After the Notre Dame fire, scientists get a glimpse of the cathedral’s origins

after-the-notre-dame-fire,-scientists-get-a-glimpse-of-the-cathedral’s-origins

The
“forest” of Notre Dame was one of Olivier de Châlus’ favorite places. That
dense lattice of timbers under the building’s lead roof epitomized the medieval
construction techniques that the engineer has spent years analyzing.

“There
was a very special wood smell, very strong, coming from the Middle Age,” de
Châlus says. “And it was very, very calm — impressive, compared to the very
noisy life inside the cathedral.” As one of the few visitors allowed in the
forest, de Châlus had the rare privilege of hearing the creaking noises emitted
by the timeworn wood and peering at numbers scrawled on the timbers by
long-gone carpenters.

That beloved forest is now gutted, lost in an April 15, 2019 blaze that destroyed the cathedral’s roof and spire and damaged parts of the masonry. De Châlus, who works for the global engineering firm Arcadis, is finishing a Ph.D. on the construction of the cathedral.

There’s
little documentation of the building process, which began in 1163 and continued
for about 200 years. De Châlus has devoted himself to teasing out the unwritten
rules of construction — how builders decided the size of columns or the height
of flying buttresses, for example. He notes that builders lifted 100-kilogram
stones more than 60 meters off the ground without the benefits of modern
technology. Exactly how this was accomplished has been lost to time, he says.

Olivier de Châlus
Olivier de Châlus studies Notre Dame construction techniques.E. Conover

“Notre Dame is my life, my whole life,” says de Châlus, who spent four years supervising the guides that show tourists around the cathedral. So, after the fire, he quickly joined an international effort organized by French scientists to use their expertise to help rebuild the cathedral and learn more about the iconic building. He is now the spokesperson for the group, Association des Scientifiques au Service de la Restauration de Notre Dame de Paris — the Association of Scientists in Service of the Restoration of Notre Dame of Paris.

The
fire has opened up access to parts of the building that could not be studied
when the structure was intact. Scientists have come together with plans to
research the history of the cathedral, as well as the fire’s environmental
impact on the surrounding city. Some will even explore what the cathedral’s
aged materials can reveal about climate change.

Getting organized

As
the flames died out, Paris despaired at the damage to one of its most treasured
historic structures. But “there’s much more to lose than what was lost
already,” says archaeologist Maxime L’Héritier of Université Paris 8. If the
materials that fell from the top of the cathedral — stone, wood, iron, lead — are
not studied, he says, the opportunity lost is “even worse than what the fire
has caused.”

The
day after the fire, L’Héritier and art historian Arnaud Ybert of the Université
de Bretagne Occidentale in Quimper, France, formed the association of
scientists. Today, more than 200 scientists are part of the group, including
geologists, archaeologists and engineers. The association aims to coordinate
work among experts in various specialties, share knowledge and advocate for
scientific study of the cathedral.

L’Héritier, who studies ancient metals, wants to know more about how iron was used in the structure, including its integration in the stone walls and the carpentry that held up the roof. While renovations in the 19th century added iron to the structure, the researchers will be searching for medieval iron placed during the original construction.

Notre Dame researchers
Researchers Lise Leroux, Aurélia Azéma and Maxime L’Héritier (left to right) are working on understanding the stone and metal within Notre Dame.E. Conover

Radiocarbon
dating is commonly used to sort out the age of materials, but for that, the
materials must contain some carbon. Luckily, medieval iron-production
techniques introduced small traces of carbon, which, when alloyed with iron,
make steel. Carbon dating those steel bits could demonstrate whether the metal
is original, L’Héritier says.

And
the iron, medieval or not, could act “like a thermometer,” revealing how hot
the fire got, says Philippe Dillmann, an archaeometallurgist at the Centre
National de la Recherche Scientifique, or CNRS. As temperatures rose inside the
fire, the corrosion on the iron — essentially rust — would have transformed
from typical rust into more unusual compounds. Analyzing that corrosion could
indicate how much heat was inflicted on the building, and so could help
scientists understand how much that heat weakened the limestone that makes up
the bulk of the cathedral’s structure.

Dillmann
is co-leader of a second effort to organize researchers to study Notre Dame,
spearheaded by CNRS. The CNRS team will also plan scientific meetings and
compile research.

Both
groups are still in the planning stages because the cathedral is still
contaminated with the toxic dust released when the lead roof burned. Most
scientists do not yet have access to the building, and all the materials within
must be sorted and cataloged before researchers can get their hands on them.

Inside the cathedral

A third group of scientists is already on the scene assisting with the building’s cleanup and restoration. Researchers from the French Ministry of Culture’s Laboratoire de Recherche des Monuments Historiques, or LRMH, develop scientific techniques for restoring monuments throughout France.

The
laboratory, located in Champs-sur-Marne near Paris, employs 23 scientists “for
all the materials and for all the monuments in France,” says LRMH’s Lise
Leroux. “We are very busy.” Even more so after the fire.

A
geologist and expert in the conservation of stone, Leroux is helping to
determine which of Notre Dame’s limestone blocks can stay in place or be
reused, and which must be replaced with new stones. “The monument is very
degraded,” she says. As the fire raged that night, the intense heat and the
deluge of water from firefighting efforts caused cracking and other damage in
the stones nearest the flames. And when the church’s spire collapsed, the
impact punched gaping holes in the limestone ceiling.

