The thread about "Hermetic Flight by Explosion" is tangentially related to a wide-ranging discussion about explosions I have often thought of starting.
First of all, do explosions exist in Ars Magica (with its arguably different physics)? Probably. Flour mills very occasionally did explode in the middle ages, though the idea provided in LoME p.135 that any spark in a mill will just trigger an explosion is preposterous - it takes exactly the right conditions for flour to explode rather than simply start a quick flash fire (that can be almost as dangerous as an explosion). In fact, I would strongly suggest errata-ing the (IMHO) poor mechanics of that section.
If explosions do exist, what are explosion "physics" in Ars Magica? This has implications both magical and mundane. We know today that explosions are, in a nutshell, A) very quick "burnups" that create a sudden increase in the local air's temperature - which in turn suddenly increases the air's pressure, making it rapidly expand with concussive force - and/or B) chemical reactions that change something (typically a solid or liquid) into something (typically a gas) with a much higher "natural" volume that it rapidly expands into. But in Ars Magica? If that still holds true, Ignem is certainly an Art that can create explosions "indirectly" (by rapidly heating air), though the case could be made about the need an Auram requisite and/or a lot of Finesse.
Subtly related: how about the sound of explosions? We know today that it's caused by air expanding faster than the sound barrier and thus creating a sonic boom. But in Ars Magica? Is sonic boom even a thing? Maybe the "crack" of a whip (you make the tip of a whip travel faster than sound and thus create a small sonic boom) has a different explanation? Does really fast magical travel create sonic booms?
Understanding explosions in Ars Magica helps understanding lightning too. Note that lightning generates a localized explosion by heating the air it passes through; thunder is the associated sonic boom. But you can create a thunderclap without a shockwave (Jupiter's Resounding Blow) and a lightning paired with a shockwave without a significant thunderclap (Incantation of Lightning is likely to knock down stuff close by, but apparently it is not deafening; then again, it's highly unnatural lightning, so maybe it's ok if it's silent?).
Last but not least ... black powder and mundane explosives! If explosions follow the same physics as in our world, black powder is possible. In the real world, it arrives in Europe, probably brought by the mongols, just a few decades after 1220 - so at a time when many sagas will be active. It seems it might have been used in battle by arab forces as early as 1260, and certainly in 1267 Roger Bacon writes about it (though not about firearms) in his Opus Majus. Surprisingly Lords of Men - that does look at weapons appearing even later - spends not a word about it. Is mundane black powder a thing in Ars Magica? And if so, what are the "mechanics" for its creation? One could imagine using the Natural Philosophy rules for alchemical reagents from A&A, but they are not very well-suited: in practice creating black powder - and quite a lot of it - takes significantly less than a season of work once you have the right formula. Of course, maybe black powder is not mundane in Ars Magica, but a faerie substance (the mongols make good faeries, after all) and producing it is faerie wizardry!
If "mundane" black powder is a thing, how about magically created black powder? It seems it would take pretty low levels of Terram magic to create vast explosive potential; this may unbalance the game and devalue Ignem further. Though perhaps saying black powder is an "unnatural" (i.e. solid and quiescent) form of Ignem would help - not the least because mundane fire using magical fuel is not magically resisted - the same should then hold for explosions! One could also say that it would take excessive finesse to magically create black powder, but in practice it's not that hard a process compared to many others that craft magic can easily pull off. In fact, on a related note, it should not be too hard (once one gets the physics right through e.g. alchemical research) for Ars Magica to create other combustible substances at just the right density for an explosion, contrary to what ToME p.135 says (did I mention how little I like those mechanics?)
So folks, what do you think? Has anyone seen a saga progress into the second half of the 13th century (I would expect quite a few to have done so)? Have you introduced black powder in your sagas to match its appearance in historical Europe?