Notre-Dame revealed its secrets to archaeologists after it burned

Recent excavations at Notre-Dame Cathedral enabled by the 2019 fire unveiled centuries-old burials, shedding new light on the iconic structure's construction and history. Following the devastating fire in April 2019, the reconstruction efforts have provided an unprecedented opportunity for archaeologists to study the cathedral, which had been little studied before due to its high visitor traffic, according to Ouest-France.

Teams from the National Institute for Preventive Archaeological Research (Inrap) have been working inside and outside Notre-Dame Cathedral for five years, including a team led by Christophe Besnier who was initially given no more than five weeks to dig under the stone floor, as reported by National Geographic. Besnier's team was only authorized to dig 16 inches beneath the floor, the depth of the scaffolding's foundation. He did not expect to find much at that depth but was happily wrong about the amount of finds. "The remains turned out to be much richer than expected," Besnier says. "It's very impressive," National Geographic reports.

In total, Besnier's team found 1,035 fragments of numerous works of art. The excavations have uncovered around a hundred graves, increasing the total number of recorded epitaphs in the cathedral to over five hundred, according to National Geographic. Each excavation of the cathedral's floor has revealed coffins, often unidentified, as well as scattered bones, highlighting the presence of historical artifacts at Notre-Dame Cathedral.

Among the significant discoveries were fragments of the 13th-century jubé (choir screen) and lead sarcophagi, one of which may belong to the poet Joachim du Bellay, as reported by Ouest-France. The excavations at the crossing of the transept yielded a very large number of elements of the medieval jubé in an exceptional state of conservation, according to Inrap. The crossing, where the transept meets the nave and choir, is a significant area for archaeological investigation during the reconstruction, National Geographic explains.

Researchers have assisted in the restoration and studied Notre-Dame Cathedral, revealing new secrets about the wood, metal, and how artisans worked in the Middle Ages, according to Ouest-France. The examination of the charred beams of the roof structure has allowed for a better understanding of "how it had been made," and the study of the wood has provided data on the medieval climate. The wood fixed the oxygen from the atmosphere during its growth, supplying information about temperatures in Paris at the time.

The medieval carpenters did not use "wood dried for many years, but green wood," and the oak trees used were "about a hundred years old, about fifteen meters tall, shaped with a 'doloire' (a type of axe) while respecting the grain to maintain its strength," according to Ouest-France. Researchers also discovered on the remains "rope holes used to tie logs together, like rafts, for transport by the Seine."

One of the important discoveries was about the use of metal in the cathedral's construction. In the 19th century, architect Eugène Viollet-le-Duc had "highlighted some iron clamps binding stones together, but it was not thought there would be so many," emphasizes Philippe Dillmann, research director at CNRS and one of the coordinators of the CNRS/Ministry of Culture scientific project, according to Ouest-France. Many iron clamps were discovered at the tops of the walls, under the burned roof structure. "They play a quite obvious belt role on the building. But we found others at the level of the choir columns, in the walls of the first-floor tribunes... Their role sometimes raises questions and continues to be studied," Dillmann states.

Several clamps, ranging from 25 to 50 cm in length, were analyzed and subjected to radiocarbon dating. The results indicate that "the oldest date from the 1160s, that is, at the beginning of the construction of Notre-Dame," Ouest-France reports. "It is now indisputably the first Gothic cathedral where iron was conceived as a true building material to create unprecedented architecture," Dillmann states. This innovation "spread to other buildings like Chartres, Bourges, or Beauvais."

The study of the stones, all extracted from limestone rocks of the Paris basin, "allowed for the collection of many pieces of information about the construction of Notre-Dame," notes Dillmann. A surprise was the "thin thickness of the vaults (15 to 18 cm in the choir and about 20 cm in the nave), which nevertheless played the role of protection against the fire," according to Ouest-France. The marks of tools left on the stones provide information "about the stone-cutting techniques." The dating of the wood and the clamps "documents more precisely the phases of the construction."

Researchers also observed a "reuse of materials in several places." The dating of the wood showed that "the second roof structure of the choir, built in the 13th century when the cathedral was raised, was made with elements of the first dating from the 12th century," Ouest-France reports. The analysis of the marks left by the stone cutters on their work "allowed us to understand that teams of artisans worked independently in the north and south of the nave," Dillmann notes.

Scientists were also able to observe, for the first time, the foundations of the cathedral, particularly those of the pillars, connected to each other by longrines, beams used to distribute the load. The complete examination of the monument "allowed us to document certain knowledge: to authenticate, for example, the medieval elements of the stained glass windows, the only ones being on the large rose windows of the transept," Dillmann states.

The massive restoration project of Notre-Dame de Paris has allowed for a better understanding of the cathedral, its construction, and its history. "Their expertise has been used for 'the analysis of materials, their resistance, assistance with structural calculations for the architects' design offices...'" according to Ouest-France. Immediately after the fire, specialists and researchers "volunteered to help with the restoration," recalls Dillmann.

Inrap archaeologists excavated the nave of Notre-Dame Cathedral to recover all the collapsed materials (beams, stones, metal) using remote-controlled machines and photogrammetric surveys, according to Inrap. Under French law, any construction project that disturbs soil where ancient artifacts or remains might be found requires intervention by government archaeologists, National Geographic explains. At Notre Dame Cathedral, it was the job of archaeologists to ensure that nothing valuable would be crushed by the 770-ton scaffolding needed to rebuild the spire.

French President Emmanuel Macron had decreed that Notre Dame Cathedral was to reopen in 2024, according to National Geographic. In his tribute to the actors of this "unprecedented project," President Macron, in the presence of Rachida Dati, Minister of Culture, and Philippe Jost, President of the Public Establishment responsible for the conservation and restoration of Notre-Dame Cathedral, praised the quality of the work and the scientific contributions of the archaeologists from Inrap, as reported by Inrap. "They have recalled its history and unveiled new aspects," the President stated.

Dominique Garcia, the president of Inrap, stated, "Notre-Dame de Paris is now one of the best-known cathedrals in Europe," according to Accueil. The entire body of work has allowed for "the collection of many recorded data that remain to be studied," Ouest-France reports. All data are "stored in a digital twin of the cathedral. This virtual double will be made available to the teams." "A group of researchers continues to work on acoustics, another on heritage emotion to understand how this fire marked the populations and how they mobilized," Dillmann states.

The findings have astounded the archaeologists, and the massive restoration project has led to "major discoveries." Fifty archaeologists worked during 14 excavation operations, examining 10% of the monument's ground surface, leading to these discoveries, according to Accueil.

This article was written in collaboration with generative AI company Alchemiq