In the Notre-Dame Cathedral in Paris, which was damaged in a fire in 2019, French researchers have discovered iron brackets that hold the stones together in different parts of the structure. Dating and metallurgical analyzes reveal that these iron reinforcements date back to the first construction phase of the church in the 12th century. This makes Notre Dame the world's oldest church structure with such iron reinforcement - and explains why its nave could reach a height unique for its time.
The Notre-Dame de Paris Cathedral is one of the most famous works of early Gothic architecture and a World Heritage Site. The five-nave church building not only impresses with its enormous size and artistic vault. When the cathedral was built in 1163, it was also the tallest building of the time, with a nave more than 32 meters high. "Until the beginning of the 13th century - two to three generations of master builders later - no comparable building was built," explain Maxime L'Héritier of the University of Paris 8 and his colleagues. Only the cathedrals of Chartres, Reims, Amiens or Bourges, built at this later time, surpassed Notre Dame in height for the first time.
What made the cathedral stable?
“Notre-Dame's enormous height was made possible by combining several innovative engineering procedures developed by Notre-Dame's first architect,” explain L'Héritier and his team. This included the five-nave floor plan, the ribbed vault with particularly thin struts and the open, also relatively thin flying buttresses on the outside of the nave, which derive the load of the building from the walls. But was that all? Later cathedrals used iron reinforcements in addition to stone and wood construction to stabilize the buildings.
Whether such reinforcements were already used in the original construction of Notre-Dame remained unclear. While there are some iron bracing, it was believed that they were added during restoration work in the 19th century. Large parts in the upper area of the building were also not accessible for more detailed investigations. But in April 2019, the roof structure of the cathedral caught fire and the entire roof of the building was destroyed. The crossing tower also collapsed and the vault of the nave was damaged in several places.
Iron brackets almost everywhere
For L'Héritier and his team, the scaffolding erected in the cathedral immediately after the fire offered the first opportunity to inspect the upper regions of the church building more closely - and also to look for iron reinforcements there. In fact, the researchers found what they were looking for: "Our investigations show that iron clamps were used at different levels of the structure of Notre-Dame," the scientists report. In the upper part of the nave, these brackets connect the pillars along the walls of the chapel to the walls, they are also found at the bottom of the galleries, on top of the arches between the nave and the aisles, and in the ambulatory.
"The number of these iron brackets can be estimated at 300 to 400 in the nave and chancel alone," report L'Héritier and his colleagues. The length of the clips, made of rectangular iron rods, varies between 25 centimeters and almost a meter, depending on the area of application. In order to be able to date these reinforcements, the researchers took samples from twelve brackets and subjected them to radiocarbon dating and metallurgical analysis. These revealed, among other things, that the metal contained numerous inclusions of slag, as was common in medieval iron production. This, combined with the position of the brackets, supports the dating of the material to the second half of the 12th century.
Oldest iron reinforcement in a Gothic building
"These rows of iron brackets are the earliest example yet of iron reinforcement used in the construction of a Gothic monument," write L'Héritier and his colleagues. “Notre-Dame de Paris was undoubtedly the first Gothic cathedral to plan and use iron as a building material from the outset,” they explain. “While most other church buildings of the period still used wood for reinforcement, the cathedral's first builder chose a daring system of a more durable material that could also be more easily concealed. This enabled him to create a whole new type of architecture.”
Source: PLOS, technical article: PLoS ONE, doi: 10.1371/journal.pone.0280945