Hum... This reminds me of something I read elsewhere.
Magical Creatures as Thaumivores, living on magic auras.
This would explain why one don't see them in mundanes areas, and could give them immunity to such mundanes deprivations.
Problem is, of course, if they don't need air to breathe, they become immune to some Auram Spells, and also to drowning ad such.
Thats a fair point actually.. The immortal magi rules in mysteries actually refer to the fact that all of the paths turn magi into various things from the realm of magic, and by happenstance, they don't age, don't need to eat, don't need to drink, nor breath... unless they want to.
.. so, in actual fact, it might well be fair enough to say that they don't need to breathe or such, indeed, you rarely hear of mythical creatures just found dead on the floor from starvation.
I'd say it was reasonable to make them immune to such things, unless it was part of their nature that they be vulnerable to a certain thing.
Wow Matt. Thanks you dork. 
I think it's always good to hyper-correct someone like that before he runs an adventure. That puts him in the just the right mood (to kill another companion character of yours).
No good turn goes unpunished. 
He didn't correct you, Mario, he did exactly what I tried to do- present, in as strong a presentation as possible, why, for him, it was the only "right" decision.
My arguments didn't convince you, and that's fine. If his did, then, likewise, fine. If not, that's fine too. Gawdz forbid any two intelligent, creative people march in lockstep just because.
It's true that Ars is conducive to discussing folk dancing, angelic beings and the prime real estate that constitutes the head of a pin. 
Another situation to add to all this questions of penetration :
A magus trasform earth beneath the feet of another magus into black powder (saltpetre, sulphur and charcoal) by using whatever combination of Arts (probably Terram + Ignem). The other magus is now surounded by an extremely explosive powder. If one of the first magus grogs "accidentally" draw a fired arrow on it... what is the damades taken by the other magus ? This is not the powder that hurt, this is the energy. I know that energy and combustion wasn't part of the medieval vocabulary, but what is their view, so ?
Light my way, please, or it seems that too many things are unclear in the Magic Resistance/Penetration problem.
Or even better, you create some flammable oil on the ground around their feet, you then set it on fire.
I would rule that even though the fuel of the fire is magical, the flames and heat are mundane. Therefore the target will be hurt even though they are protected by MR.
The specific example isn't appropriate to the setng as presented in the books because the setting runs on the "science" of Plato and Aristotle so there is no black powder.
The way to adjudicate any such question is to ask if the heat or substance or motion is in any way magial.
Sounds like an alchemical mixture, and magical (given the early 13th century).
Interesting question I never ask myself. well, let's see.
Nothing in the book about that, i'm pretty sure. The fire is not independant of the fuel, different fuels change the fire (a grass fire VS a coal fire), so a magic fuel would probably change the fire to be magical as well. (with the same penetration as the oil). After all, the fire is sustained by magic so it's no more natural.
It's arguable but some people like to argue. 
At a little scale the oil thingie doesn't seem to be unbalancing, but change your spell for something that changes rain into oil, blazing entire cities. I have a hard time considering that fire would be natural, even if it's lighted by a completly mundane torch.
Well, let's the debate begin ! 
There are given examples of magically created stuff reacting with normal stuff to make more normal stuff, such as the horse which eats real food and leaves a real body at the end.
I certainly have no problems with setting fire to magically created oil, though you might have even better luck if you made greek fire. Just go have a word with your friendly local alchemist and learn how to make it ^^
Yeah, absolutely nothing.
Everyone knows that saltpeter doesn't burn, nor sulphur very well- and charcoal would take a while to catch. What are you talking about???
Here's another- a mage takes a large, long iron cylinder, wraps it in a tight spiral of a copper cable, and sends a lightning bolt down the cable.
Does he create a huge magnet? If the lightning has a duration, does he create an electromagnet? If he creates an electromagnet, can he create an electric motor???!!!
Nope. He probably melts the copper onto the iron, and heats the mess a bit. (What's an electromagnet? What is this word "motor"?)
Modern Physics does not apply in Mythic Europe! (What part of that do people not understand?)
Since when does lightning go into metals?
Since mages started making magic items invested with Cr Ig. 
Everyone except the chinese
Therein lies part of the problem though, as an order of very intelligent magi, many of whom also know various alchemical processes (tho related to magical theory rather than mundane alchemy admittedly), why -shouldn't- they try, or be able to invent more complex things? .. certainly blackpowder had been around since the 9th century, and greek fire from even earlier.. .. simple mixtures of various things are well within the potential of any alchemist.. and mages have an advantage that many human alchemists do not... magic.
The primary reason black powder was not more quickly invented is I imagine due to the fallout.. how do you -invent- such a thing without blowing yourself up? ^^ .. mages can get around that with just a simple ReIg ward like many magi will use in their labs around experiments.
Anyhoo.. sorry, rambling again.. I suppose what I'm getting at, is a lot of people use the medieval paradigm as an excuse against ever learning anything, which is basically just a simplification so that we don't have to try and figure out what they -could- learn.. which is pretty hard to conceptualise from our modern point of view.
But yeah, chemistry is a growing science even back them, when alchemy had started becoming popular.. so I've got nothing against people discovering stuff and using them, but it's just tricky to decide just what is possible and what isn't
They should and do invent more complex things.
But black powder isn't part of the paradigm. It isn't available to be invented. In the same way that the steam powered ether machines of Space 1889 aren't available for characters to invent in a Traveler game and Characters in a Legend of the Five Rings game can't get magic resitance from their faith in Christianity.
Black powder works because of chemical reactions. In a world based on the four elements chemical reactions are meaningless.
???
Since when does it not?
Or are you suggesting that since the lightning rod wasn't invented until Ben Franklin, then in Mythic Europe lightning is not conducted by metal? 
Meh, not my choice, but that works along the same logic of Medieval Paradigm.
Or am I misunderstanding your objection?
Lightning Travels through the air, not through little rods, It's far too big to fit.
Why would it travel through rods?
I totally disagree with you there Erik.. .. and it's a prime example of what I was on about...
Just because people don't know the underlying principles behind chemistry, does not mean things will simply not mix and do things. I already gave an example of greek fire, which has been around for -ages- .. that works.. so how do you explain it in the medieval paradigm if things are totally unmixable?
The lack of the knowledge of chemistry doesn't stop things working.. .. thats part of the whole thrust behind alchemy.. trying to figure out correct combinations of stuff that -does- work
No, I can see this point. The lightning travels "along" the metal, but never "in" it. (After all, how would anything get "inside" metal???)
My point above had more to do with using magic to create technology, than the details of that technology itself.
However, I used the preposition "down"- "sends a lightning bolt down the cable", so Fruny's actually the one guilty of projecting modern conceptions on the statement.
J'accuse! 
The problem is that there is a gulf of difference between knowing the proportions of mixing sulphur, charcoal and saltpeter, and having the insight to try the proper combination of those three random substances.
The Aztecs didn't have the wheel. Not because a wheel wouldn't work, but because it wasn't part of their culture, they just never made the mental leap. If we were playing an Aztec game, what's to stop anyone from building a wheelbarrow, or wagon?
It's far easier to make that leap in hindsight. That's why it shouldn't work- because it's not reasonable to expect anyone to do what no one else would have thought of for centuries. That, and it's not the game as designed.
If a SG wants to introduce gunpowder, or steam engines or electromagnets, or orcs and halfling nations and and underworld cities of scimitar-wielding drows, then that's fine- but it's not canon Ars.