I have a very newbie question. How does item penetration work? I haven't found it explicitly spelled out in ArM5 so either I'm looking in the wrong place or I'm imagining that it is more complex than it is. I expect it is he latter.
Here's how I think it work. Please correct me if I'm wrong.
All penetration comes from the item and is added at the time the effect is invested as per ArM rules (2 points of penetration per level of effect). So an effect with zero penetration bounces harmlessly off the least PM or Might.
There is no calculation of penetration involving technique and form of the caster and the level of the effect is not subtracted from the penetration. So an item with an effect that has a penetration of 22 will always penetrate a PM+form or Might of 20. Correct?
Except that the penetration of the item is affected by the local aura, so if you use the item in a Divine Aura (say) it will have a penalty. The Magic Resistance of the target is also affected by the aura, but depending on the realm alignment of the target it might be affected more (or less) than the item.
No and yes. There are no rules allowing it, but there are rules for changing the rules, in game. Look up Original Research. If this is something you want in your game, an NPC has done it already, and the PC's just need to find the lab texts to recreate the change. Otherwise, this is one of the cool things about Ars Magica. The PC who wants to do this can, if he devotes the time and effort, with a big payoff in power and reputation. Might even get the coveted title "ArchMagus"!
It's quite a long and expensive process but it is very much worth it from a story perspective to say nothing of the sense of player achievement when you get it done (speaking as a player of a character only one project away from integrating Heronic magic). You get to stretch your item/spell design muscles along the way, then you get the thing you're aiming for, and you get recognition from a whole load of magi interested in the same effect.
Sounds like a lot of work, but I say give it a go. If you find you want to short-cut the process you can always find another magus' laboratory notes and piggy-back off his research.
I think it could be a lost Mystery from Veriditius maybe. Come up with a decent Initiation script along with appropriate Flaws and you can add the escalating Penetration maybe.
However, fyi, in the Magi of Hermes book, the Flambeau Hugh added a secondary enchantment to his Talisman that effects two of the enchantments he made earlier in his career. The only thing this secondary enchantment does is add a higher Penetration to the earlier effects when he triggers it. So technically there's already a way to do it.