Original subtitles

Notre-Dame de Paris,

a treasured icon of Gothic architecture

and medieval construction.

The feat of engineering in those days, just extraordinary.

It is one of the most important monuments in France.

But on the 15th of April 2019,

disaster strikes.

A huge fire rips through the cathedral,

reducing the roof and spire to ashes.

For the last 3 years,

our cameras have been following teams

of mastercraft's people and engineers

as they battled to bring Notre Dame back to life.

Wow.

This is amazing.

I've been given an astonishing opportunity

to visit the work site here at Notre Dame

and this is such a critical time for the team.

They've met the challenge of making the structure secure

and now they're moving on towards rebuilding it.

Hundreds of workers here are locked in an ambitious race

to restore this medieval masterpiece

in time for a grand reopening ahead of the Paris Olympics in 2024.

Ooh, what have you got here?

As we track their progress,

we'll also meet the historians and scientists

working to solve Notre Dame's architectural mysteries.

- Have you seen this before? - No, never.

It's a kind of walking backwards in time.

What's this lady doing? Is she riding upon a dragon?

They're revealing ancient technology hidden for nearly 800 years.

We're dealing with unknown structures

that are so far unique in Gothic Architecture

And struggling to save the fragile fabric of the building.

The loss of material is catastrophic for us.

Three years into this extraordinary

5-year restoration project

this is the next chapter

in the race to save Notre-Dame Cathedral.

Paris,

a city of churches.

Basílicas,

and almost 2,000 historic monuments.

On the 15th of April 2019,

one of the city's most famous buildings,

the 850 year old Notre Dame de Paris

was undergoing a 5 million pound restoration of its spire.

A fire broke out inside the oak framework of the roof.

After 90 minutes,

the 360 ton timber and lead spire gave way

and crashed through the stone vaulting

causing catastrophic damage.

It's only thanks to the bravery of the firefighters that night

that anything of the cathedral survives at all.

And for weeks and for months after that

its future still hung in the balance.

The roof and spire are completely gone

and there are three gaping holes in the vaulting.

Teams of engineers raced

to install supports to shore up the fragile structure

but the cathedral remained open to the elements.

Rainwater saturated the vaults,

adding weight to the weakened stonework

and increasing the chance of a total collapse.

Before they could make the structure watertight,

they had to remove 40,000

burnt and melted scaffolding poles,

left over from the spire restoration.

Now, they've built a temporary sliding roof.

It's called the big umbrella,

to keep the stonework dry.

The cathedral looks very different today

than it did when I first visited.

I'm historian Lucy Worsley, chief curator at Historical Palaces.

I first came here, age 16, French exchange trip.

Here's a picture. Ah, so young!

And I remember going through the doors

of Notre Dame for the very first time,

it was breathtaking.

It was one of the formative moments that set me on my course

to wanting to spend my working life with historic buildings.

That's why for me,

like for so many other people,

the night of the 15th of April 2019,

was such a horrible shock.

We still don't know how the fire started

but 3 years on,

the ambitious project to restore the cathedral is being ramped up.

In recent months, the team here has almost quadrupled,

bringing together ancient techniques and modern engineering.

200 workers are now battling

to bring Notre Dame back to life.

The first time I came here, after the fire,

it was devastating.

There was a real sense of tragic loss.

But now,

things are changing. It's like a new chapter beginning.

The cathedral looks clean.

I can see many, many people working away inside.

It's like the wounded old lady is learning to walk again.

The French state has ruled

that Notre Dame will be rebuilt exactly as it was before the fire.

Donors have contributed 700 million pounds towards this effort

but the task ahead is enormous.

The team must remove tons of toxic lead dust left over from the fire.

Then, clean and restore

the fragile stained glass windows.

To reconstruct the roof identically

they must first fill the 3 gaping holes in the stone vaulting

and rebuild the timber framework

from almost a thousand oak beams.

Then cover it with 3,000 square metres of lead sheeting

before they can raise the 65 metre high iconic spire...

finally, to make Notre Dame whole again.

Beyond rebuilding identically,

for conservationists,

restoration is all about preserving the story of a monument.

Old buildings matter to us because

they embody not just our history

but also our future.

So there could be an argument in conservation

that if you're conserving a building you don't need it to be like the original.

You know the story of the building

can best be told by showing where the differences are,

how the building has changed and responded

all across the centuries.

The challenge for the team right now

is to clear up tons of toxic lead dust.

