Effect expiry can not be used for lesser enchantements and even for an invested device, expiry wouldn't come in to play when using a lab text. A lab text already allows the "magus" to instill the effect in a single season so no time could be saved.
This pair of items were originally created for a covenant in the highlands of Scotland, where the covenfolk (in complete defiance of history) wore plaids. They would work just as well for cloaks, though.
I'm kind of proud of this pair of items: my magus created them as a demonstration of Doing Cool Things Despite Terrible Relevant Arts. The covenfolk loved us for them.
Ha! I was just skimming the spell guidelines thinking of a few items. You know me too well.
The trick is to have items that are thematically appropriate for apprentice characters.
The guideline for this effect reads as follows "Create a corrosive substance doing +(Level) damage. Increasing the range to more than Touch is a very good idea." I'm wondering if duration momentary is sufficient. Might the corrosive substance be gone before it has a chance to actually corrode anything?
Yes, I've previously thought that a device like this, or casting tablets for the spell, would have to exist for the game to work as presented. It's very apprentice appropriate.
I'd wager that bronze charm is in the shape of a mirror with a tiny little reflective surface so our apprentice can get a shape bonus for it.
Unless you're like me and think the purpose of the shape and material rules is to encourage the creation of devices that make thematic sense, a charm placed upon a door to hide it makes sense, a mirror less so.
The rego imaginem guidelines don't address controling volume.... but they should.
Edit: I see Aurelentus from Magi of Hermes did this with Perdo at base 3. I'm judging that putting a cap on volume is equivelent to moving the sounds 5 paces away (out of the room), is that too low?