AILSA CHANG, HOST: 

We're going to turn now to someone who knows Notre Dame Cathedral inside and out. Art historian Caroline Bruzelius has spent years studying its Gothic architecture. Welcome.

CAROLINE BRUZELIUS: Hello.

CHANG: Now, I understand you actually spent years inside the cathedral. Can you tell us; what is it made of? Are you surprised that it caught on fire like this?

BRUZELIUS: Well, there are several things to talk about there. My work was on the cathedral while they were cleaning the interior. And I was taking advantage of that to go up a scaffolding up the height of the vaults, which are 108 feet above the ground, to try to study how the building is put together - to measure the stones, to record the moldings, to think about process of building what I call a mega-building. The cathedral was...

CHANG: Yeah.

BRUZELIUS: ...The biggest building of its time in Paris especially but in that whole region.

CHANG: And can you tell us what kind of stone it's made out of?

BRUZELIUS: So it's made out of Parisian limestone, which is quarried right there under and around the city. It's a beautiful quality of stone. But when it's exposed to fire, stone is damaged. It doesn't actually burn, but it loses its surface. It becomes friable. It chips, and it's no longer structurally sound.