This post is more meta-magic system thoughts for ars magica – on the complexity in Muto based materials creation and manipulation, raised from a forum discussion…
Switching one form to another should include a degree of flexibility in the final shape, as this is the declared purpose of the spell. As long as the spell is creating a specific object or thing. Surrounding air to a natural plant is fine and declaring the type and shape of the plant is reasonable in the Formulaic formula at base.
Then modifying for complexity in shape seems appropriate, as:
- I see the complexity as quasi- geometric shapes (+0 mags) like lines, cubes, boxes, or walls. Exploiting a feature of the new material should be free too – like using a plant to create a natural shelter.
- The allowing sub-shapes in the main shape (1) like windows, basic stairs, spikes.
- Then almost any other ordinary detail (2).
- Then finally very ornate details as (3) like artwork.
Having the additional mags should allow forgiveness in the Finesse check but not remove a basic “fit for purpose” check. A serviceable but ugly level for a simple item is vs 6 (a sword), and a more complex item is vs 9 (a suit of armor). Less than this and the finesse check might still be needed (vs 3) , but is really at the “don’t botch” level (a cube of stone).
So a basic sword is +1, and a full suit of armour might be +2. Then add mags for moderate flexibility in choice of form when casting. Making a spell which can pick when cast from a dagger, knife, or sword should cost something, whereas two different styles of swords which are serviceable seems plausible as a spell without too much added complexity. Sword vs axe should definetly need complexity modifiers, if not be allowed at all.