Thanks for the reply. I can understand it at least!
N.B. I think we are both thinking in caloric theory-like terms here – I know this is out of period, but I don’t know any period theory of heat. Perhaps Eric is using one as I cannot understand his position at all.
In summary:
The temperature of an object depends on the quality of heat contained within it. Thus if you add heat to an object it increases in temperature and if you remove heat it decreases in temperature. For instance, if a PeIg spell destroyed all heat in an object, the object would be as cold as ME allowed it to be.
A CrIg spell can act directly to increase the temperature of an object - adding heat to the object.
This added heat is magical and only exists for the duration of the spell.
This heat will flow into other objects exactly like mundane heat flows – unless blocked by MR
Any object into which this heat flows will be subject to MR. So exploding jars are resisted.
When the spell expires, this magic heat ceases to exist and any object which contained this heat will change temperature to accord with the mundane heat they contain – if the spell was D:Mom they would return to their original temperature.
However, this leads to some interesting issues for longer duration heating. Imagine a magus using a D:Sun CrIg spell to make a fire to heat his lab. The natural heat flow of the room would be from the fire to the walls and then outside. This stream of heat will sweep the mundane heat outside too and soon all the heat in the lab would be magical. When the spell expires the lab and its surrounding will become as cold as ME will allow. Disaster!
So magical heating is actually extremely dangerous if this position is correct.
I am not convinced though that this position is nominal canon. It looks to me that as standard in ArM magically created heat is assumed to persist after the end of the spell. If it persists that it should be mundane imo. It looks like others interpete the text in a similar way.
Been a while since i played Ars Magica (we have moved into a few other games recently, even if I plan to change that in brief time) but Parma can work if it does not stop anything, just resists it.
Parma Magica adds its level x5 to your soak score for resisting damage coming from a magical source. So a parma of 3 adds +15 to your soak vs magical damage, be it a fireball, you hitting a magical bridge, or to resist a sword enchanted with edge of the razor. This is regardless of the magus being the target or the spell or not. It does not dispel the effect either, it just prevents the effect affecting the magus.
Parma adds its magnitude to natural resistance rolls.
Penetration erodes the protection that parma provides. So a parma 3 targeted with a penetration total of +12 would only offer a +3 bonus to soak.
Now THAT is a house rule and the rest are just wannabe house rules LOL :mrgreen:
It totally changes the way how parma work from the "all or nothing" apporoach it has now to a more "armour" approach. It softens the attack, but does not necessarily prevent you from getting damage from it. Probably the multiplier can be tampered with, making parma less or more powerful changing the x5 that it actually has.
Probably there are loopholes in this rule, so consider it food for thought
The biggest loophole is of course that there are no natural resistance rolls in the game any more so without significant further changes to the system anything that doesn't cause damage sails right through.
It seems like there's refreshingly original thinking going on here, you could institute a sort of universal saving throw system and make it work.
But does this really solve any of the problems regarding what is resisted and what isn't. It seems to me that the pink dot and the poisonous air issues exist in this system too.
LOL, then we have been playing Mentem magic wrong for a few years 
