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The archaeometallurgy of Notre Dame de Paris

by Craig Bettenhausen
May 16, 2024 | A version of this story appeared in Volume 102, Issue 15


Stapling Notre Dame back together

A photo of Notre Dame Cathedral at night shows construction scaffolding as well as Gothic architecture.
Credit: Craig Bettenhausen/C&EN
Iron maiden: Iron reinforcements join flying buttresses as architectural innovations featured in Notre Dame de Paris.

Maxime L’Héritier has sliced up more than 500 historical metal artifacts over the course of his career. But the first time he cut into ironwork from Notre Dame de Paris, the famous medieval cathedral in the center of the City of Light, the moment was still special. L’Héritier, an associate professor of medieval history at the University of Paris 8 Vincennes-Saint-Denis, uses elemental analysis and other techniques to study the metals in historical buildings, a field of study called archaeometallurgy.

L’Héritier’s specialty is iron and lead in medieval construction, especially in Gothic structures, so he was eager to help with the restoration of the iconic French church, which was built between 1163 and 1260 and severely damaged when the roof caught fire in April 2019.

The collapse of the roof revealed hundreds of iron staples holding stones together along the very top row of the walls. The stones were damaged beyond repair, but officials wanted to know if the hand-forged staples could be reused. L’Héritier’s lab ran several staples through a battery of mechanical tests and determined that the iron was in good shape. Any staples that were physically intact had enough mechanical strength to resume their structural purpose. Overall, he says, about 80% of the staples will return to their place of honor, high above Paris.

The research reaches back into history even as it informs the reconstruction. L’Héritier and his team cut slices from 10 of the staples to map the elemental composition of the iron. The results suggest that builders got iron from at least six different forges. The supply chain also seems to have completely changed around the turn of the 13th century. The swap corresponds in time both with the death of the bishop who commissioned the cathedral and the rise of Paris’s middle class, the original bourgeoisie (PLoS One 2023, DOI: 10.1371/journal.pone.0280945).

The team was also able to answer a fundamental chronological question: When were the staples installed? Other than nails, iron reinforcements didn’t show up in French church construction until the middle of the 13th century, so historians assumed the staples were added during previous renovations. Carbon dating of Fe3C inclusions in the staples, however, showed they were manufactured as early as the 1160s. It’s possible that the success of iron structural support in Notre Dame inspired its more extensive use in subsequent Gothic cathedrals in nearby Chartres, Bourges, and Soissons, L’Héritier says.


Putting the lead back in

Image of an old staple that is helping to hold together the walls of Notre Dame Cathedral.
Credit: Maxime L'Héritier
Ironwork: This staple, about 45 cm long, has been helping hold the top of Notre Dame's walls together for around 800 years.

When most people remember Paris’s Notre Dame fire of April 2019, the metal that might come to mind is lead. Because the roof was made of lead tiles and sheets, the blaze sent as much as a metric ton of lead billowing into the air above Paris.

Nonetheless, archaeometallurgist Maxime L’Héritier explains, the current thinking on historical restoration is to return the building to its last known intact state. Thus, the new spire is an exact replica of the one that fell, and the new roof is lead once again.

It’s not as irresponsible a choice as it may seem. Researchers have struggled to clearly attribute spikes of lead contamination to the fire. L’Héritier says most of the lead dust from Notre Dame may have simply washed away in the rain a few days later.

And on an ongoing basis, “lead is not that bad of a material” for a roof, he says. A grad student he helped supervise analyzed the rainwater coming off comparable lead roofs to estimate how much of the toxic metal gets shed into stormwater. The pessimistic but pragmatic answer was that the mass flow from Notre Dame’s new roof will be background noise compared with the innumerable other lead sources in the 2,000-year-old city.

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