The French Iron Lady – iron innovations revealed in the Notre Dame Cathedral

The French Iron Lady – iron innovations revealed in the Notre Dame Cathedral.

The renovations to the Notre Dame Cathedral in Paris after the April 2019 fire has revealed the architect’s innovative use of iron. It was ahead of its time – high tech and ultra modern – for Gothic architecture! This is a surprising and unexpected revelation for the renovation team. 

So, before the United Kingdom’s Margaret Thatcher – known as the Iron Lady – prime minister from 1979-1990, there was the French Iron Lady – or rather, the French Iron Dame – the Notre Dame Cathedral. 

Maxime L’Héritier at Paris 8 University Vincennes-Saint-Denis in France and the renovation team have unearthed design innovations that have been unknown until now. Renovations are continuing – estimated to continue into 2025 – and this is not the time for analysis, says Maxime L’Héritier, but glimpses of architectural design elements already reveal the cathedral’s innovations.

Notre Dame Cathedral was the tallest building in Paris when construction began in medieval times – during the 1160s. And now the renovation team has estimated that the construction used thousands of iron staple reinforcements from the floors to the upper walls. This is the first Gothic cathedral known to have used this method of reinforcement.

The fire on 15 April 2019 was caused by a spark while undergoing planned repairs and renovations to the roof. The fire destroyed the whole roof and the spire, which fell spectacularly during the fire. Water and fire-fighting materials led to water damage and the fear of total collapse. Subsequently, massive renovations to restore the cathedral in its truest form have continued almost immediately after the assessment of the fire damage. 

During the post-fire renovations, the team found large iron staples holding together the building’s stone blocks – making the cathedral’s architecture a ‘modern marvel,’ the New Scientist wrote in March 2023.

Maxime L’Héritier says that the iron staples were not visible before, but due to the framework burning, it made some staples visible to the renovation team. 

They found 12 iron staples, with each one measuring about 50 centimetres (almost 20 inches) long and weighing 2-4 kilograms.

Sample of iron staple reinforcements; Source: 10.1371/journal.pone.0280945


Researchers conducting preliminary analysis of the iron staples and the cathedral’s history show that they were used in the earliest stages of construction. Researchers used radiocarbon dating analysis on the staples – made of an alloy of carbon and iron. They dissolved the iron staples to leave behind the carbon that originated from charcoals used in medieval ironworking furnaces. They documented their findings in a research paper in Plos One (15 March 2023). Therefore, the use of iron made the building lighter than the Roman stone cathedrals of the time, and was crucial to the Gothic design of Notre Dame’s tall structure.  

The research also shows that many of the ironworking sites, where the original architect sourced the iron staples, were within a day’s walk from Paris, says L’Héritier. This ‘archaeological sleuthing process’ involves using lasers to pulverise the iron samples so that they can undergo analysis with a mass spectrometer, which allows for the chemical signature comparison. 

But, as Maxime L’Héritier says,

‘Now is not diagnosis time – it’s restoration time.’

There will be time later for further diagnosis to determine more of the Iron Dame’s architectural secrets.

Journal reference PLOS ONE DOI: 10.1371/journal.pone.0280945

Reference: L’Héritier M, Azéma A, Syvilay D, Delqué-Kolic E, Beck L, Guillot I, et al. (2023) Notre-Dame de Paris: The first iron lady? Archaeometallurgical study and dating of the Parisian cathedral iron reinforcements. PLoS ONE 18(3): e0280945.


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