As the lead on the roof melted

and the spire collapsed,

it was sucked inside the cathedral.

I've worn lots of different...

costumes, outfits, for telly over the years.

Never quite one like this before.

Workers must put on full protective clothing...

to pass into the dirty zone.

The way onto the site is actually...

through the shower.

This is the clean zone and through there it's the dirty zone.

When you come out, you come through the shower, too because everything has to be washed.

All your equipment, all your clothes and obviously yourself.

It's then just a short walk into the cathedral itself.

Wow.

This is amazing.

I have never seen this much scaffolding.

Goodness me. There's an army of people

marching about doing things.

It's freezing, it's really cold.

And it's dark and there's banging and it's--

it's a bit like hell, actually.

This is quite the opposite of what a church is normally like.

There's a lift going up into the nave.

My goodness, that's a lift made out of scaffolding.

- Hello. - Hello.

Deputy director of operations, Jonathan Truillet.

coordinates the work to bring Notre Dame back from the brink.

Jonathan, there's a huge amount of scaffolding in the cathedral.

What what does it allow your team to do?

The scaffolding not only allows access to clean up the toxic lead dust

but also plays another critical role.

Before the fire, the vaulting

made from hundreds of cubic metres of limestone,

supported the massive timber and lead roof above.

But now the vaulting is extremely fragile.

As the team begins to repair the stonework,

it could trigger a collapse.

To avoid this, one of their first tasks

is to erect 52 timber support arches beneath the vaulting

so that they can rebuild safely.

Installing them 33 metres above the ground is no easy feat.

Well, can we take a closer look?

Thank you.

The scaffolding enables the team to build a temporary floor,

so they can install the timber supports.

The crane must lower each one and a half ton ton support arch

down through the hole in the vaulting.

Each support arch is fitted with steel brackets to secure it in place.

Hydraulic jacks raise it to meet the underside of the vaulting.

With the temporary supports in position,

it's safe for me to take the lift to the top floor.

Oh.

That was a mistake to look down.

Oh, look, here's the window.

We're at the level of the windows.

Wow. This is not normally how you get to experience a cathedral.

This glass is so... rich.

So intense.

Oop.

We're now as high as the lift can take us.

Close to the underside of the shored up vaulting.

Oh, wow!

This is incredible. It's like being in a timber barn.

This same technique was used to build the vaults

850 years ago.

The scaffolding and supports will stay in place

until the vaulting roof and spire are rebuilt.

The scale of the support work is-- it's just astronomical.

It's incredible.

I noticed there's quite a lot of fire extinguishers up here.

Now these gigantic timber supports are in place underneath the arches,

the cathedral is structurally safe.

It's definitely not going to fall down.

Which means that the people working here can move on

to restoring and rebuilding.

You can hear them hard at it, as I speak.

And this is going to be a real race against time

because they want to reopen the cathedral by 2024.

All eyes will be on Paris

when the Olympic games are held here in 2024.

President Macron has agreed

that the landmark must be rebuilt within 5 years,

in time for a grand reopening before the games begin.

But with the cathedral's vaults, roof and spire to rebuild

and the entire site still contaminated by lead dust from the fire,

the task seems near impossible.

To make the site safe the team must remove all the toxic lead dust.

Clara is one of 40 decontamination specialists,

meticulously cleaning

all 6,000 square metres

of Notre Dame's vaulting, walls, pillars and floors.

This is to keep you safe. It cleans... cleans the air.

Yeah.

And where does all the lead dust that you've collected go.

My vacuum.

Oh, into the--into the backpack.

- Yeah. - Yeah.

With this equipment we only work two hours and a half at a time

and we need to stop to take a break.

How long will it take to clean the whole cathedral?

- There are different spaces of cleaning - Yeah.

now we're in the first one.

Only the first phase. More-more cleaning to come.

- Yeah. - Oh, my goodness.

The vacuuming of the lead dust will take 8 months.

Then the restorers can move on to deep clean the stone.

My dream was to come here and work here, it's an opportunity.

You-you never expect to be working

in one of the most important monuments

in France, so...

Your dream has come true.

I think you deserve a cup of coffee now.

Well, I already have three cups of coffee today.

Go enjoy your break. You deserve it.

- Bye. - Bye.

You know it's obviously tragic that the cathedral burned

but I hadn't really thought about this;

it's given opportunities

to young people, to learn new skills like Clara

but wearing a vacuum cleaner on your back

for two, four, six hours a day,

sucking up poisonous lead.

