I think it's definitely right that a finesse roll is needed - there's some room for debate on what its level should be, but flour is an artifical product, and therefore needs a finesse roll by page 77 of the main book.
If I was the magus designing the spell I think I'd try to define the geometry at that stage rather than leaving it to finesse at casting. X number of tetrahedrons made of flammable plant material of side Y in a face centered cubic arrangement of unit side Z. Platonic forms are easy for creo and you don't want to be able to vary Y and Z at the time of casting so if there is a finesse roll required it will be low.
If you are trying to improvise a dust explosion then yes, high finesse is called for, and probably a check on natural philosophy or milling or baking (or mining if you are a terram specialist using coal dust) as well.
Yeah, i´m not really sure which would be correct. Canon spells/guidelines end up on both sides.
In this case it would be a matter of there always existing X individuals of flour in the air, if it´s exploded it´s gone so the spell compensates by remaking it. If you throw barrels of water over a big CrIg fire, will you be able to put the fire out before the Duration of the spell creating it is over? I would most certainly say "NOT a CHANCE!".
So i´m actually not at all sure how to handle a situation like this.
Nope. :mrgreen:
It´s because except under certain circumstances, the cigarette isn´t hot enough to cause ignition. Lack of oxygen for the reason you guessed can contribute however.
Outside of optimal you might still have something that burns more or less, but you´re unlikely to get anything resembling an explosion.
I probably would.
Not so sure about that.
Mechanically, the fact that an explosion is big (area of effect) does not mean it has an impressive damage.
What does hurt more IRL? Being struck a sword blow, or being near a flour dust explosion? I don't know, but I don't think the "damage" is very different, and, given the chance, I'd avoid the sword blow.
=> I'd say probably more something like "Does +10 damage to all in x area, which must also make a Str roll of difficulty 9 to avoid being blown back, 12 to avoid falling over". It might also destroy/damage buildings, as per the rules in LoM
If you want a high ignition method it might be better to soak people in auto-replicating lamp oil (diameter duration) and set them on fire with the spark. Higher level, that is sure, though.
I also think that +30 is really high. being trespassed from side to side by a foot-wide javelin of stone only causes +10 damage. +30 is equivalent to being hit by a LAVA flow. I think that the extra damage in a dust explosion is because of the flying shrapnel that it causes when it blows stuff from the things around it in a confined space, not because the explosion is specially powerful in an open environment. As a related note, people did not fear (much) canon balls in the napoleonic naval battles, they feared the wood shrapnel the canon ball created.
Cheers,
Xavi
Hardly. Your suggestion might work very well for persons that are some distance AWAY from the explosion, but if you´re inside a dust explosion this big, you´re either extremely lucky or you die, end of story.
The fire part would probably be enough for +5-10 damage by itself, but it´s the shockwave that kills.
Standing inside a large dust explosion would be something like having several hand grenades explode at maybe 2m distance in a cluster around you. Hit by shrapnel or not wont make much difference, the combined shockwave is deadly most of the time.
Remember, a small dust explosion is enough to make a silo or a mill to become 100% wrecks, and this spell would use over 100 times more material, even at the low end. 1/10th of a barrel of flour is enough to destroy a normal wood building if it explodes inside.
Only reason this was never historically used for anything is because it´s so hard to cause the explosion when you want it, it´s highly unpredictable and even an error in the spread so minor you cant see it can make it into a slow burning fireball or make it not catch fire at all. But when it explodes optimally, it´s no different from any slow(like cordite) to medium(black powder) type explosives. Only thing you wont get from it is something like really fast explosives, like Pentyl or similar(8400m/s).
I am not much into pyrotechnics, but isn't it ^RECISELY because it is an enclosed space that you can have an explosion? IIRC if you have a bunch of blackpowder and ignite it, it just burns fast, while if you do the same with the powder in a contained space it goes BOOM. It might be that the same happens with an enclosed space like a mill or silo?
Xavi
Confining burning gunpowder gives you explosion in two ways.
