40d:Fire

Fire is not an individual item, but a condition that affects creatures and items. An item or creature is either on fire, or not. Adjacent items can catch fire if they are flammable - wood and cloth are examples of flammable items, stone and gems are not. Some items are especially flammable, such as coal or alcohol.

Fire is exceptionally dangerous, more so than most creatures, in part because dwarves will not avoid fire nor will they react to it in any way, and so will happily walk into flames, catch fire and then spread it without a word of complaint. No announcement will be generated until a dwarf dies ("Urist McHotstuff has died in the heat"), which might be the first notice you get of the problem.

(click to enlarge)
!!Dwarves!! on fire (flashing red/yellow), and trailing smoke.

A border of exclamation marks on an !!item!! or !!creature!! indicates that it is on fire. Items on fire also release smoke and can ignite adjacent items. Most burning items will eventually vanish, being completely consumed by the fire, but some take extremely long to do so.

In the units list, the name of burning dwarves or creatures will flash, alternating quickly between yellow and red, similar to the graphic, right. (And may do so forever after the dwarf dies.)

The biggest danger is that fire can easily spread, setting other dwarves (who don't care) and items alight. Once something is on fire, either an object or a creature, there is very little you can do about it except quickly isolate them like some plague carrier, and wait for the inevitable. ^[Verify]

A typical example

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While you're not watching (of course), a furnace operator gets hit by fireball from a fire imp while working at the magma smelter.
The furnace operator then runs around on his business, smelting coke, and sets fire to both the charcoal stockpile, and a bituminous coal vein.
As he does this he takes burning damage.
Now wounded, he heads for the barracks for a rest.
The dwarf dies on the way, and his (still burning) clothes fall to the side.
(Somehow, let's say you miss the announcement of his death - the game doesn't pause or re-center, so it wouldn't be the first time.)
A peasant comes and picks up his !!giant cave spider silk shoe!! and puts it on. His clothing catches on fire.
He goes to the surface and starts a forest fire, which distracts you for some time. You think the forest fire caught him on fire, not the other way around.
Meanwhile, your legendary armorer grabs some !!charcoal!! for a new steel project. Soon, he's a !!legendary armorer!! (but not for long...)
A hauler grabs some armor in the forge where the charcoal is burning, catches fire, and sets fire to the leather armor items in the armor stockpile, and/or the rows of wooden bins there.
A miner finds the burning coal vein, and becomes a !!miner!!.
One of these dwarves soon feels hurt also, and heads for the barracks to rest, getting on a wooden bed and lighting that up.
Another goes and has a drink, setting fire to the highly flammable booze stockpiles - kaboom.
Nearby beds and food stockpiles light up; dwarves sleep on those beds and the others near it and get lit up themselves, as well as the dwarves going for a drink or meal, and for the !!cave spider silk robe!!.
The announcements about all the dwarves that have "died in the heat" start pouring in - you don't miss these.
You find the burning coal vein and miner, and blame it on him, wondering when and why he went outside into the forest fire.
Now a fair portion of the fortress is on fire, but you can't do anything about it, and can't see what's happening very well for all the smoke.
To save what dwarves you can, you <A>ctivate them, and so some and go to the burning armor bins to get !!armored!! up.
Those few who do manage to get out unlit have all lost several friends and loved ones, and a tantrum spiral is now unavoidable. Those few soon go insane and throw themselves into the river in a final ironic act.

Wasn't that fun?

Forest fires

Origins

Creatures such as Fire imps can cause a forest fire by igniting a creature with a fire attack. If grass also ignites, the fire will slowly spread to all connected grass, generating smoke as it burns. Fire turns grass into ashes, but not usable ashes. Anything that is flammable and stands on burning grass (if you look at it you will see "A fire") has a chance to catch fire each turn.

Preventing forest fires

It is recommended to guard the surroundings of an exterior magma pool with Marksdwarves so that creatures don't have a chance to light anything up (except the Marksdwarves themselves, but this is unlikely if they are well trained). Having walls or better yet fortifications all around the pools is likely to prevent all possible forest fires. Strangely grizzly bears seem to be ineffective against such a threat.

Alternatively, pour water on top of the pool to turn it to obsidian, sealing it. (But what a waste of magma!)

Protecting from forest fires

Since dwarves will not pay attention to fires , and dwarves will rush to move their dead friends to the graveyard, you may end up very quickly with heavy casualties if you do not order all dwarves to stay inside. If you have precious items and buildings outside, it is also possible to dig a channel to stop the fire from spreading any more (they are cheaper then walls, offer the same protection and, most importantly, are much faster to create).

Know however that merchants will not pay anymore attention to the fire than your own dwarves, so you may also want to protect them in the same fashion, even if it should change their path, otherwise they could all happily walk into flames when leaving the region (or worse, stand for days on "A fire").

Irregularities

Forest fires present certain irregularities that you should take in consideration ^{v0.28.181.40d}:

Trees, plants and other map tiles will not be destroyed in the fire. That makes them very easy to spot after a forest fire, and also a possible (if unlikely) natural defense. Items, however, may burn.
Grassy slopes will not catch on fire, thus preventing the fire from spreading across z-levels.
Forest fires, no matter how big, will never cross a channel or a non-wooden road. However, they will go through closed doors. ^[Verify]
In you start a fire in adventure mode, it will not spread if temperature is turned off.

Use as a weapon

This is not recommended, although some success has been reported with the Booze Bomb trap (which consists of a fire imp in a cage linked to a lever, standing near an alcohol stockpile so that it can be released when the enemy get near). It should be noted that this is a criminal waste of good liquor (unless you use crap like Prickle Berry Wine, Sewer Brew or Gutter Cruor..).

Adventure Mode

In adventurer mode, it's possible to ignite trees and shrubs with g. The possibilities for killing wolves with the burning corpses of other wolves should not be overlooked. (It should be noted that the Travel function puts out any inconvenient fires on yourself and your collection of "!!wolf head!!"s, which will probably end up as "Xwolf headX" after your trip). Since steel items do not burn, it is possible to kill friendly guards by setting the right location on fire, and when the hapless speardwarf perishes, heading back and taking his spear to use as a throwing weapon.