To further complicate this, I note that there is such a thing as invisible fire that emits little if any light; the sphere of fire is made of it. That is pure fire, though, not the usual terrestrial fire.
Normal fire is released from the compound it is part of, and it is visible and generally gives off light. When you set a fire, you are releasing that fire from the matter, which has normal qualities. If you set fires with an invisible flame, it's mysterious; fires are starting for no obvious reason, but the fires can be seen.
Fire is not a spreading plague, it's an element in matter.
Which is ... ? I lack your encyclopedical knowledge, callen, and I am far too lazy to look it up
Then again, there is a Perdo Ignem (so, the actual Arts from the OP) spell in the corebook, Soothe the Raging Flames, that:
Eradicates the heat of a bonfire, which, however, continues to burn until the fuel already covered in flame is consumed. The flames do not spread, or harm anything beyond what they were already burning.
This muddles things a bit, in the sense that it seems that the affected fire will treat the fuel it's already burning differently from new fuel.
Not really. This even somewhat works from a modern perspective. It's an exothermic process, so you don't have to keep supplying energy for the original fuel to burn. But you do need to ignite the new fuel, which takes drying it and warming it. Drying the fuel is an endothermic process, as is warming it. Now, to a certain extent applying "endothermic" and "exothermic" are ridiculous, as we should really look at a molecular level and say that each molecule must be heated for it to combust so a fire that can't heat anything can no longer consume its current fuel, either. But you can see how modern and microscopic we're getting. So this really doesn't seem problematic for Ars Magica.
