The box on HoH:MC p.24 deals with Heartbeasts, specifically having effects that activate based on whether they are touching an animal or not. "The solution is the following well-known [InAn] effect, instilled into the item, and then connected with a Linked Trigger..." There's nothing to solve if a regular trigger would could handle the situation.
Chris
Yes, that's my issue: the trickiness. It's a slippery slope. I don't disagree with what you wrote earlier. I just don't see any distinct line between the ends of the spectrum. I like to be able to draw a line so as not to feel arbitrary to my players.
This combined with the canonical lack of being able to identify animal from non-animal touch leads me towards requiring more InXx effects. That's why I would design the item to activate when anything touches it, because for our troupe you'd need an InTe effect to identify the thing as metal, and that's just not needed if you can let the effect fail.
Chris
I'm not sure it's as alarmingly overpowered as all that if you cost it rigorously. I think a MuX spell that confers upon X the quality of behaving as if it were a lesser enchanted device with effect A should be (lab total to enchant a device with A including trigger, penetration and frequency costs) + magnitudes for Range between magus and X + magnitudes for spell Duration + magnitudes for spell Target + base for giving X a highly unnatural quality and have requisites of Vi and TeFo of A.
The contagious magic required for a rats of doom scenario would then be (lab total to enchant a device with Contagious Rats of Doom including trigger, penetration and frequency costs) + 1 magnitudes for Range touch + 3 magnitudes for spell Duration: moon + 0 magnitudes for spell Target + base for giving rats a highly unnatural quality and have requisites of Vi and TeFo of Contagious Rats of Doom. Since the level cost of effects and spells that do the same are the same this means level required = level required + lots, ie. infinite, ie. impossible.
If you think contagious spells actually sound quite cool then perhaps a guideline of (lab total to enchant a device with A including trigger, penetration and frequency costs) + magnitudes for Range between magus and X + magnitudes for spell Duration + magnitudes for spell Target (needs to be at least group) + base of 40 and have requisites of Cr,Vi and TeFo of A and must be ritual. More traditional hermetic nukes will be cheaper but maybe not as stylish.
The metal armour goes "clang" when hit with metal, and the metal armour goes "thud" (or at least a different kind of "clang") when hit with wood.
Precisely, you get different sorts of "clang" depending on what substance you hit the cymbals with. Ergo, the metal knows what sort of material it has been hit with. The armour shouldn't, probably, be able to tell the difference between a metal sword and a metal mace (say), but the armour can tell the difference between being hit by metal and being hit by wood.
I thought that might be what you were looking at.
I wouldn't read that as prohibiting other solutions. That is just one solution. It's not necessarially the best or only solution. In fact, it's actually a terrible solution, because the talisman ends up Warping the Bjornaer (as it is a constant effect monitoring the Bjornaer's Form).
The physics side of me is squirming at that complexity. Anyway, I'm not saying you can't use this trigger. But for me requiring this kind of understanding involves InTe magic.
Oh, I don't see it as prohibiting other solutions, nor do I see it as a good solution for the Warping reason you mention. We're in total agreement there. However, clearly just a regular trigger is insufficient, otherwise this effect need never be created and certainly wouldn't be used so much. The big question would then be whether its creator would be laughed out of the House faster than he reaches final twilight from the warping the item causes.
Chris
I don't see why it prohibits a carefully worded regular trigger.
Actually, for the Bjornaer talisman, even a "command word" style trigger should do, as long as the Bjornaer remembers to say it.
Well, it is the solution, not a solution, though I would like to leave other options open to clever players. Plus there's nothing to solve if it just works without adding effects.
Yes, I would go with a command word (or phrase or similar), too. And then I would use the same command word for all items meant to transform in such a way.
Overall, I think it would be very nice to say Intellego magic acting on (as opposed to used by) something doesn't cause warping. Then we can have our mental triggers and shapeshifting triggers and other coolness without all the warping that would tend to preclude their use.
Chris
Yep. Warping is what hkeeps most intellego items out IMS. Time and vis arew not as much a consideration as the "I will get more warp than the Enterprise in a few minutes" one.
Thirded.
Appearantly, one (or more) of my players are reading this?
I just got a mail wherein my Bjornear player wants to invent Inscription on the Soul as a Bjornear inner Mystery. 
On the OP question I voted Major - Armaments is a wide class of objects made from all sorts of materials and their defined functions of personal attack and defense can be accomplished by a variety of methods - there is lots of scope to apply the focus.
It is listed in canon as Minor.
However, as a Minor Virtue it was also not applied to all enchantments placed in weapons and armor. If that were to be done I think we all agree it needs to be moved up to Major.
Chris
We really need some kind of clarification on this...
Sorry, what's a TeFo of A?
a variable from his previous explanation:
Technique and Form combination ~TeFo.
TeFo stands for "Technique + Form". Creo Auram (or CrAu, for short), for example is a TeFo combination 
Noliar is talking formulas in that message, so I guess "A" is something he had referred before.
Cheers,
Xavi
If you read the entire quote, "A" = any one effect.
TeFo is shorthand for an unspecified Technique/Form combo.
So (it seems), "TeFo of A" is "the Tech+Form needed to achieve effect A".
exactly
Hi,
Personally, I have no difficulty with 'Swords' and 'Arms & Armour' both being Minor Foci, even though the former is clearly narrower than the latter. The Virtue system is coarse-grained, having only two categories; it's all a matter of style. The Confraternity of Roland is all about swords, so a Minor Focus in Swords is perfectly reasonable. However, Hugh Hostis comes from valkyrie blood, and armaments are more his style. For me, neither are Major because 'Iron' is Major, and 'Iron' is a lot bigger than either.
If Hugh Hostis was a character in my saga (and he is...), I'd ask for a clarification to specify 'Metal Arms & Armour' -- and still allow another character to take 'Swords' or 'Helmets' or 'Shields' as Minor Foci if it was thematically appropriate.
For me, no Focus automatically applies to the Lab Total when enchanting an effect in an object under its purview; only if the effect has an appropriate Target will the focus count. So an enchantment to strengthen a sword gets the focus, but an enchantment to make the sword burst into flames does not.
Mark