspheres, covering the entire cathedral. It was therefore necessary to carry out a major cleaning using compresses and a paste based on kaolin, the formula of which was specially developed. The stones then revealed the forgotten polychromy decorations that Viollet-le-Duc had imagined. “We discovered ornaments, colors in the greenery that are really typical of this period of the 19th century”enthuses Philippe Dillmann.

The scaffolding also provided a close-up view of the stained glass windows, the vast majority of which date from the 19th century, alongside three medieval ones on the west, north and south rosettes. Any information can supplement and modify the census of Paris stained glass, published in 1978.

Finally, the site made it possible to identify the footprints on the stones. And to show, for example, that two teams worked independently, north and south of the main ship. The association of scientists serving the restoration of Notre-Dame will now use this experience to apply it to other monuments in France. This project in a nutshell “has inspired and structured research in all these areas”.


Leave a Comment