The 2019 Notre-Dame Fire Revealed Iron Staples Holding the Cathedral Together

The Paris landmark is the first known Gothic cathedral to use iron in this way, researchers say

An aerial view of the exterior of the cathedral, its damaged roof mostly covered by white tarp. An enormous crane looms over the building
An aerial view of the ongoing efforts to reconstruct Paris' Notre-Dame Cathedral, pictured in June 2021 Bertrand Guay / AFP / Getty Images

The 2019 fire in Paris’ Notre-Dame Cathedral destroyed the Gothic structure’s wooden roof and toppled its iconic spire. But the disaster comes with a small silver lining: Due to the fire—and the ongoing restoration process—researchers are getting an unprecedented look at the cathedral’s inner workings.

According to a paper published in the journal PLOS One this week, large iron staples have been found holding together the cathedral’s stone blocks. In light of this unexpected discovery, scientists say that Notre-Dame is the first known Gothic cathedral to make such extensive use of iron as a construction material.

Several old-looking iron staples against white backdrop
A side view of the iron staples L’Héritier et al. / PLOS One

Crews began work on the cathedral in 1163 and finished in 1345. Researchers now know that they used hefty iron staples—measuring nearly 20 inches long and weighing between 3 and 9 pounds—to reinforce the stone blocks that make up the cathedral’s walls, nave aisles and tribunes. Prior to the fire, the staples supported load-bearing areas of the cathedral, “indicating the iron crucially improved the cathedral’s structural integrity,” as Gizmodo’s Isaac Schultz writes.

Iron also served an aesthetic purpose: The staples allowed medieval builders to construct a large, sturdy structure that appears to be slender and delicate.

Illustration of iron staples holding stones together next to photo of stone cathedral wall
The iron staples are nearly 20 inches long. L’Héritier et al. / PLOS One

“Compared to other cathedrals, such as Reims, the structure of Notre-Dame in Paris is light and elegant,” says Jennifer Feltman, an art historian at the University of Alabama who was not involved in the research, to New Scientist’s Jeremy Hsu.

Builders in France installed iron reinforcements in other medieval cathedrals, including those in Beauvais, Chartres and Bourges. Those structures, however, were built after Notre-Dame.

Several images spliced together of iron staples at Notre-Dame
Researchers found the staples in the upper walls, nave aisles and tribunes. L’Héritier et al. / PLOS One

Researchers analyzed 12 of the cathedral’s historic staples. Using radiocarbon dating, they learned that they were made during the early phases of construction, around the year 1160. They also hope to discover where the iron came from.

“We’re trying to figure out if it’s local or more distant,” says lead author Maxime L’Héritier, a historian at Paris 8 University, to Live Science’s Jennifer Nalewicki. “There also seems to be different ore sources depending on whether the construction occurred in the 12th or 13th centuries. We do know that the [cathedral’s] bishop died at the end of the 12th century, so it’s possible that a new ore resource was used years later. We should know more in a year or two.”

In addition to the iron staples, restoration work has uncovered some other surprising secrets: Last spring, archaeologists found two lead sarcophagi buried underneath Notre-Dame’s floor. In December, they announced that they’d identified one set of remains as Antoine de la Porte, a high priest who died in 1710. The other coffin held the skeleton of an unidentified man, likely a horseback rider, from an earlier period.

Top of iron staple in stone blocks
The iron staples bound the cathedral's stone blocks together. L’Héritier et al. / PLOS One

More discoveries may be to come. Scientists remain hard at work investigating and repairing the cathedral, which is slated to reopen in December 2024

For many of the experts involved, including L’Héritier, the process of carefully repairing one of the world’s most celebrated buildings can be emotional. The first time he saw the destruction up close, L’Héritier was “speechless,” he wrote in a 2021 essay for the anthropology magazine Sapiens.

“The atmosphere was oppressive,” he added. “Although I had watched the fire many times on television, only at that moment did I truly realize how devastating the blaze was and how demanding the task was before us.”

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