Fire and stone

[skip.knox]

I have a practical question for folks. Can fire bring down a stone wall?

Specifically, if the attackers were able to bank a hot fire against the walls of a city, hot enough that it burned for ten or twelve hours, would that be enough to weaken the wall so that it collapsed?

I'm thinking of a Mediterranean city wall, faced with mortared limestone maybe a couple feet thick, with rubble behind. Parts of the wall might be faced with marble and parts of the wall might have brick behind the facing.

What I'm envisioning is, the fire burns the marble and the mortar. The accumulated heat is enough to cause some of the stone blocks to crack and maybe even to explode. This weakens the exterior to the extent that the wall collapses outward.

I'm reasonably sure of the facts here, but I wanted to consult the Assembled Wisdom before hanging too much of my plot on this.

 

[TheKillerBs]

Limestone is fairly heat resistant, as is marble. Not overly so, but I think you would need a very big, hot fire to damage the stone. I think you're right about the mortar though.

 

[CupofJoe]

I thought that it was a fairly standard siege technique. But that might also include at least so sapping under or at least to the base of the wall to get as close as possible and then use the weight of the wall itself to bring it down...
I have read that tunnels under walls were packed and propped with wood and then fired. As the props burned out, the wall would collapse.

 

[skip.knox]

There will be no sapping. My attackers don't know how (they are not human).

As for size of fire, picture a thirty-foot wall with a fire banked against it at least twenty feet high and a few hundred yards wide and kept burning for hours. I can keep it fed by magic, if need be.

 

[DragonOfTheAerie]

There will be no sapping. My attackers don't know how (they are not human).

As for size of fire, picture a thirty-foot wall with a fire banked against it at least twenty feet high and a few hundred yards wide and kept burning for hours. I can keep it fed by magic, if need be.

During the Great Fire of London, stones in buildings exploded and shot out of the walls due to the heat..but i have no idea if that means anything to you.

 

[TheKillerBs]

There will be no sapping. My attackers don't know how (they are not human).

As for size of fire, picture a thirty-foot wall with a fire banked against it at least twenty feet high and a few hundred yards wide and kept burning for hours. I can keep it fed by magic, if need be.

... Yes, that sounds like it should do a number to the wall.

 

[skip.knox]

I did not know that about the Great Fire. Should have; it's at least in the vicinity of my research field. What you describe, DragonEtc. is exactly what I wanted to envision.

 

[DragonOfTheAerie]

I did not know that about the Great Fire. Should have; it's at least in the vicinity of my research field. What you describe, DragonEtc. is exactly what I wanted to envision.

It may not actually be a historical fact, now that I think about it. It was in a historical fiction book I read. However, I doubt that the author would just make something like that up.

DragonEtc.? Hahahaha

 

[RedMetalHunter]

Rock and stone can usually withstand high heat. That's why it's used in fireplaces and chimneys. The mortar is often the weakest part of masonry work. Poorly mixed mortar could fail under high heat. As far as stone goes, they can explode if there is moisture in them when they are heated rapidly. The water vaporized and boom. Stone can also crack when it is heated and then rapidly cooled. This was used by some prehistoric people as a mining technique. They would light a bonfire, let it heat the stone, then douse it. The cracked stone was then easier to break.

 

[skip.knox]

@RedMetalHunter - Hannibal.

Maybe I'd better lay off the exploding stone, but I think this will still work. The wall in question has been attacked by clawed monsters. They've been hurling themselves at it (no siege equipment) for a couple of days, so the facing is extensively gouged from the ground to about fifteen feet up.

I should be more specific here. I'm talking about the Constantinian (not Theodosian) walls of Constantinople. The outside consists of limestone, in some places faced with marble, on the outer and interior, with mortared brick and rubble filling in between. The wall was, if memory serves, anywhere from six to twelve feet thick. It was three miles long.

