The fire that engulfed Notre-Dame Cathedral in central Paris four years ago exposed a long-hidden secret about the Paris landmark: it was the first Gothic cathedral in which iron staples were used as reinforcements during the construction of what was the tallest building of its time.

It took the near destruction and the ongoing restoration project to allow a team of archaeologists to discover the iron reinforcements.

The construction of the famous cathedral in the heart of the French capital began in 1160 and was not completed until almost a century later.

In the absence of detailed plans and documentation, architects have long been mystified as to how their medieval counterparts managed to build such thin walls to a then-unprecedented height of 32 metres, and have them support the cathedral's massive vaulted roof structure.

A study led by Maxime L'Heritier, an archaeologist at University Paris 8, shows that builders used as many as 1,000 iron staples to stitch the soaring structure together.

The 2019 fire exposed some of these previously invisible reinforcements, while others fell to the ground as they were dislodged by the heat of the blaze.

Dynamic Gothic architecture

The staples come in varying sizes, ranging from 25 to 50 centimetres long, some weighing several kilos.

They were found in many different parts of the cathedral, including in the walls of the nave, the choir tribunes and in parts of the roof-level cornice.

"This is the first truly massive use of iron in a Gothic cathedral, in very specific places," L'Heritier says.

Iron staples were used in construction since Antiquity, including in Rome's Colosseum and Greek temples.

But in those cases they were simply used to keep large stone blocks secure on the lower floors.

Notre-Dame shows a "much more dynamic conception of architecture", according to L'Heritier.

More than 200 scientists are among the teams of specialists working on restoring Notre-Dame, with the project due to be completed by the end of 2024.

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Originally published on RFI