Hoo.

That's tough work.

In places you catch a glimpse

of how dramatically changed Notre Dame will be after the cleaning.

Beneath the lead

and centuries of smoke from the millions of candles,

lies pristine limestone.

This is how the cathedral looked 850 years ago

and will again soon.

As the cleaning progresses,

crafts people and technicians,

from mural specialists and glass historians

to metal scientists and carpenters,

get to work.

These are France's foremost experts in their fields.

In charge of guiding this unprecedented

concentration of medieval knowledge,

are chief architects, Pascal Prunet

and Philippe Villeneuve.

But this team will need to pull out all the stops

if they want to reopen the cathedral in 2024.

In April 2019,

this was the heart of the inferno.

This is where the spire fell down into the nave.

It's only when you get up here

that you appreciate the scale of what happened.

And to rebuild that spire, my goodness.

It's going to need a tremendous amount of wood.

Above the stone vaulting,

Notre Dame's roof was completely destroyed by the fire.

One of the most complex challenges the team here now face

is entirely to rebuild

the 1,000 ton roof structure.

Remi Fromont is the chief architect

tasked with reconstructing the medieval roof

which is known as the forest.

During a research project in 2014,

Remi manually measured

the dimensions of every beam in the forest

to create a comprehensive survey of Notre Dame's roof.

This allows them to replicate it perfectly.

-- used 850 oaks

to reproduce the Gothic roof trusses.

The most intricate element,

the cathedral's iconic spire,

will be built from another 1,200 trees.

The spire was a 360 ton

engineering masterpiece.

Hidden beneath 16 copper statues

and 140 tons of lead sheeting.

With a complex skeleton of oak beams,

some as long as 20 metres.

The secret of its strength,

a dense lattice of oak tied into the rest of the roof,

supports the entire structure.

The spire came to be

the embodiment of the building and of the Paris skyline.

Which took huge imagination and levels of engineering

and creativity and architecture, which is quite exceptional.

Identically reconstructing this wooden wonder, is no simple task.

The market town of Chesterfield

bears witness to the perils of building with green timber.

Its sports the 660 year old crooked spire

at the parish church

could be due to beams that have warped as they've dried.

For Notre Dame's spire,

this precarious lean must be avoided at all costs.

In public and private forests across France,

the hunt for 2,000 perfect oaks for Notre Dame begins.

These will form part of France's annual forest management quota.

One-third of the country,

65,000 square miles,

is covered by forests.

This team of forestier has their work cut out.

They must source 60 flawless oaks for the spire

from this 8,000 acre forest.

On the other side of the clearing another candidate emerges

but it must be wide enough and tall enough

to form a spire beam.

Lumberjack, Ahmat Seerpan,

begins by making a cut

that will direct the tree to fall into the clearing.

Notre Dame's medieval carpenters

etched Roman numerals on their beams

to reassemble them correctly up on the roof.

Today this team attaches a bar code

to each oak destined for the cathedral,

so they can track it from the forest

to its final position in the new spire.

It took the forestiers several months

to complete their painstaking search

to fill their quota of 60 trees for the new spire.

Notre Dame's builders

did not only innovate with timber

they also pushed the limits of what could be made with glass.

More than 3,000 square metres

of spectacular stained glass

circled the cathedral in three levels.

One of the things I really remember

about coming to Notre Dame for the first time

was the otherworldly light from all the stained glass.

Today most of the natural light's been

blocked out of the cathedral by this huge scaffolding structure

but there's a silver lining.

The scaffolding gives experts the chance to get up close

to Notre Dame's astonishing rose windows.

This is the first time it's been possible in 160 years.

It is a very unique opportunity

because we don't see them again

in the same way. Never.

When you're so close from the panels,

you have a kind of intimacy

with the work of art and

even you can see sometimes the mistakes of the medieval painters.

The 3 rose windows date from the 12th and 13th centuries

and together they're made up of 1,100 panels.

Protected by the stone vaulting,

they survived the fire unscathed.

These kaleidoscopic wonders

are filled with depictions of ancient legends

biblical scenes

and saints performing incredible feats.

The panels have been restored multiple times

over 800 years

as they've been damaged.

Like time capsules they contain messages

passed down across the ages.

They also hold many unsolved mysteries.