My notion is that these attackers have been dying at the base of the wall for two days. Their bodies have piled up (think ants or locusts) to a height where new attackers are starting to come over the top of the wall. The defenders get the bright idea of setting fire to the bodies. They use magic fire, but they also use plain old oil and wood, which in their enthusiasm they pile deep.

The fire drives back the attackers, sure enough. And it burns for many hours, though less than twenty-four. My hypothesis is that the three assault points (so, three bonfires) have already been slightly weakened--mainly places where the heat can distribute inward. The limestone transmits the heat inward where it builds up over the hours. At some point, somewhere in there, something will collapse and a section of the wall falls in upon itself. It doesn't have to be a massive failure. It falls maybe ten or fifteen feet. Enough so there's a breach and, more seriously, enough so that the fire defense must be abandoned, leaving the city entirely at the mercy of the invaders.

Don't worry, the physics won't find their way into the story. I just want to know that the informed reader won't instantly scoff at the notion. I know where to go to get the history right, but I'm at a loss on the thermodynamics of stone!

 

[skip.knox]

OK, I may have an answer. Rain. It's a bit of nimbus ex machina, but not too awfully heavy handed. Besides, it lets me go all Chandler-esque.

Previous days have emphasized heat (it's August). But this is Constantinople, so I can have clouds build up. Torrential rain over the night. This dampens the fires, but the center one was started by magic and was fed magically, so it burns hotter, longer. The rain supplies the rapid cooling (it's a cloudburst) and gives me those satisfying exploding stones, around about dawn. This also lets me have the repair of the breach, plus the sharp battle inside the city (about a thousand monsters get through) take place in rain. Very satisfying.

Any objections from the physicists? You know who you are...

 

[Ireth]

I'm no physicist, but I do hope your defending army doesn't rely heavily on archers. Bowstrings are unreliable when wet. *side-eyes Peter Jackson's battle of Helm's Deep*

 

[RedMetalHunter]

I think that sounds great. Limestone is quite porous, if they were wet from rain and/or blood and heated quickly by magic - exploding rocks makes perfect sense to me.

Or the rapid cooling cloudburst, that makes sense too.

 

[skip.knox]

Nope. Although, until the morning of the breach, the weather was dry (though humid, what with the Black Sea and all). I do use archers there, and catapults. It was like swatting gnats, but if you swat enough of them, they will pile up. Once the storm breaks, though, I'll be sure to downplay or just plain not use bows.

 

[Legendary Sidekick]

They use magic fire

Right here… this is all that matters. The facts are on your side, blasting stone and burning bodies should work without magic. But the fact magic is involved means most readers (who read for enjoyment, not to scientifically challenge a fantasy story) shouldn't question whether the scene is possible. There's magic, so it isn't. Now stop asking questions and be immersed in the scene like a good reader.

And by the way, this does sound like an awesome scene.

this is Constantinople

Istanbul, not Constantinople. So if you've a date in Constantinople, she'll be waiting in Istanbul.

I had this song stuck in my head this morning. (The non-Speak-and-Spell version.)

 

[DMThaane]

I can't help with the main question (although I don't think I've ever heard of fire being applied during a siege to break down a wall my siege warfare is hardly perfect and the use of magic changes the rules) but I do know of a brick fort, Fort Zverev, that actually experienced some melting as a result of an intense fire, creating some rather haunting and extraordinary images. I also ran across the vitrified fort mystery which are a number of forts found in Europe that appear to have been deliberately semi-melted. Exactly why this was done seems to be under debate but the going theory I read is that it was done by capturers to render it less useful or occupants as some sort of ritual act when the fort was abandoned. Not sure how useful that is but they're the only examples that come to mind.

I may do some research into exploding stone as I'm using a sort of super napalm in a current WIP and this has raised my curiosity.

Picture of Fort Zverev:

 

[skip.knox]

>Istanbul, not Constantinople

The year is 376AD (1131 AUC), so it's Constantinople. And thanks for the ear worm!