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Last week, I posted various opinions about the mysterious cause of the fire at Notre-Dame Cathedral in Paris.

This second part largely concerns the oak beams in the roof and what one architectural expert said in an interview afterwards.

I have also added a Twitter thread identifying the mystery man from the cathedral’s tower.

The oak Forest

On Tuesday, April 16, 2019 — the day after the blaze — Canada’s National Post reported on the cathedral’s roof, known as the Forest (emphases mine):

Among the biggest challenges facing the reconstruction of the iconic church is rebuilding the intricate latticework of wooden beams that made up the roof’s frame, known as the “Forest.”

The 800-year-old oak beams were added to the cathedral in 1220. Because of the building’s gothic style which called for high vaulted ceilings, tall, sturdy oaks were sourced from nearby forests.

Each beam that held up the lead roof was constructed from a single tree, requiring about 13,000 individual trees in total, CNN reported.

When workers began constructing the roof hundreds of years ago, they cleared 21 hectares of oak trees. To reach the heights required for the style, carpenters needed to use massive trees. That meant when the trees were cut down, they likely would have been 300 to 400 years old. In other words, the trees used to build the cathedral — immortalized in Victor Hugo’s 1831 novel “The Hunchback of Notre Dame” — sprouted in the eighth or ninth centuries.

To get the dimensions and structure right in the Middle Ages, workers first built the frame on the ground. Then, it would be disassembled and hoisted to the ceiling with lifting gear, where it was reassembled. The oak beams would be set at 55-degree angles as the gothic style called for.

“Its dimensions are impressive,” the church’s website says. It’s more than 100 metres long, 13 metres wide and the transept is 10 metres high.

The article has impressive photos, by the way.

A 210-pound lead roof went on top of the Forest. The builders had to use lead instead of traditional clay, because Paris is not near any clay deposits.

Therefore, it is not surprising that samples of the oak are being investigated.

I will come back to the oak and an architect’s interviews later in the post.

What was and wasn’t happening

The following Twitter thread clarifies various aspects of the fire, including the identity of the man walking around one of the towers early on.

By Wednesday of Holy Week — April 17, 2019 — some questions were answered:

These two relate to the mystery man in the tower:

Now on with the rest of the thread:

This is the final tweet:

Former French chief architect speaks

On Tuesday, April 16, Benjamin Mouton, Chief Architect of Historic Monuments in France from 2000 through 2013, gave an interview to one of France’s news channels, LCI (La Chaîne d’Info):

Benjamin Mouton has spent his entire career working on France’s historic monuments, including their restoration. He has also been awarded some of France’s highest accolades including Knight of the National Order of the Legion of Honor, Officer in the National Order of Merit and Commander of the Order of Arts and Letters.

NewsWars has this important soundbite, translated, from the video:

“So, you’re telling us that this type of timber doesn’t burn like that?” Mouton was asked by an LCI host.

Oak that is 800-years-old is very hard – try to burn it,” Mouton said. “Old oak, it is not easy at all. You would need a lot of kindling to succeed… It stupefies me.”

Batiactu, a website that reports building trade news, was also able to interview Mouton that day. Excerpts follow (translated):

As Chief Architect of Historic Monuments, Benjamin Mouton was in charge of Notre-Dame Cathedral from 2000 to 2013, for which he led the heavy fire detection project. He has not yet been able to go inside the building, but already fears the impact of the fire and the collapse of the frames on the overall stability of the building.

Mouton repeated the words of his successor Philippe Villeneuve, with whom he stays in contact:

Benjamin Mouton affirms to Batiactu that he was “totally incredulous” in the face of this fire that could have spread from the renovation site, and that suspicions could be focused on the “valley”, where the nave and the transept of the cathedral meet.

He explained that:

the melted scaffolding will not be doing the vaults any good, as this creates a significant mechanical shock

He said that the stones that made up the vaults were turned to ash. They were the most important part of the building:

The stones are going to turn into lime, and the water fired by the firefighters creates a second heat shock.

As for the fire detection and prevention systems in place:

The fire protection set up in the cathedral was at its highest level.

When I took care of the fire detection, which was very expensive, it took only a few minutes for a fire officer to resolve any doubt. We had many wooden doors replaced by fire doors [and] we limited all electrical appliances, which were forbidden in the attic.

He also told Batiactu:

In my 40 years of experience, I have never known a fire of this sort.

He also pointed out the slow burn of oak:

The fire could not start from a short circuit, from a simple incident. It takes a real heat load at the start to launch such a disaster. Oak is a particularly hard wood.

Different types of wood have different burning temperatures

On Thursday, April 25, RMC’s Les Grandes Gueules had a segment on their news and talk programme about the fire.

I was quite cross to hear that they thought a smouldering cigarette butt could have started the fire!

I have a plank of seasoned oak that I almost posted to them with a letter asking them to leave a lit cigarette on it to see if it would burn the wood. While there might be a burn mark left, the oak is highly unlikely to ignite!

South Yorkshire Firewood has a page on different types of wood and how they burn. The reason Benjamin Mouton brought up the maturity of the oak was that thick, 800-year-old oak planks would not burn very quickly at all.

South Yorkshire Firewood deals in firelogs. The company rightly points out that logs need to be ‘seasoned’ before burning. Afterwards, one generally wants a slow yet steady burn in the fireplace. Therefore, one needs kindling and one needs good wood. They explain:

Hardwoods are generally more dense than softwood and therefore burn for longer and produce more heat …

Despite providing a more efficient fuel source, hardwood can be difficult to ignite from cold. Softwood kindling is therefore best used to get a fire started, the resinous and fibrous nature of softwood helping it to burn from cold. Once a fire is established and there is some heat in the base of the fire, it should be fuelled with hardwood to maintain a slow burning fire with a good heat output.

Softwood can produce a very pleasing flame to look at but it will burn very quickly and you will get through a large volume of wood in a very short time.

As for oak:

The density of the wood also affects how long it needs to be seasoned for. Oak is a very dense wood and can take up to 2 years to season fully.

One of the best firewoods but needs a long seasoning period due to its density. Burns slowly and is long lasting. On smaller stoves it is best burnt in smaller pieces than other woods.

Now, I am not for one moment suggesting that someone put lit kindling on the cathedral’s oak beams, but if anyone thinks a lit cigarette caused this fire, they need to read up more on oak, particularly old oak!

Although Benjamin Mouton was not in a position to say so, there’s something mighty suspicious about this fire.

Tomorrow, I will report on what rebuilding Notre-Dame might mean and what such a reconstruction would entail.

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