Notre Dame netting
Falling debris punched holes in the cathedral’s vaulted ceiling. Scientists are assisting with efforts to determine which of the remaining stones are damaged and need to be replaced.Brian Katz and Mylène Pardoen/CNRS

Finding
stones to replace damaged or destroyed ones will demand great care. Placing
stones of different compositions next to one another — for example, distinct
types of limestone quarried from different parts of the world — can cause water
or pollutants to accumulate in one stone more than another, weakening the
structure.

Even
before the fire, “the monument was very, very dirty,” says LRMH metals expert
and chemist Aurélia Azéma. Now, LRMH researchers are devising and testing
techniques for removing lead, which was strewn throughout the cathedral when
the roof burned. Metal, stone, paint and other materials require tailored
methods to extract the lead without causing damage.

A fire’s fingerprints

Problems
with lead extend beyond the cathedral walls. During the fire, extremely high
temperatures caused the lead to aerosolize into small particles that billowed
into the air and fell as dust nearby. That gave geochemist Sophie Ayrault, who
studies toxic metals, a new project.

Ayrault, of the Laboratoire des Sciences du Climat et de l’Environnement in Gif-sur-Yvette, France, previously searched for metals in the sediments of the Seine, the river that runs through Paris. Analysis of sediment cores from the river’s floodplain reveals how contamination has varied over the last 100 years.

To
pinpoint the origins of the lead she detects, Ayrault measures the relative
concentrations of its isotopes — different versions of the element with varying
numbers of neutrons in the nucleus. The ratios are a fingerprint that can be
used to trace the contamination’s source.

Notre Dame fire cleanup
After the fire, lead contamination near the cathedral mandated cleanup efforts (above). Researchers are measuring the isotopes of lead in samples taken from the Seine and other spots around Paris to tell which contamination came from the fire and which came before.Francois Mori/AP Photo

For example, in a paper published in 2012 in Chemosphere, Ayrault and colleagues reported that the signature of leaded gasoline was detectable in older Seine sediments, but faded away in sediments deposited after leaded gasoline was phased out in the mid-1980s.

Before
Notre Dame went up in flames, Ayrault had hoped to search the Seine’s sediments
for runoff from Notre Dame’s roof — which, when intact, contained as much as
460 metric tons of lead, she says. But Ayrault hadn’t yet procured the roof
samples she needed to discern its fingerprint. Now, to understand the fire’s
impact, determining that signature has become more important.

After
the fire, tests in parks and schools near the cathedral found lead levels high
enough to endanger children. But it’s not clear if all of that lead was a
result of the fire, or if some contamination predated it. To resolve that
question, Ayrault aims to collect samples of melted lead and dust from the
fire, as well as remaining intact parts of the roof. Then she’ll search for
signs of that lead in future tests around the city.

Into the woodwork

The
charred remnants of de Châlus’ beloved forest can also tell a story.

The oak trees that became the roof’s wooden frame grew during a hot spell in Europe known as the Medieval Warm Period, which lasted from the 11th century to the early 14th century (SN: 8/17/19, p. 6). Studying that wood could reveal details about that natural warming — such as how often droughts occurred — and may lead to a better understanding of what to expect from modern-day climate change, says Alexa Dufraisse of CNRS.

Dufraisse
plans to analyze tree rings within the burnt timber. The width of the rings and
the amounts of various isotopes found within the wood reveal the conditions
under which the tree grew. That could include how wet or dry the climate was
and the approximate geographic location of the forest.

Notre Dame supports
The “forest” of Notre Dame held up the cathedral’s roof and spire. It was destroyed in the fire, but researchers hope to study the charred remains of the medieval oak beams to learn about climate change.F. Epaud

She
and colleagues also hope to learn how builders chose the trees and whether the
forests were managed in some way. “This is a study that … could never have been
conducted without the destruction of the structure by fire,” says Dufraisse, a
dendroanthracologist, a scientist who studies tree rings within charred wood.

Other researchers are investigating less-tangible aspects of the cathedral, like its acoustics and its sociological significance. Anthropologists plan to interview people affected by the fire, including tour guides and musicians who’ve performed in the cathedral, to understand the psychological toll of the fire. “We all remember what we were doing when it was burning,” says molecular archaeologist Martine Regert of CNRS, who leads the CNRS group alongside Dillmann.

Regert compares the Notre Dame disaster to the 2018 fire in Brazil’s National Museum in Rio de Janeiro, in which millions of artifacts and preserved specimens were lost or damaged (SN Online: 9/7/18). In the Rio fire, “for me, we lost more” in terms of the scientific value, she says. Yet, emotionally, “I was probably more upset by Notre Dame.”

The
cathedral holds an outsize place in the hearts of Parisians and people around
the world. If another cathedral had burned, says de Châlus, there would have
been less interest. Determining how to rebuild requires understanding our
relationship with it, too, he says.

Subject
to bouts of emotion himself, de Châlus says he cried when he first entered the
cathedral after the fire. He felt an unfamiliar wind at his back, sweeping into
the church and up through the holes where parts of the ceiling had collapsed.
He says of Notre Dame: “It was much more than a church … much more than a study
subject for me.”

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