On April 15th 2019, firefighters charged into the
Notre Dame cathedral to quell a blazing fire.
They took careful precautions while
spraying down the 850-year-old Parisian monument,
like using lower water pressure,
and avoiding stained glass vulnerable to shattering.
They also followed a plan to save priceless relics and
works of art.
In the days, following firefighters gave way to
a new team on the front line:
restoration scientists from the Laboratory for Historical Monuments.
These researchers were tasked with evaluating the
condition of the damaged, stone, mortar and metal, and
guiding engineers on the best way to restore the cathedral.
Even as they try to reclaim what was lost,
the lab researchers and others are also
analyzing samples of wood and stone for
clues to the cathedral's medieval past.
Their first order of business: stabilizing the cathedral.
The lead roof and the wooden timbers that supported it
were mostly gone, and there were holes in
the vaulted stone ceiling.
That threatened a delicate balance of forces
between the vaults, which push outward on the cathedral walls,
and the flying buttresses which provide an inward stabilizing force.
In order to steady the structure, temporary wood
supports were installed to prevent the buttresses from
toppling the walls.
To restore the balance of forces, researchers also need
to understand how the stone in the walls and vault
were altered by the fire.
Most of Notre Dame was constructed with limestone--
an iconic stone used all over Paris,
including at the Louvre.
At 800°C, limestone can turn to a
soft white powder known as lime.
Even before this point, at 300°C,
fire can begin to reduce its strength.
One way to estimate these changes is by
looking at the color of the stone.
Beige indicates a sturdy structure, while a red or black
surface means the fire has damaged its internal structure.
The high porosity of limestone presents another complication.
As firefighters hosed down the Cathedral, some stones
gained almost a third of their weight in water--
further changing the balance of forces between the vaults and wall.
The laboratory researchers are also grappling with what
happened to the lead roof.
Parts of it reached the melting point of 300°C,
dripping into the vaults and gutters,
but some of the lead became airborne.
Yellow smoke billowed from the Cathedral as a result.
The lead powder that settled after the fire
is toxic to humans, making researchers take careful
precautions when working in the cathedral.
Before restoration even begins, much of the
cathedral must be decontaminated, and the mangled
scaffolding put in place for renovations prior to the fire
must be removed.
Meanwhile, the exposed interior and fallen fragments of
the cathedral have provided new opportunities for research.
One researcher is using stone samples collected from
the cathedral to learn about the quarries they came from.
Other materials, like the wood from the attic beams,
tell the climatic history of the 12th and 13th century.
The long and narrow shape of the beams also indicate
that these 100-year-old trees were
grown alongside many other trees--
and maybe were even farmed with the intention of using
the wood to build the Cathedral.
These finds provide glimpses into the detailed
construction of Notre Dame.
It took a 182 years to complete, with signature pieces
like the spire added well after in the 19th century.
Centuries from now, this fire will be just another chapter
in the story of Notre Dame.
But the collaborative research to understand its past,
and the careful plans to reconstruct the cathedral,
will allow future generations to continue to
be inspired by the iconic structure.