The gestures used in spellcasting in Ars Magica are not arbitrary, as they are in some other roleplaying games. We know this, because a magus who sees a rival casting a spell can deduce its effects from the gestures, and cast a counterspell for protection. This means the two magi share a semantic framework: the symbols they are using are intelligible to each other. In The Mysteries, we learned that spells work by communicating with, or creating then commanding, airy spirits. This is why the gestures are mutually intelligible: there’s a vocabulary built in the foundational nature of the universe, and the magi are using it.
So, if this axiom is true, what are the casting tools doing? There are several options.
Are the tools entirely arbitrary? Superficially they appear so. If you want to make a casting tool that lets you fling a ball of fire, it can have any shape or material: it doesn’t need to be a ruby or a wand. Many Verditius seem to choose material which is thematically linked to the spell that the tool is linked to, but they need not. They can, for example, make each tool a link in a chain. They need to be able to distinctly sense the individual link they require, but this just requires a superficial mark. If the casting tools are just psychological props, a perfect forgery of a casting tool should work normally for a magus, provided they are unaware of the substitution. Magi could test this.
The casting tool may boost the signal of the Verditius, to make them intelligible to the airy spirits. The function of the season of enchantment might be to make the item highly visible to airy spirits. If this is the case, can these magi use each other’s tools? It does not appear that they can: why? Communication with airy spirits may require the tool to be part of the extended mind of the Verditius, so that unless it has an Arcane Connection it cannot be used.
Superficially, this idea, that casting tools are a sort of semaphore flag for airy spirits, suit what’s been published. That being said, Arcane connections can be created with Creo Vim magic, and it seems not to be used, despite its obvious importance in travel and combat.
There’s the possibility that the casting tool is a magic item, temporary in nature, that is powered, then expended, to create a signal for the airy spirits. The season of enchantment could be to make the item suitable to act as a sort of magical capacitor. In this case, rather than the magus communicating with the airy spirits directly, instead they are sending pre-recorded signals using these devices. Again, if this is correct, why Verditus cannot use each other’s casting tools is unclear
Spells that affect casting tools
In one of my campaigns, my Verditus maga had a gold plate imbedded under the skin of her upper arm, which was linked to Leap of Homecoming. Why are implants so rare in the House?
Why do duelling magi not destroy the casting tools of Verditus enemies? I’ve never seen a mastered spell that targets them, and its an obvious weakness in their tradition?