Learning from the ashes of Notre Dame

Nature takes a look at how the tragic fire at Notre Dame last year created a unique opportunity for researchers to study medieval construction:

The structure was modified in the Middle Ages and extensively restored in the nineteenth century by the architect Eugène Viollet-Le-Duc. But it has been the subject of surprisingly little scientific research, compared with other Gothic monuments in France and elsewhere, says Martine Regert, a biomolecular archaeologist at the CNRS’s CEPAM centre for the study of historical cultures and environments in Nice, who is one of the Notre-Dame project’s leaders.

The fire on 15 April, possibly caused by an electrical fault, destroyed the cathedral’s roof and spire, and caused part of its vaulted ceiling to collapse. The walls still stand, and the building will eventually be restored — although this is likely to take longer than the ambitious five years initially forecast, and is set to cost hundreds of millions of euros.

The absence of tourists might also make it possible to use radar imaging to probe the foundations, which have been little investigated. Even some parts of the structure that were largely undamaged are now more accessible for inspection, says Philippe Dillmann, a specialist on historical metal artefacts at the CNRS Laboratory for Archaeomaterials and Alteration Forecasting in Gif-sur-Yvette, who is coordinating the project with Regert.

The CNRS project will focus on seven topics: masonry, wood, metalwork, glass, acoustics, digital data collection and anthropology. In all, the effort will involve more than 100 researchers in 25 laboratories and will last for 6 years.

Gallet’s team will study Notre-Dame’s stones to identify the quarries that supplied them and “reconstruct the supply networks and the economy of the site”. Studying the mortar used to bind the stones together could reveal how different compositions were used for the various structural elements — vaulting, walls and flying buttresses. The mortar used lime prepared from sedimentary limestone, which might contain fossil remnants that could reveal where it originated. A better knowledge of the historical materials could inform choices made in restoration, says [Yves Gallet, a historian of Gothic architecture at the University of Bordeaux-Montaigne].

The team will also analyse weaknesses in the remaining structure caused by the high temperatures of the fire, the fall of masonry and the water used to extinguish the flames. Damage to the stones was exacerbated last July by extreme heat waves in Paris, which “brutally dried” and weakened the masonry, says Gallet.

“The burnt structure constitutes a gigantic laboratory for archaeology,” says Alexa Dufraisse, an archaeologist at the National Museum of Natural History in Paris, who will lead the multidisciplinary wood team. The group will include archaeologists, historians, dendrochronologists, biogeochemists, climatologists, carpenters, foresters and engineers specializing in wood mechanics.

Tree-ring dating of timber beams could reveal the year and location in which the trees were felled, filling in gaps in knowledge about the sequence of construction. “Each tree records within its tissues the environment in which it has grown,” says Dufraisse. This kind of study “could never have been conducted without the destruction of the structure by fire”, she says.

In particular, says Regert, the wood is a climate archive. “Isotopic analyses of oxygen and carbon in the rings make it possible to determine the temperature and rainfall over time,” she says.