Glass historians Karine Boulanger and Elisabeth Pillet

hope this access will help them reveal

the secrets of the largest window in Notre Dame,

the gigantic south rose window,

measuring 13 metres in diameter.

They're busy mapping every shard of glass.

Karine, what's this lady doing? Is she riding upon a dragon?

It's, um, Margaret,

she's tamed the dragon.

- She's tamed the dragon? - Yeah.

I can see, she's telling it "down dragon, down."

It's fabulous, I love it.

And you can tell me how old this window is, can't you?

This is your super power. - Yes, no way.

It's a 13th century glass

but there are some restorations, of course.

You see the the colour of the glass is different.

So, I'm guessing that that bit there.

Yeah.

That looks like it's quite dark. Is that medieval? - Yes, it is.

- All the red is medieval. - Yes.

But if you look at this red here, you see how different it is.

Oh, so that's-that's not medieval, that...

No. It's 19th century.

You see, this one is more translucent.

- It's like a jigsaw puzzle. - Exactly.

It's not only the subtle differences in colour,

the style of painting on the stained glass changed over time.

Medieval glass painters

applied a solution of copper and iron oxides

to the coloured glass before it was baked to harden the paint.

The style evolved,

a 13th century painter

won't paint the same way as a 15th century painter, obviously.

At the same time for the 19th century.

What causes the medieval glass to go cloudy like that?

It's linked to the composition of the glass itself.

In order to make glass,

- in 13th century, you use potash. - Yes. - And it causes corrosion.

That potash, over 8 centuries, corrodes like that.

Yes.

It's like a time limited artwork, isn't it?

Yes.

Potash, a potassium-rich blend of plant ashes

reduced the melting point of the ingredients used to make glass.

By the 19th century, sodium carbonate

combined with calcium oxide, were used instead.

And produced more stable glass that didn't corrode.

This factory, on the banks of the Loire in southern France,

is one of the last places in the world

that can produce stained glass using medieval techniques.

We produce glass for major historical buildings

as Versailles or the White House.

Workers start by mixing sand,

metal oxide for colour,

sodium carbonate

and calcium.

They heat the mixture to 1,300 degrees Celsius

and build up layers of the molten glass

on the end of a blowing pipe.

The glass blower forms a sphere from the red hot mass.

He rolls it to maintain this shape

which is critical to form an even thickness of glass.

It's a manual know-how.

There is absolutely no machines

and the glassblowers uses a sense,

a feeling, to blow one glass sheet.

The team enlists the help of gravity,

they swing the 7 kilo ball of glass

in a 4 metre deep pit

so it elongates to form a tube.

Herve, has blown glass here for more than 33 years.

Once cooled, they cut the cylinder

and send it to a special furnace where it's unrolled.

Extreme heat of 750 degrees Celsius

and a wooden block

smooth out the glass and minimise imperfections.

They carefully inspect each pane

and remove any rough edges.

Every glass sheet is different

and has the spirit of the glass blower.

Losing this patrimoine and know-how would be a disaster.

The factory marries these ancient techniques

with the latest technology

to reproduce stained glass accurately.

We can fit perfectly

with the old colours

by using tools like

spectrophotometry,

like x-rays,

and this allows us to know exactly

what are the elements that are in the glass

and reproduce it for the future.

By analysing samples of ancient glass

to learn the chemical composition,

they can match its unique combination of metal oxides.

This team is ready with the skills

passed down through the centuries

to reproduce any of Notre Dame's stained glass that may be beyond repair.

Until the 20th century when a glass was

too badly damaged,

we had to replace it.

There are restorations every 50 years

or 100 years.

So, a stained glass window is always a mixture of original glass,

here, 13th century glass

and restoration from...

every century almost.

As the historians map the south rose window

they uncover an unusual trend.

This window should contain glass from many restorations

spanning almost 800 years.

But they're finding only

original 13th century glass,

glass installed during the 19th century

or panels containing both.

Where has all the other glass gone?

We are finding lots of things

they altered quite strongly the design of the panels,

the roses have been extensively restored

and we have to...

study them again to understand what has been done.

Karine and Elisabeth must work fast

while they still have this unprecedented access to the south rose window.

They will continue their investigations to try to solve this mystery.

Just above the south rose window

is one of 3 holes in the vaulting

created as the spire collapsed.

Incredibly the top section of the spire survived the inferno.

Its slender pinnacle is lodged in the vaulting stone.

The team gently nudges this spire section free

and carefully winches it down.

