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 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.
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 walls.
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 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 tree,
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.
On April 15th 2019,