Tuesday, April 16, 2019

Notre Dame: What's With That Roof?

     It was the roof "frame" and the structure of the spire that burned.  They were covered in lead, great stuff for the job from the 1100s (when the building was underway) through the 19th century (the last major restoration): it's waterproof, doesn't burn, and sheets of it are easily joined together.  Hundreds of tons of lead -- and the environmental laws and rules in the EU aren't any softer on lead contamination than the ones in, say, California.  Expect to hear more about that in coming weeks.

     The trusses themselves were a "queen-post" design, fairly strong for their weight and with an open space in the center, allowing for access (as seen in this CNN article).  The roof mainly keeps water away from stone arches (six-part rib vaults) below it, arches that carry compressive force from the buttresses on each side.  When news articles talk about the danger of interior collapse, damage to those limestone arches is a big part of the worry; while I snarked on Facebook about the kind of idiocy that manages to set a stone building on fire, the heat of structure fires is a great danger to limestone architecture.

     I'm not finding a lot on the interior structure under the spire in a quick search.  The dramatic interior photograph that has shown up everywhere seems to show a great lump of debris right under the crossing and it could be very bad.

     At the base of the spire was a group of statues: the twelve Apostles, in four groups of three, each group preceded by one the animals symbolizing the four evangelists, and all of them -- all but one -- facing out towards Paris.  The lone exception?  St. Thomas, patron saint of architects. He was looking up at the spire -- with the face of Eugène Viollet-le-Duc, mastermind of the extensive 19th-century restoration.  All of the statues had been removed only days before the fire; no doubt both the saint and whatever records Viollet-le-Duc left are going to be getting a lot of attention in coming months and years.

     This isn't the first time Notre Dame has been badly damaged; the 19th-Century work largely addressed damage caused during and in the aftermath of the French Revolution.  Nor is it the first time the modern world has had to address this kind of disaster: Reims Cathedral was reduced to ruins in a very similar manner during World War One.  Restoration started immediately after the war and the cathedral re-opened twenty years later.

     As of this morning, over $339 million had been pledged by private donors to rebuild the cathedral of Notre Dame.  During the fire, brave firefighters rescued irreplaceable artifacts.  As dreadful as things look, it is not lost.  It will be rebuilt.


pigpen51 said...

I have not followed the fire too closely for fear of hearing the dreaded word terrorist. While I have not heard it, the things that I have heard have seemed to point to accidental fire due to electrical problems. It probably is immaterial.
What is important is that as you said, many brave people rescued irreplaceable artworks, of great importance to many of the Catholics who truly love the church of old.
I feel sorry for them, myself, even though I am not Catholic. The Cathedral served as a very important focal point in Paris for the many faithful, and to see it burn is a horrible feeling. I saw the church that I helped grow and turn into a booming congregation burn to the ground due to arson by a man who was released from a psychiatric facility, and who wanted to go back, and so went inside, and set several fires, which burned the entire large building to the ground. Watching it burn, with a large group of members, was heartbreaking. I can imagine the sense of loss, at the burning of a monument such as Notre Dame Cathedral, which had stood for 800 years or so.
Often, we say our thoughts and prayers are with someone, and those who tend to be dreary say that it is pointless to offer thoughts and prayers. In this case, I find that it is perhaps the best thing we can offer.

Matt said...

I saw an episode of The Woodwright's Shop once where they made quicklime (Calcium Oxide) to use as mortar. The process was, briefly, load up a furnace with limestone and set a fire in it that burns all night. The fire burns out some component of the limestone, chemically changing it to quicklime.

Which dissolves easily in water.

If the fire got hot enough, they have a serious problem when the next rains come.

pigpen51 said...

We used to use (Calcium Oxide CaO) in a process in making steel. We also added Magnesium Oxide, (MgO), to the vessel, before introducing the bath of molten steel. It made a slag of the same material as the vessel brick was made of. The steel would attack the slag, instead of attacking the bricks, to help extend the life of the vessel.
The slag also held metallic elements during the time we were blowing carbon out of the alloy, to reduce cheaper iron down into more expensive stainless steels. Rather than lose the metallic elements by simply throwing the slag away, we would add silicon and aluminum to the bath and stir once the carbon was low enough, and it would reduce the metallics back into the steel. It is all done on a mathematical formula, to ensure you use the correct amount of aluminum and silicon based upon how much Oxygen you used to blow the carbon out, which combined to form Carbon Monoxide.
After the steel was tapped and the heat was finished, we dumped the slag outside to cool, and indeed, if it rained, it got as hard as concrete. We used a very large front end loader to break it up and load it into a hopper to haul it away.

Montana said...

Electrical fire or whatever, I'm always keen to read about exactly how things went sideways. As someone who works in construction, I can tell you that fires in the construction zone are far to common. I'm always interested in how to avoid them.

markm said...

Matt: Limestone is calcium carbonate. At high temperatures, it breaks down into carbon dioxide and calcium oxide (quicklime). Add water and you have calcium hydroxide (slaked lime), which is used in lime mortar. And that gradually reacts with carbon dioxide from the air, turning back into limestone. (Modern mortars generally also include sand and Portland cement to control the setting time and make stronger and longer-lasting joints.)

So yes, if any of the limestone masonry got hot enough to calcine to quicklime, there are problems ahead. First, quicklime itself is somewhat hazardous. Skin contact may result in chemical "burns", probably painful but not too serious. The bigger hazard is that quicklime is formed as a fine powder, easily stirred up and inhaled. In your lungs, it will suck water from your cells, and then forms a strongly alkaline paste that will dissolve more of the lung lining. Wetting it down suppresses the powder, but then you'd better don rubber boots, etc., and shovel it somewhere before it turns back to stone.