Editor’s Note
The restoration of Notre-Dame de Paris highlights a critical, often overlooked challenge in heritage conservation: sourcing authentic materials. As this article details, the urgent need for compatible stone was a major logistical hurdle, underscoring the fragile link between our built history and the natural resources required to preserve it.
The reopening of Notre-Dame de Paris Cathedral on December 7, 2024, would not have been possible if replacement stones could not have been found so quickly and in such large quantities.
The fire at Notre-Dame de Paris Cathedral in April 2019 led to major restoration work, with an exceptional need for “new” stones to replace the damaged original stones.
The project managers faced an unprecedented priority: to guarantee the supply of suitable replacement stones available in sufficient quantity. And a crucial question: where to find them?
The French Geological Survey (BRGM), in close collaboration with the public establishment Rebâtir Notre-Dame de Paris (RNDP), the architects, and the Laboratory for Research on Historical Monuments (LRMH), found solutions to secure this supply within extremely short deadlines.
In France, the number of building stone quarries has been constantly decreasing since the end of the First World War. Today, there are fewer than 500 active operations. Only half extract limestone, the other half being made up of granite, sandstone, and other rocks.
Restoration projects for historical monuments and other old buildings are thus faced with a growing difficulty in finding the “right replacement stones,” i.e., stones that are analogous from a physico-mechanical and aesthetic point of view to the original stones of the buildings.
The fire at Notre-Dame de Paris Cathedral in April 2019 caused very significant damage to the original stones of the building. The restoration project thus required a volume of replacement stones without comparison: approximately 1000 cubic meters (m³) of “finished” blocks, compared to a maximum of a few tens of m³ for “ordinary” monument maintenance and restoration projects.
This exceptional volume of “new” stones also had to be made available to the site within a particularly tight deadline and schedule given the scale of the work. Finally, the expected replacement stones had to present precise physico-mechanical and aesthetic characteristics.
Faced with this challenge of supply chain control, a first action led by BRGM in 2020-2021 made it possible to assess the capacity of the nine still active Lutetian limestone quarries to supply certain types of stones compatible with the cathedral’s original stones.
To do this, each quarry was the subject of field investigations by BRGM geologists. Laboratory analyses were carried out. All the results were presented in the form of a methodological guide for selecting stones for the restoration of Notre-Dame de Paris Cathedral, accompanied by a collection of reference samples for the architects.
Regarding the hardest stones, representing by far the largest needs of the Notre-Dame project with about 750 m³ of cut blocks, only one quarry, that of Croix-Huyart in Bonneuil-en-Valois in the Oise, was found to contain a rock layer sufficiently high.
The public establishment RNDP and BRGM quickly, from mid-2021, launched a second action focused on securing the supply of hard stones from Croix-Huyart for the project. A rigorous and particularly innovative procedure was developed and then applied by BRGM, based on quality, conformity, and traceability controls of the stone.
Over a period from March 2022 to March 2024, nearly 90 controls were carried out by BRGM.
The building is now adorned with “new” stones juxtaposed with stones laid several centuries ago, without future visitors being able to distinguish the “old” from the “new” with the naked eye.