We can still see the structure

of how the spire was made

with these fine lead sheets

of a few millimetres thick

that were used

on the entire structure of the spire.

Six decorative lead roses

remain attached to the spire's lead sheeting.

We will be able to study

how this decoration was made

touching the spire that was

just taken down from the vaults today.

It's a magical moment.

The fire has given scientists and historians

an opportunity to look deep into the fabric of the structure.

Notre Dame pushed the limits of Gothic architecture.

Advances in stone construction techniques

such as the introduction of flying buttresses,

allowed medieval masons to build incredibly tall and thin.

As the team examines the structure closely,

they discover metal hidden throughout the cathedral

that could unlock more of its architectural mysteries.

From the thousands of nails that joined timber beams

to iron bars that brace and hold secure

the stunning medieval stained glass.

I was amazed that there's so many iron in, in this building

that was never truly studied before.

The staples that we see here,

they're embedded in the...

maybe the oldest part of Notre Dame's masonry.

These 45 centimetre long iron staples

secure the great arches beneath

and prevent the stone blocks from being pulled apart by the enormous forces.

20 metres above,

along the very top of Notre Dame's walls

the destruction of the roof

has revealed previously concealed iron work

that may hold the secret to this cathedral's gravity-defying height.

It's really exciting because we're dealing with

unknown structures on the top of the walls

that are so far unique in Gothic architecture.

Medieval builders may have been worried

that the top of Notre Dame's tall slender walls

could be an Achilles heel.

The weight of the roof could push the stones apart.

The destruction of the roof has revealed

the builders tied these stones together

with more than 500 staples,

creating a ring of iron, holding the walls together.

This engineering master stroke

has remained hidden under the roof of Notre Dame for hundreds of years.

The staples with the flying buttresses

are two ways of preventing the stones to collapse.

It's...

an ancient form, a form which is known since antiquity

but it looks like in Notre Dame,

we're trying to use ancient forms of reinforcement

such as the staple

in order to build a new form of architecture,

really high, really thin, Gothic structures

of which Notre Dame is kind of the first true example.

Maxime uses pioneering technology

to unlock the secrets of the iron staples.

Like we're acting as some kind of detectives,

trying to find out the digital prints,

the digital signature of each of these staples

and try to rebuild their path

from the workshop to the building site.

Radiocarbon dating confirms

they were installed in the early 13th century

when this part of the cathedral was built.

These are the oldest pieces of iron used in a Gothic church

that we know of so far.

That's a huge discovery.

This is a revolution in Gothic architecture.

No other Gothic monument had used iron

in such a way before Notre Dame.

Maxime reveals the microstructure of the iron.

Multiple pieces of iron have been forged together to form each staple.

The world is the result of the mixing of scrap iron

to make a brand new iron staple.

We're maybe dealing with the richest building site,

at that time,

and knowing that it might have used almost

90 percent recycled iron, opens new perspective.

The team has shown

that the recycling of iron

may have been commonplace on the building site of Notre Dame.

Shedding new light on medieval building practices.

The labs electron microscope

reveals further clues

to how far Notre Dame's builders went to source the iron.

What we discovered is that every single staple

has a different chemical signature.

All the staples,

they come from different iron that was made in different places.

It means that there's a truly active

iron market in Paris,

gathering iron from many, many different origins.

These hidden iron staples

may also have played a critical role

in the aftermath of the fire of April 2019.

The staples were placed by the

medieval master mason

to reinforce the upper main walls,

they might have helped the walls

to prevent collapsing during the fire.

To create the most advanced church of its time,

builders of Notre Dame turned to ancient construction techniques

only now fully understood in the wake of the fire.

As a conservationist,

it's teaching us how

expert these builders were in those days.

It's a testament to their

technical competence and their vision

that they put in these structural elements

which have preserved the building for us.

Their knowledge might have kept the cathedral standing

but the damage to the vaulting reeked by the fire

is shocking to see.

For the restoration of Notre Dame,

it might just be the stone of the vaults that's the biggest problem.

The thing is, it got soaked with water,

firstly from the fire fighting

and then because the roof was missing for months and the rain came in.

They've got a temporary roof up now and it's starting to dry out

but even that has presented an unforeseen problem.

As the stones dry,

salts are crystallising on the underside of the vaulting

and they're breaking off the outer layers of limestone.

All the damage in this part

and on the-on the vault is the result of the salt

you can see the loss of material is

2 or 3 centimetres.

