Where is the Notre-Dame scientific project at one year on?

On 15 April 2019, Notre-Dame Cathedral in Paris was destroyed by a fire. The whole world was gripped by a sense of great loss. This included the scientific community, which immediately offered help. For scientists, it was first and foremost a question of offering their expertise within the framework of the cathedral's restoration and preservation in the future. The research, some of which is being carried out at Saclay, has begun to uncover some new insights into this landmark building.

As of May 2019, Martine Regert, Scientific Assistant Director at the Institute of Ecology and Environment at CNRS, and Philippe Dillmann, Head of the Laboratoire archéomatériaux et prévision de l’altération (the Archaeomaterials and Weathering Prediction Laboratory (LAPA - Université Paris-Saclay, CNRS, CEA), were appointed to lead the Notre-Dame scientific project CNRS/ Ministry of Culture in close collaboration with the administrative public body created for the restoration of the building. Together with their counterparts at the Ministry of Culture, they coordinate 175 scientists in about 50 laboratories and various institutions across France. Most of these scientists belong to the Association des scientifiques pour la restauration de Notre-Dame (Association of Scientists for the Restoration of Notre-Dame), which was created shortly after the fire.

Cross-disciplinary groups from a wide range of backgrounds

Eight research groups have been set up. Four are focusing on the materials of wood, stone, metal and glass. They are made up of chemists and scientists specialising in ancient materials, as well as archaeologists and historians. Two further groups are focusing on civil engineering (structural design) and acoustics. Another group is involved in digital engineering and models all the data which is collected. The final group, made up entirely of experts in human sciences, is studying the “emotional legacyˮ which the event left behind and is comparing it to other similar cases around the world. All these scientists from different backgrounds have been working together for a year. They are careful not to interfere with the restoration work, and each scientific field has its own objectives and agenda. “The research will take place over the next five years,ˮ says Philippe Dillmann. “After the safety phase (the central scaffolding is being dismantled), an assessment of the state of the materials and the building will take place up until autumn 2020.ˮ

Modelling Notre-Dame

In order to study the materials, they first have to be sorted, removed and then stored in the best possible conditions. Employees from the Laboratoire de recherches des monuments historiques (Historical Buildings Research Laboratory (Ministry of Culture) and DRAC (Direction régionale des affaires culturelles - Regional Directorate of Cultural Affairs) were the first to arrive on the site to sort and catalogue the remains which are to be studied. “Carrying out analysis on a stone is pointless if you do not know where it came from. Before removing it with the help of robots, we have to pinpoint from where each stone fell. We have used photogrammetry techniques, with drones and cameras placed in different locations to photograph the objects from different angles in order to ‘triangulate’ them. As a result, we know the exact location of each object found. This is then entered into the digital database developed for this purpose,ˮ concludes Philippe Dillmann. Laser scanning of the building has also been carried out. Ultimately, all analytical data collected will be stored in this digital ecosystem.

Lead - a twofold problem

Sophie Ayrault, a member of the ‘metal group’ and a scientist from the Laboratoire des sciences du climat et de l’environnement (Climate and Environmental Sciences Laboratory) (LSCE - Université Paris-Saclay, CNRS, CEA, UVSQ), is attempting to answer two questions. One concerns (for health reasons) the contamination of the city by the lead from Notre Dame, and the other (for historical purposes) concerns its origin. This specialist in the release of metals into the environment is also taking part, together with 300 fellow scientists, in the PIREN-SEINE project. She has been studying the contamination of urban environments, and Paris in particular, by lead for some time. This phenomenon can be found in other old cities in the world, such as Chicago.  “Lead has a special characteristic in that it possesses an isotopic signature which remains intact from the mine from which it was extracted right up to in its pollutant state. Analysis of this signature following the fire (the roof contained several tonnes of it, which were partially vaporized), will allow us to determine whether the positive lead levels detected in the blood of people in the immediate vicinity in the months following the fire were the result of the fire, or whether they came from a previous source.ˮ The likely outcome in a few months will be, once the isotopic signature of the fire has been determined, to simply compare it with the traces of lead found in Paris, as the buildings designed by Haussmann already contain a lot of lead.

Back to the Middle Ages

Another issue revolves around the lead used during the construction of Notre-Dame itself. “It's like a police investigation,ˮ says Sophie Ayrault, who is working alongside historians and archaeologists, including Maxime L'Héritier, who is in charge of the “metal groupˮ. Lead is found everywhere (in artefacts, stained glass), and was used in the roof and pillars right at the very beginning of the cathedral's construction. Small metal cushions were inserted to compensate for variations in the ground. Some of the sheets did not completely melt. “How was so much lead procured in the Middle Ages and where did it come from?ˮ, wonders the scientist. “Although we do not know the trade routes used during this period, specialists can now compare this lead’s signature with that found in other medieval buildings in different locations where lead was used.ˮ Archaeometric scientists have known for a long time that “the use of metal with wood was part medieval building designˮ. Delphine Neff, a specialist in the corrosion of old metals at CEA and a scientist at LAPA, will attempt to determine the temperature of the fire by studying the change in the corrosion profile of the iron ties. Other scientists in the “wood groupˮ are trying to find traces of carbon in the wooden structure (including the charred sections) in order to trace its origin and age by using cutting-edge methods from the Laboratoire de mesure du carbone 14 (Université Paris-Saclay, CEA, CNRS, MCC). Valérie Daux, from LSCE, is collaborating on the project to go back in time and study the climate of the Middle Ages…

An unknown girdle of ties

“No one's ever seen a cross section of a medieval vault,ˮ points out Philippe Dillmann. Yet today, the exposed pinnacle of Notre-Dame is now a life-size example of this. This has allowed a lot of information to be extracted and it did not take scientists long to make a discovery. “At least three girdles of ties encircle Notre-Dame, perhaps right from its original creation. One of these, located in one of the cornices, had already been uncovered during restoration work by Viollet-le-Duc, but another, located at the top of the walls which create the heart of the cathedral and at the level of the galleries, has been revealed for the first time in 800 years!ˮ, says Philippe Dillmann. Even in its battered state, this building is continuing to reveal its mysteries and enthuse scientists, restorers, loyal supporters and the wider public alike.