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VonKatzen

Magical Fire and Accidental Arson

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Crosspost from TDM forums about magical fire and how crazy they could potentially get.

The devastating and deadly wildfires that have yet again hit California made me think about the fact that characters in fantasy games will often be in grasslands, wooden villages and forests. They're carrying around lamp oil, have flaming swords, etc. In a fantasy world where D&D or RuneQuest-style magic is commonly available it seems like there would be some pretty intense wildfires started by a handful of idiots fighting over gold dubloons and resorting to lava bolts in the struggle. As a GM I occasionally bring this up, but only now did I consider the scale to which an ultra-hot flame (of the sort used by sorcery to inflict instant and deadly wounds) would probably also turn an oak tree into so much lit kindling if the person it was shot at happened to be near one.

In one instance (this was OSRIC) my players got into a fight in a bar (with some local thugs) and the party wizard used a flaming hands spell and almost burned the bar down. They managed to get a room in the inn because the innkeeper was terrified of them, but they got little love in that town. Now just imagine if Tim the Enchanter goes a bit overboard when he's dispatching the Jabberwocky and sets the whole of the Ruritanian Royal Forest on fire? Not only would this devastate the people of the region and the ecosystem generally (arguablly, it would be good farmland afterward) but there would probably be some pretty serious law enforcement/wizard guild pressure bearing down on people who toss white-hot bolts around like arrows.

Mythras has rules for setting people on fire (it wouldn't be a fantasy game if it didn't!) but what about burning down buildings, forests? It seems to me that in order to do as much damage as a sword in a similar amount of time a magical flame would have to be pretty freakin' hot, which means whatever it heated up would probably be hot enough to get ordinary flammable materials going, the exception being if it was like a hot laser where it burnt so fast that it snuffed itself out by oxygen consumption.

I know that in D&D the fireball spell and Dragon's breath are hot enough to instantly liquify gold, which would turn a green forest into a campfire just as fast.

Edited by VonKatzen
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No, pouring on liquid gold won't turn a green forest on fire instantly.

Even if you manage to shock-dry a piece of green, living wood to catch flame, that flame will have to spend quite a lot of heat to dry the adjacent potential fuel, which then has to get evaporated, too, to feed the flame. Once the magical ignition is gone, the flame will propagate only as far as its heat can release fuel from the substrate, and that means to get rid of humidity first. Any evaporation will cool the flame, and without providing new fuel, the flame will quickly fade out.

Glowing embers are similar.

For a greater fire like currently in California, you need a prolonged period of drought that has pre-dried the available fuel. It is quite brown vegetation that is burning there, or if green, high on volatile organics that will promote fire and low on water content.

The presence of fire entities like elementals may change that equation, otherwise it needs magic or innate ability to kindle the area again and again.

In Gloranthan RQ, Oakfed is a greater spirit that can be summoned out of the bonfire and the well-aerated smithy. A fire like the one ravaging California is a full-blown Oakfed activity, an entity with blind hunger and a (random) will of its own.

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2 hours ago, VonKatzen said:

I know that in D&D the fireball spell and Dragon's breath are hot enough to instantly liquify gold, which would turn a green forest into a campfire just as fast.

2 hours ago, Joerg said:

No, pouring on liquid gold won't turn a green forest on fire instantly.

I read the OP as saying "Dragon's breath that is hot enough to liquefy gold instantly would turn a green forest into a campfire just as fast". I think that's a little more reasonable, since fire that is hot enough to liquify gold is going to have a ton of heat in it that is not being used to liquify the gold since the fire is unlikely to be focused entirely on and into the gold. Depends on how damp the "green forest" is.

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25 minutes ago, PhilHibbs said:

I read the OP as saying "Dragon's breath that is hot enough to liquefy gold instantly would turn a green forest into a campfire just as fast". I think that's a little more reasonable, since fire that is hot enough to liquify gold is going to have a ton of heat in it that is not being used to liquify the gold since the fire is unlikely to be focused entirely on and into the gold. Depends on how damp the "green forest" is.

I don't doubt the immediate area being burnt while magic increases the fire, but I was talking about expanding the fire once that initial energy has been spent and a fire has been started.

Heat transfer through an evaporating substance is quite tricky - I had a couple of dis-illusioning experiences with impact plasma of lasers on minerals. A well-fueled fire might dry out neighboring vegetation over time, but that creates a slow expansion of the fire.

Sooner or later, the best heat a fire produces is a reddish-glowing piece of coal, which can explode away and drop into yet unburnt areas. You can test how well a fire spreads under such circumstances by taking a piece of burning charcoal or a hand-held gas burner and sear away the grass between the paving of your courtyard. Good luck getting a self-propagating fire there even in dry conditions.

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Yes, in order to get to big proportions it would have to be a sustained flame or a really dry area - or a lot of it would probably disperse from water evaporation and so forth. However, starting a fire in an ancient/medieval city would probably be easier, since the timbers in buildings are already dry. Although some cities use quite a bit of stone the roofing is usually wood.

Edited by VonKatzen

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14 minutes ago, SDLeary said:

Or thatch. Fun for the medieval pyros!

Or in the case of a Roman tenement, dry wood with thatch roofing, walls filled with wood dust and soaked with the body oils of occupants and their meals. Combine that with the warm, dry conditions of the 1st c. BC-AD and you've got a lucrative money making scheme for your fire brigades.

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3 hours ago, VonKatzen said:

Or in the case of a Roman tenement, dry wood with thatch roofing, walls filled with wood dust and soaked with the body oils of occupants and their meals. Combine that with the warm, dry conditions of the 1st c. BC-AD and you've got a lucrative money making scheme for your fire brigades.

Yup! There is a reason a good chunk of Rome had to be rebuilt so often. It wasn't just Nero. 

And of course, we can also talk about London. Old cities love untended, indiscriminate, or unintended flames.

SDLeary

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As Joerg says, you need more than just a fire and fuel to make a wildfire. Wet or damp fuel stops the fire from spreading, as does still air.

Orlanthi weather-workers can control wildfire by causing rain or by controlling the wind, blowing the fire back on itself.

A full Oakfed Wildfire is exactly what is happening in California, including spawning elementals that jump between areas and spread the fire. 

Instead of heroic firefighters, Gloranthans have Ernaldans who throw earth on the fire, Orlanthi who blow the fire back or make it rain, Shamans with a big Extinguish spell and Lodrili who just say "Now, now, just stop that nonsense and come hide in this jar, will you".

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One caveat that occurs to me in the case of some Gloranthan dragons, is that they are (in themselves) of sufficient scale to have a single breath be a multi-mile-wide disaster.

Furthermore, they are so cosmically-potent that I'd expect the fires ignited would spread however the Dragon wanted them to spread, regardless of the wetness or dryness of the wood/grass/whatever...

Very few settings have creatures on quite that scale, of course!

I'd expect the worst PC-driven accidental fires to be in towns.  Wooden buildings, cracks sealed with pitch and tar, ancient and very-dry wood, close-set buildings, etc.  As VonKatzen notes, some cities have lots of stonework, which obviously inhibits fires; but I'd expect most of the poorer quarters to be made as cheaply as possible (and almost everywhere, it's far cheaper to build with wood than stone).

I'll see the OP's "Ruritanian Royal Forest" and raise him "the Ruritanian Capitol" ...

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On 11/27/2018 at 4:40 PM, g33k said:

Very few settings have creatures on quite that scale, of course!

Glorantha dragons, Wakboth, the Crimson Bat and some others I am forgetting pretty well dwarf the monsters in most RPG settings.

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