- it confines the heat and gas of the burning gunpowder so the temperature + pressure increases which increases the rate at which the gunpowder burns which means it burns hotter in a positive feedback loop - this is the main reason. This is also why a straw fuse filled with gunpowder is faster than the same amount of gunpowder laid out in a line.*
- The heat and pressure is not felt outside the container until it bursts releasing them all at once giving a nice sharp shockwave.
Given the spread out nature of a fuel-air explosion factor 1. can't really come into play because by the time the heat reaches the confinement most of the burning has already happened. A fuel-air blast gives a much longer pulse of overpressure which does not shatter resistant materials but pushes over walls and fills space. This makes it useful against cave complexes because the energy follows the tunnels rather than being wasted making gravel. (a bit like an AD&D 2E fireball 
)
- This is also why, if you are cooking your dinner over a bit of burning C4, you should not stamp on the fire to put it out
I will say I do like this argument. I am not a huge fan of the idea of a flour explosion, it seemed cumbersome the last time it was discussed, but I absolutely LOVE it's use in the fiction I have read and I can see it being a story "moment" where a dice roll would hinge the entire event. A botch would be insane and imagine a exploding series of "1's"!
Good stuff
I'm surprised nobody has brought up MR.
A created momentary flame needs to get by MR. But what about when a created momentary combustible cloud ignites and explodes from a mundane source? MR? No MR?
If it's created ritually (using a ritual, requiring Vis and time, minimum level 20) it is real flour/dust - no MR.
If created non-ritually, it is resisted.
I think that because the flour is created (creo) then MR applies to all of it.
If it's created ritually (using a ritual, requiring Vis and time, minimum level 20) it is real flour/dust - no MR.
If created non-ritually, it is resisted.
Bad guys just need to hang out for an hour then!
Whereas my weather specialist will dump flour CrAu some wind and then ignite it.
You could argue that the shockwave is actually secondary and bypass MR while the fire part do hit the MR.
I wouldn´t allow that but it makes dangerously much sense.
The answer to that is pretty much yes AND no. You get a shorter, sharper and nastier explosion if it´s contained.
But as Noliar also mentioned, a dust explosion is more like a FAE bomb, a Fuel Air Explosive, which i might add is a very nasty thing simply because it creates very, VERY BIG booms.
You are essentially making a medieval thermobaric weapon.
en.wikipedia.org/wiki/Thermobaric_weapon
A thermobaric weapon, which includes the type known as a "fuel-air bomb", is an explosive weapon that produces a blast wave of a significantly longer duration than those produced by condensed explosives. This is useful in military applications where its longer duration increases the numbers of casualties and causes more damage to structures.
en.wikipedia.org/wiki/Dust_explosion
Mount Mulligan mine disaster in Australia 1921. These cable drums were blown 50 feet (15 m) from their foundations following a coal dust explosion.
Above is 1.5m high STEEL cable drums. A few hundred kg, and they were blown 15m...
Anyway, i dont think you´re thinking the same scale when comparing gunpowder burning with a dust explosion.
Try igniting a barrel worth of gunpowder instead. Or several. Rremember, blackpowder is only 20-30% "fuel", the rest is oxidizer, a flour explosion is 100% fuel, using the air for oxygen. In short, you have to use ~4 times as much gunpowder as flour(or coaldust or sawdust etc) before you can start talking about "same scale".
With a blackpowder bomb, the combustion all happens inside whatever container is used because if it´s spread out outside it´s much less effective. However, a FAE works very differently as it relies on oxygen in the air to burn and a perfect spread to maximize burnspeed(that as Noliar said works the opposite for blackpowder).
The shockwave is much longer, but it´s also double since it creates a vacuum as a secondary effect.
All in all, FAE, which a dust explosion effectively is, is a very effective and powerful weapon. Seriously dangerous.
Hmm, good question. I would treat it exactly as: if you have a mundane fire fueled by a magically created log, is the fire blocked by magic resistance? This itself is quite unclear. Phylosophically, I would be inclined to say no: the fire feeds on a magical source, but the fire itself stays mundane. In terms of game aesthetics (more than balance), I would be inclined to say yes, the fire is blocked by magic resistance: otherwise the premier fire-wielding school would be one specializing on the creation and control of fuel, which ... feels wrong.
Can a magus (parma up) keep herself warm by a fire burning log made with CrHe?