This is catastrophic for us.

In the 18th and 19th centuries

restorers cast sacrificial layers of plaster

on top of the vaulting in case of a fire.

This protected the stonework from the heat of the 2019 inferno

but not from the water used to fight it.

This, along with months of rain water

drew salt from the plaster into the porous limestone.

As the moisture evaporates, the salt crystallises

and forces the limestone apart,

destroying the inner surface of the vaulting.

To find the solution, you have to remove the salt.

- Remove the salt. - As the salts are soluble salts,

you will have to use water...

- With water. - ...and some other tricks.

To extract the salt,

the team will coat the vaulting with a paste of clay, sand,

and purified water, called a poultice.

The water from the poultice is drawn into the stone

where it dissolves the harmful salt crystals.

As the clay of the poultice dries,

it draws the salty water out of the stone,

saving the vaulting from further damage.

The poultices will be removed

when they will all have dried.

Workers chisel away the mortar between the stones

to allow the poultice to reach deep into the blocks.

Next, they load the sticky mixture into a compressed air gun

and spray it into every crevice.

Finally, they carefully smooth the poultice

across the face of the vaulting stone.

You can see that it follows very, very closely the surface of the stone

and what we recommend is that poultice should not be thicker

than half a centimetre to one centimetre,

otherwise there are risks that it falls down.

So, do you have enough time? You have to open the cathedral.

Can you have it there as long as you need to?

If you wish,

all the poultices to be extracted,

we need to-to have a very slow process

until the vaults are dry.

And this will take time, much time,

so we have to get the gusset ready

in 2024.

I understand those time constraints

because it's a worldwide known symbol

and this building has to live again.

The poultice may stay in place

until the missing vaulting has been rebuilt

and the cathedral is permanently watertight.

It's the end of another long day on site.

Notre Dame falls silent once again.

Well, this is quite a remarkable experience

because I've got the deserted cathedral

to myself.

It's extraordinary.

And just over there I can still see the remains of a burned

beam.

Which is a reminder

that amidst all the activity that goes on in the daytime.

This is where the fire actually started.

This is where nature was out of control.

The medieval timber roof, known as the forest,

was built section by section over almost 100 years

as the cathedral grew beneath.

Hand axes were used to craft each individual beam

for a specific position in the roof structure.

The fire, which started in the forest

took just hours to reduce this medieval masterpiece to ashes.

The team will soon begin an unprecedented challenge

to rebuild the forest in a matter of months.

Notre Dame's spire is relatively modern,

built in the 19th century.

These beams were cut with saws,

so, it's down to the French sawmills

to transform 1,200 of the oaks

into flawless beams

with the precise dimensions needed for the new spire.

Francois Feillet, runs this sawmill in Normandy

that processes more than 30,000 tons of timber each year.

It's one of 45 sawmills across France

that has answered the call to cut the beams for Notre Dame's new roof.

The team wastes no time in getting to work on the beams.

First stop, the debarker.

This machine excoriates the outer layers, removing the loose bark.

Now, the 3-ton tree trunk enters the cutting shed

and rolls onto the saw carriage.

The band saw blade is a 1,000 pounds,

high speed, ribbon of steel

that runs at 43 metres per second.

This is a laser-guided, precision operation.

Francois removes slices just 3 centimetres thick,

to trim the beam down to the exact dimensions

requested by the Notre Dame architects.

The beams from Francois' sawmill are stacked,

ready to join more than a 1,000 others

coming from across France for Notre Dame's new spire.

Inside Notre Dame,

the first chance in 160 years,

to get up close to the vast south rose window,

has revealed a puzzle the glass historians need to solve.

We should be finding glass from all different periods.

We know the window's been restored lots of times.

But the glass all seems to be either

original 13th century glass or

relatively modern 19th century glass.

Where's the rest of it got? It's a bit of a mystery.

The chief suspect is architect, Violet Leduc.

In the 1840s,

he was tasked with breathing new life into Notre Dame.

At the time, it was not the beloved building we know today.

The cathedral was ransacked during the French revolution.

Statues of kings on the facade were decapitated

and it was used as a warehouse,

lying derelict and unloved for decades.

Over 20 years,

Leduc reinstated the statues of the facade,

he rebuilt the sacristy.

He designed hundreds of new gargoyles

and he raised the ill-fated 65-metre tall spire,

a replica of the medieval original.

