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[Xvareon]

There appears to be a very interesting glitch with items made of lignite/coal/graphite.  Build a mass of floor grates in an area, set up a magma floodgate then trigger it. The grates will catch fire immediately, incinerating anyone on them.  Now here's the fun part; because the materials used to make said grates have an ignition point but a melting point higher than that, they will *NEVER* stop burning. (At least I think that's the cause... look it up on the DF wiki to be sure.)

I have a theory of how to use this for defense, but I need someone to test it to be sure it works.

Make a sideways drawbridge with a gap running perpendicular to it, link it to a lever and raise it.  Fill the space the bridge used to occupy with lignite floor grates, then light them on fire.  Nothing except fireproof enemies will get through that alive, and because the AI currently doesn't understand being on fire very well, they will happily charge right into the blaze.  What I need tested is if the grates will remain flaming after the bridge covers them, or remain in place after the bridge drops.

Note; the problem with this is that floor grates do not support each other, and so cannot be used to make 3x3 walkways.  The best you can do is a 2x2 hallway, and even then it'll be a bit iffy.  To get around this you could simply remove the gap and let them sit on the floor, or have a few sections of floor underneath for support.

[ext0l]

Make a sideways drawbridge with a gap running perpendicular to it, link it to a lever and raise it.  Fill the space the bridge used to occupy with lignite floor grates, then light them on fire.  Nothing except fireproof enemies will get through that alive, and because the AI currently doesn't understand being on fire very well, they will happily charge right into the blaze.  What I need tested is if the grates will remain flaming after the bridge covers them, or remain in place after the bridge drops.

Sadly, you can't build underneath a bridge.

You don't need to build grates over a channel, so you can have as large a hallway as you like. Then flood it with magma to start the fire. Then the only way to remove the fire is if you wash it with water. The setup isn't too challenging to build.

[PopeRichardCorey]

You could use the Burning Hallway as an "alternate" entrance to the fort; a standard trap hallway, really.  But if you can block it off while already burning in such a way that when you open it up  onto a flammable spot, you could create a GrassFire cannon without needing Magma perpetually at the ready...

(The fact that lignite burns forever is an ancient bit of knowledge; it's been used to crash framerates nearly empty oceans.  I'm not sure if it's been used in Floorgrate traps, though!)

[Girlinhat]

What happens to a flaming lignite hatch/floor gate that's opened?  Does it stop burning?  If so, you can place a burning lignite floor hatch somewhere, and then make a hallway of similar.  The burning one is NOT on a lever, but the desired burning ones ARE.  Result: Pull the lever and the hatches extinguish as they open (and effectively disappear).  Pull it again so they close, and catch fire from the one burning one within range.  Thus, toggling burning hallway!

[Psychobones]

I seem to recall that the items will eventually be destroyed due to taking wear, unless the item is an artifact.

[PopeRichardCorey]

I seem to recall that the items will eventually be destroyed due to taking wear, unless the item is an artifact.

Oooh yeah; it was Artifact Lignite Bins which were needed to evaporate half the sea.

There's also another obstacle...  I have yet to see any Lignite.

[Xvareon]

I love how this game allows us to break the laws of physics over our collective knees.

I usually see both; wherever there is coal, there will be lignite.  But yeah, items taking wear from fire damage sounds about right...  still, this could be an interesting defensive option.

[Urist Da Vinci]

I seem to recall that the items will eventually be destroyed due to taking wear, unless the item is an artifact.

Oooh yeah; it was Artifact Lignite Bins which were needed to evaporate half the sea.

There's also another obstacle...  I have yet to see any Lignite.

It is a normal iron or steel bin containing bars/blocks of lignite or coal.

What happens to a flaming lignite hatch/floor gate that's opened?  Does it stop burning?

I have set many wooden doors on fire in adventure mode. They still open and close, until the wear destroys them. The opening or closing didn't put out the fire.

Non-magma-safe raising drawbridges can be destroyed by magma that touches the area that they would occupy when open, for similar reasons. The door is "still there".

[Flaede]

It may be still there. does it's heat from being on fire still transfer to things around it?

[Sphalerite]

Now here's the fun part; because the materials used to make said grates have an ignition point but a melting point higher than that, they will *NEVER* stop burning.

Not actually true.  Burning objects suffer wear and eventually are destroyed.  The melting point has nothing to do with it.  Only artifacts burn forever, since they are immune to wear.

[Quietust]

Now here's the fun part; because the materials used to make said grates have an ignition point but a melting point higher than that, they will *NEVER* stop burning.

Not actually true.  Burning objects suffer wear and eventually are destroyed.  The melting point has nothing to do with it.

This. Burning items without a HEATDAM_POINT (or with one above the usual temperature of fire) will take about 9-10 months of in-game time to wear away into nothing. I've tested it with boulders, bars, and crafts, and they all took approximately the same duration to burn away.

[Nameless Archon]

Now here's the fun part; because the materials used to make said grates have an ignition point but a melting point higher than that, they will *NEVER* stop burning.

Not actually true.  Burning objects suffer wear and eventually are destroyed.  The melting point has nothing to do with it.

This. Burning items without a HEATDAM_POINT (or with one above the usual temperature of fire) will take about 9-10 months of in-game time to wear away into nothing. I've tested it with boulders, bars, and crafts, and they all took approximately the same duration to burn away.

One could actually work around this propensity to wear to nothing, if one built a couple of "strips" of burning floor grate that could be independently replaced. When strip #1 burns through, close off that tunnel to invaders, replace the floor grates and reignite. Meanwhile, open tunnel containing strip #2, and ignite it. Invaders will come through the open tunnel.

Given the relative death of materials in 31.19, I might consider investigating wood grates as a suitable replacement. Wood can be farmed, can be grown underground where it's safe, and can be made into grates just like lignite.

[Quietust]

Given the relative death of materials in 31.19, I might consider investigating wood grates as a suitable replacement. Wood can be farmed, can be grown underground where it's safe, and can be made into grates just like lignite.

There's just one problem - wood has an IGNITE_POINT of 10508 but a HEATDAM_POINT of only 10250, which is why wooden goods only last a few days (or, if exposed to magma, a few minutes) before burning up.

[Sphalerite]

The obvious solution is to deliberately produce flammable artifact grates.

Artifact materials and types can be influenced to some degree.  Dwarves pick moods based on their highest skill, and when picking materials usually pick those closest to the workshop.  Train every dwarf in your fortress in masonry, by churning out hundreds of blocks or some similar method.  This should get you plenty of Masonry moods.  Surround every masonry workshop in your fortress with a stockpile that only takes graphite, lignite, or bituminous coal.  Make sure those stones are disallowed for normal masonry use.  When you get a masonry mood, they should take the flammable stone first, being closest to the shop.  Only a few of those moods will turn out artifact grates, but other flammable furniture can be useful too.

[PopeRichardCorey]

One could actually work around this propensity to wear to nothing, if one built a couple of "strips" of burning floor grate that could be independently replaced. When strip #1 burns through, close off that tunnel to invaders, replace the floor grates and reignite. Meanwhile, open tunnel containing strip #2, and ignite it. Invaders will come through the open tunnel.

Given the relative death of materials in 31.19, I might consider investigating wood grates as a suitable replacement. Wood can be farmed, can be grown underground where it's safe, and can be made into grates just like lignite.

A needlessly complicated, limited use weapon which has to be reset every 9 months?  THAT is true dwarfiness.