THE GOAL OF RESTORING Paris’s Notre Dame cathedral by 2024 has encouraged scientific analyses of this historical building. Science, March 13, 2020, devoted a major article to these endeavors. Here are tidbits assessing Our Lady’s stone, wood, lead, and stained glass.
Her Earlier Restoration. As cited in “To All of Us, She is ‘Our Lady,’ ” here at SimanaitisSays, Notre Dame had extensive restoration after it was heavily damaged by mobs during the French Revolution. In 1844, architect Eugène Emmanuel Viollet-le-Duc designed its spire of wood covered in lead. The roof of the cathedral was of lead as well.
The April 15, 2019, Blaze. A conflagration arose from an electrical fault in the attic, quickly spread through dry timbers, and melted the lead roof and tower. The tower’s collapse pierced part of the limestone vaulted ceiling.
This and the following images from Science, March 13, 2020. The graphics are by Chris Bickel.
Of Limestone, Lead, Stained Glass, and Flying Buttresses.The Historical Monuments Research Laboratory is in Champs-sur-Marne, an eastern suburb of Paris. LRMH researchers are studying damaged materials retrieved from the Notre Dame wreckage. Robots were used for the retrieval to protect restoration experts from the cathedral’s unstable structures.
As one example, in “Saving Grace,”Science, March 13, 2020, Christa Lesté-Lasserre writes, “Heat can weaken limestone, and knowing the temperature endured by these fallen stones can help engineers decide whether they can be reused.”
Initially, heat causes a chemical breakdown of iron crystals in the limestone. As temperature rises, these crystals turn into black iron oxide. With yet more heat, the iron oxide is transformed and the stone crumbles into powdery lime.
Lead’s Legacy in Paint, Auto Exhaust, and Notre Dame’s Roof. Lead doesn’t vaporize until around 3100 degrees Fahrenheit. However, it melts at around 570 degrees Fahrenheit and transforms into a microscopic aerosol at twice this, a temperature reached during the blaze (and also in automotive combustion of leaded fuels).
Lesté-Lasserre writes, “Because of lead’s toxicity, especially in children, France’s national health agency imposes a legal limit of 0.1 micrograms per square centimeter on the surfaces of any building, including historical monuments.”
A researcher examined surfaces of the catherdral’s oak confessionals and choir seats. After the fire, one sample was 70 times the legal limit.
Lesté-Lasserre writes, “People entering the cathedral must strip naked and put on disposable paper underwear and safety suits before passing through to contaminated areas, where they put on €900 protective masks with breathing assistance. After a maximum of 150 minutes exposure, they peel off the paper clothes and hit the showers, scrubbing their bodies from head to toe.”
One researcher noted, “We’re taking five showers a day….”
Baby Wipes to the Stained Glass Rescue. The cathedral’s 113 stained glass windows survived the blaze, but inspection revealed contamination by the lead aerosols. Commercial wipes were fine for wood surfaces, but the wipes’ traces of acid could damage stained glass.
Glass researcher Claudine Loisel examines a piece of the cathedral’s stained glass.
Instead of commercial wipes, “chemical-free” baby wipes from the Monoprix grocery store chain were employed.
Flying Buttresses. The mass of the cathedral’s limestone walls, lead roof, and tower is stabilized by exterior support provided by its flying buttresses. Missing portions of the roof and vault upset this delicate architectural balance. Temporary wood bracing has been added; otherwise, the flying buttresses could force the walls to collapse inward.
It’s all painstaking work, but as one researcher said, “Notre Dame will come out of this experience enriched. And so will we.”
I am reminded of another current challenge around the world, about which I share similar sentiments. ds