Violet Leduc, god bless him,

would have been what we consider to be a starchitect, you know?

He was a man who knew his mind,

he was a man who was highly respected, really determined

and saw himself as a powerful leader.

During his restoration,

Leduc removed all glass in the south rose window that was not original

and replaced it with modern glass.

Elisabeth is also finding

that he made brutal changes to some of the original glass panels.

What do you observe in this...

And he's lost his aureole, his halo.

I've come to the Paris media tech of Architecture and Heritage.

These archives may hold clues to why

Leduc made such dramatic changes to the glass in the south rose window.

Ooh, what have you got here?

The spire. The spire.

- Have you seen this before? - No, never.

So, I guess your colleagues, the architects,

will be looking closely at this to get clues for how it should be.

Oh, look. Is this Mr Leduc?

- No, I don't think. - No?

I think it's monsieur Leduc, he's put himself in the drawing.

The windows of the tribune...

- Look, look, look, it's the famous- - Gargoyles.

it's the famous gargoyles. Here they are.

Gargling.

What creature do you think that is?

The gargoyles not only frighten away evil demons

they also protect Notre Dame's walls

by directing rain water away from the cathedral.

That folder is a complete treasure trove.

It's full of these really beautiful drawings by Violet Leduc,

showing his plans for Notre Dame.

And it's such a treat to see them,

not only because they're beautiful

but because this folder contains such important information about...

France's idea of itself,

that's captured in Notre Dame cathedral.

But it's Leduc's plans for the south rose window

that may hold the key to unravelling its mysteries.

All this part here

- was destroyed in the 19th century. - Yes.

They threw it away.

Violet Leduc recognised that it was an original medieval design

but for him maybe it wasn't the ideal 13th century rose

that's why he changed the design.

So he looked at the medieval window

and he thought: "No, that's not medieval enough."

Yes.

Yes, I think he said that.

But such radical changes to the glass panels

suggest Leduc was motivated by more than mere aesthetics.

After hours of research,

the team makes a stunning breakthrough.

It sounds like you're coming across

completely new information. Is that fair?

In fact we just discovered that

there was a change of structure of the rose,

there was an iron reinforcement

in the centre of the rose, but obviously it wasn't enough,

so Violet Leduc put it further away from the centre.

When we moved the iron work here,

I had to change the form of the panels.

Leduc's vision for a more medieval south rose

combined with the need to beef up the iron reinforcement,

required him to change the glass.

But the historians find evidence

he actually altered the entire stone structure of the window.

It's like two different windows.

In fact, he slightly...

turned the rose.

So, it's gone like this and he's sort of gone--

Why did he do that?

That's a big change, isn't it?

Leduc's restoration was sweeping.

He removed a smaller structural ring of iron

and replaced it with a bigger ring

to strengthen the core of the window.

He removed all trace of previous restorations,

to replace them with panels of new glass.

And he turned the whole window through 15 degrees,

to make it structurally stronger.

These two drawings in the archive have been revelatory for me

because they've helped me understand why Violet Leduc

may have messed with the structure of the window.

This showed what it was like before

and you see this vertical axis, here we've got panes of glass,

that's not structurally very safe.

But after, in this drawing, he's put in a big strong stone arm,

that's going to hold the whole thing together.

I mean he wasn't just doing things for aesthetic reasons,

he was also helping Notre Dame

to withstand the centuries.

Thanks to the scaffolding,

this team is painting an intimate portrait

of how the south rose window,

one of the wonders of this cathedral,

evolved to survive.

It's been such a privilege to spend time

in this wounded

but wonderful cathedral

and it's clear how utterly dedicated

the architects, the restorers, the scientists

are who've been tasked with bringing it back to life.

I'll be amazed if they do make their deadline of 2024.

But you know what?

There's no shortage of ambition here.

This experience marks another chapter

in my relationship with Notre Dame. Oop.

A building that has inspired me and countless others.

I've seen the vaulting secured,

the interior cleaned of toxic lead dust,

and the mysteries of its windows solved.

Next time, I'll be back to witness the rebuilding of the vaulting,

the new roof,

and the spire under construction.

And maybe,

just maybe,

the reopening of this astonishing building to the world once more.

And check out Lucy's Ladykillers over on BBC sounds,

giving Victorian women a voice

with a contemporary crime twist.

And civil war makes civil hands unclean,

dirty with inspiration, Art That Made Us,

next here on BBC2.