Can she smell the smoke?
I would say yes to both and so both fire and pressure damage bypass MR. Other people disagree. A case can be made for either the fire or the pressure bypssing MR without the other or for the explosion being magical and thus resisted within the original volume of the dust but being natural beyond it. Deoxygenation, partial vacuum implosions, scenery set on fire and collapsing buildings are all definitely secondary and likely lethal though. And it can be done with a 5th magnitude spell so getting decent penetration is not that difficult.
An Aegis of the Hearth is a pretty limited protection too unless you go with the most magical interpretation of the effect because that 5th magnitude version can go into multi-tonne yields and flatten everything above ground from outside the boundary. If you spend one magnitude to up the range to AC (highly recommended!) that's another four that can go on yield while still being a formulaic spell - 10-100 KT.
Many ST will look at this and be tempted to ban dust explosions or add a fudge factor that makes them practically impossible. BUT this is not the only way to make a spell of mass destruction, it may not even be the easiest. SMDs are only the most obvious of ways that the ability to invent your own spells and a little lateral thinking can disrupt the setting but it's also something you don't want to squelch because playing with the spell creation rules and seeing just what you can do is one of the best things about the game.
It's a bit of a dilemma. Probably the best way to deal with it is by social contract "that spell does seem possible by my reading of the rules and the metaphysics of the setting but it is literally a game changer so I agree it is inventable by your character if you agree not to use it until the whole troupe is ready for the saga to go in a world changing, possibly apocalyptic direction."
I imagine there is something similar IC. Many magi are are horribly aware that it wouldn't be that difficult to invent an SMD. Quaesitors and Trianomans know that they don't know how many mages can already cast such spells but every improvement in magic theory and natural philosophy increases the percentage who could invent something nasty. The knowledge sharing of the OOH accelerates and spreads the risk but it is also one of the main social binding forces of the Order and the Order provides the social context in which they haven't been used already. Sleepless nights all round.
A 5th magnitude spell penetrating an Aegis, it's not impossible, but it starts at a minimum CS of 45 (25+ 20 for the minimum Aegis) and goes up from there. So, that 30th level Aegis, which is pretty common means a CS of 55. Again, it's not impossible, but there are huge implications, socially, within the Order when magi start bombing covenants. That is so ironic, based on what I just kicked off in Bibracte. If a character wants to do it, it may not be game changing, he may end up having his character turned over to the Quaesitores on the first use of such a spell, kicked out the covenant and the player being forced to create a new character.
Lots of ways to handle this...
I always love this sort of discussion about wierd effects...
Adding my opinion now:
I have regularly set off small dust explosions (custard powder, finer than flour. In single teaspoon quantities) within large metal coffee tins. This makes a nice 'bang' and the lid flies off, often hitting the ceiling. If you do the same thing in the open, you get a pretty fireball that does no damage but makes onlookers go "ooh, that's nice".
(These experiments are standard ones for British science teachers to demonstrate how explosions work and to illustrate how food can be a fuel.)
If you scale them up, I think that an uncontained flour explosion will do +5 fire damage to those inside the explosion. Very little else. Average people will be burnt, but it is unlikely to be fatal.
Now, put that same explosion within a container and the container will be destroyed - houses, large buildings, boxes, etc. The shockwave of the blast being contained will probably kill anyone inside (perhaps the original +30 damage?). You could also make (large) grenades by exploding something that will splinter into schrapnel and hurt nearby people. Making the created amount of flour bigger would not enable it to do more fire damage - it's still just flour burning - but could blow apart a bigger structure or affect a bigger army.
Limitations: if you don't get the dust and air to mix well, nothing happens. +1 magnitude or a basic finesse roll to get it right, in my opinion (it's fairly easy after a bit of practice even for a klutz like me). I'd also require almost perfect timing to get the fire in at the right time. Better to add an Ignem requisite which would also avoid the 'how momentary is momentary' problem. In which case, you're probably better off simply making a CrIg spell!
As far as MR is concerned, I'd rule that the actual explosion is resisted, along with the original shockwave since, in my opinion, both of those are 'part of the spell effect' but any schrapnel equivalents are not resistable.