I have a question about the intensity of heat. The guidelines state that it is more difficult to destroy the heat of fire, the greater the intensity is. I feel though, that it is not clear whether or not you can partially destroy/reduce the intensity of heat/fire?
Given a source of heat/fire that does +10 damage to it's surroundings; will a perdo ignem level 15 that destroys fire with intensity up to +5 damage do anything to this fire? Will the flames be reduced to doing +5 damage until extinguished, or will nothing happen since the fire's intensity is greater than the spell?
One last thing; does destroying fire in this manner behave differently if the fire is magical? If you want to fast cast this level 15 perdo ignem spell on a pilum of fire targeted on you.. will it need to penetrate anything? will you reduce the damage of the pilum of fire by -5 damage, or will the perdo ignem spell not work since it's not designed to destroy fire up to +15 damage?
These are nice questions. My answer to your exact question would be: a run-of-the-mill PeIg spell designed to destroy fire with intensity up to +5 damage would not affect larger fires. However, a different PeIg spell that is designed to reduce the size of fires - including, of course, a possible spontaneously cast spell - would affect fires of any size, even magical fires.
For example, the guidelines for PeIg spells (ArM5 page 142) include "Level 3: Reduce the size of a fire without destroying it completely." I think a reasonable interpretation is that the damage done by the fire is reduced by 5. So a level 10 spell (base 3, Duration Momentary, Range Voice, Target Individual) could reduce the damage of a Pilum of Fire from +15 to +10. But note that a fast-cast level 10 ReIg spell would completely deflect a level 20 Pilum of Fire, as per the fast casting guidelines on page 83. (Is ReIg the right combination of Techniques, or would a PeIg also work? ReVi? Fast-casting rules always confuse me.)
The level 4 guideline "Chill an object" also seems to be able to diminish fires in a similar way. Note part of the description of the spell Conjuration of the Indubitable Cold: "House fires become as small as campfires, bonfires become as small as torch fires, and campfires and smaller fires go out." In general, I would say that PeIg spells can be designed to reduce a fire's damage by 5 at base level 3, and every additional magnitude added to the spell can reduce the damage by an additional 5. One would have to rule at what point such fires are actually extinguished rather than diminished.
Note also, according to the heat and corrosion injuries rules on page 181, that the amount of damage done by a mundane fire is a function of how much of the target is within the fire, rather than the size of the fire itself (this doesn't seem to be the case for magical fires). This makes sense, I guess: sticking my hand in a fire isn't likely to kill me no matter how huge the fire is. Therefore it might be the case that PeIg spells designed to diminish fires don't really reduce such fires' damage at all, but only their size.
A-ha, yes. Your reasoning sounds spot on, I had not read the heat and corrosion rules on page 181. With that thinking, a Pilum of Fire spell is constructed of a fire with intensity of +5 damage, but large enough to cover a limb so it's damage is multiplied by 3. Reducing the size of a fire will according to the heat and corrosion rules always reduce the fire's damage.
Then I'm thinking about a modified version of Conjuration of the Indubitable Cold. A Perdo Ignem level 25 (base 4, Duration Momentary, Range Voice, Target Individual, intensity +1, size +2), this spell should then be able to affect fires of a very large size and also high intensity. It should cover fires doing up to +60 damage (base intensity +15 and quadrupled for covering an entire persons body). It should rather easily extinguish a Ball of Abysmal Flame, which I would gather is a fire doing base damage +10 and tripled because it covers half a persons body.
I think this spell could have a lot of applications; extinguishing fires in your home, a nice defense vs ignem magic, and a nice defense against a dragons breath?
So personally, I find a bit of dissonance in the ArM5 rules on fire damage.
On the one hand, the heat-and-corrosion rules seem to indicate that (mundane) fires are all essentially the same intensity and that the damage modifier is just a function of how much of your body the fire covers.
On the other hand, the Creo Ignem rules seem to indicate that (Hermetic) fires differ in intensity, in that higher-level spells produce fire that does more damage. I looked through the spell descriptions and it didn't seem to me that they were agreeing with the body-coverage paradigm. For example, Coat of Flame certainly covers the whole body but does only +5 damage. Ball of Abysmal Flame's +30 damage doesn't fit into the body-coverage paradigm at all.
I'm content to accept that the game mechanics are different for mundane fires and magical fires. But I would be wary of combining the two mechanics, by postulating for example a fire that did +60 damage because it was a base +15 intensity and covered the whole body for a quadrupling of the damage bonus.
My thinking would be to rule that your level 25 version of Conjuration of the Indubitable Cold (which is 4 magnitudes higher than a base 3 Momentary/Voice/Individual spell would have to be) would either, depending on what kind of fire it was cast upon:
reduce a mundane fire in size by 5 factors of ten (so would probably extinguish any mundane fire of size up to a burning mansion); or
reduce the damage modifier of magical fire by 25.
I'm definitely interested in hearing other people's interpretations.
The heat-and-corrosion rules states that spell damage already has the size/exposure multiplier built-in. Ball of Abysmal Flame would be fire with a base damage +5, one magnitude up for base +10 and given it's size covering half a persons body you get a final +30 damage.
The rules do indeed say that, but when I look at the CrIg spell descriptions, I don't believe that this rule has been applied consistently. Coat of Flame, with its mere +5 damage, is the most obvious example.
Still, if you've already gotten it straight for yourself, don't let me derail you!