When a fire destroyed Notre Dame Cathedral’s famous spire and most of the roof in 2019, the immediate damage and subsequent renovation efforts unexpectedly revealed large iron staples holding many of the building’s stone blocks together. Analysis has now shown that Notre Dame was the first Gothic cathedral to use such iron reinforcement throughout its entire structure, a fact that highlights the iconic building as a modern, high-tech marvel of its day.
“You realize they were doing things that were like the Empire State Building around 1930, or like the Burj Khalifa in Dubai, really extraordinary things for the time,” he says. Roberto Bork at the University of Iowa, who was not involved in the analysis. “You can compare it to the moon launch in the 1960s, and you can compare it to some of the big high-tech initiatives now.”
Notre Dame was the tallest building ever built when the project began in medieval Paris during the 1160s. Examination of the cathedral during its ongoing renovation has allowed Maxime L’Héritier at Paris 8 Vincennes-Saint-Denis University in France and colleagues to estimate that the building has thousands of staples from the floors to the upper walls.
“The fact that the structure burned caused us to see some staples that we couldn’t see before,” says L’Héritier. He described each staple as being about 50 centimeters long and weighing between 2 and 4 kilograms.
A closer analysis of 12 staples showed that they were used in the early stages of the cathedral’s construction, according to the researchers. They performed radiocarbon dating analysis on material samples taken from the staples (each sample is an alloy of carbon and iron) dissolving the iron to leave behind carbon that originated from the charcoal used in medieval kilns to work the iron.
The use of iron to reinforce the building’s stones and other features, such as the stained glass iron connections, was key in creating the cathedral’s Gothic architectural style, Bork says. Unlike the stone block architecture of Roman times, medieval builders of Gothic architecture took advantage of this iron innovation to create structures that appear lighter and much more detailed.
“Compared to other cathedrals, such as the one in Reims, the structure of Notre Dame in Paris is light and elegant,” he says. Jennifer Feltman at the University of Alabama, who was not involved in the analysis. “This study confirms that the use of iron made this lighter structure possible in Paris, and therefore the use of this material was crucial to the early Gothic architect’s design of Notre Dame.”
The team also began comparing the elemental composition of different iron commodities to see if the iron was produced at specific iron working sites; many sites are within a day’s walk of Paris, says L’Héritier. This archaeological research process involves the use of lasers to pulverize the iron samples so they can be analyzed by a mass spectrometer, allowing for comparison of chemical signatures.
Analysis of the current strength of iron staples will even provide modern architects with information on how to reuse intact iron staples to rebuild Notre Dame Cathedral to its former glory. “Now is not the time for diagnosis, it is time for restoration,” says L’Héritier.