Notre-Dame de Paris is asking for new stones. The catastrophic fire on April 15, 2019 actually caused serious damage to its mineral composition. “The fire locally experienced infertility, which led to the fire and the stone turned red, an incident observed during the fire in 1914 at the Cathedral of Rheims.” Jonathan Trulet, deputy director of operations of the public establishment responsible for the restoration of the cathedral, explains. The stone is weakened, which is not necessarily visible on the surface. And it is also necessary to keep in mind the stones falling on the ground: about 15% of the surface of collapsed vaults will not be reusable. “ Researchers and architects are now in charge of evaluating the possibilities – or not – of reuse of vault elements.
The health status of the cathedral stones was first diagnosed by radar reading. Thus a rover equipped with radar equipment surveyed the vaults to identify their surface and internal parts. Several dozen cores with visual analysis make it possible to mark the position of the stones. “For those who fell, before Christmas we did tests to reassemble an arc. It was a question to see how the blocks were re-arranged between them, if the blocks were possibly reinforcements. Were reusable with Note Jonathan Trulet. A broken block, for example, needs to be doweled [resolidarisé ndlr] With fiberglass or carbon, which provides mechanical bonding between the pieces.Today, maintenance and restoration projects for historic monuments attempt to replace only minimal stones. But the Notre-Dame site is on a different scale and requires the renovation of a much larger number of blocks than a traditional site.
Limestone structures that appeared 45 million years ago
The stones of Vaux-le-Vicomte’s palace come from the same geological site as Notre-Dame. Credit: Zairon, CC BY-SA 4.0, via Wikimedia Commons
To meet this need, public installations and BRGM (Bureau de recherche géologique et minière) are joining a joint survey program in the mines of the Paris Basin, looking for new stones that are aesthetically and physically compatible with damaged stones . “Stones from medieval times and used during maintenance and renovation work that followed each other until the 19th century, which come from the limestone structures of the Lucian era” Jonathan insists on Trulette. The Lutheran deposit was formed about 45 million years ago from marine invertebrates, corals, sea urchins, molluscs, etc. These buried marine sediments were then compacted and transformed into limestone geological rocks, formed by grains of calcite and animal shells.
A white stone is also present in Versailles, Vaux-le-Vicomte or Chantilly
The Parisian quarries and its perimeter make it possible to erect a large part of the Parisian buildings. Hence its name (Lucres is the old name of Paris). Luteian limestone has the advantage of easily working in more or less large blocks and being resistant. In addition to a wide variety of Parisian monuments and buildings, this blonde stone is found in the châteaux of Versailles, Vaux-le-Vomte, Chantilly, etc. Or in the churches of Rheims, Soissons, Senlis … These deposits had the advantage of being close to sites and offering various properties of stone: hard stones that could support the weight and forces of architecture, and others for it. Do the sarcasm. Sculptures for example. But this career has shrunk over time like a grain of salt. “In the early 18th century, in-situ resource shortages were to be compensated, Jonathan Trulet indicates. The supply sector has gradually moved to the inner suburbs, increasing the number of extraction sites. “ Today, the supply of Paris building stones depends on a dozen mines located in Oise and Eizen, north of Paris.
Parisian career map (after Caron et al 1986). Sincerely: MNHN
“The strategy of the project is therefore to search for the stones closest to the origin of the construction of Notre-Dame,” David Desandier, a geologist at BRGM, explains. This requires the specimen to determine the characteristics of the specimen. “ In the laboratory, the color, grain, physical and mechanical properties of the specimens will then be characterized, particularly their compressive and capillary resistance to the transfer of water between the stones. The program will also focus on unused deposits in the Paris Basin. It should be completed this summer.