When creating the obj you can always drop the duration to concentration and pay the +% for concentration

I find creating magical obj in general needs the interventing from the game master.

Any comments on that ?

ps. I'm new in this forum but used to Arsmagica !

welcome to the board.

Clearly for enchanted devices with uses per day or with many charges effects with Durations beyond Sun make only sense, if one intends to have the same effect from the device active several times in parallel.
If a device can maintain concentration for the wielder (ArM4 p.87), it is still an SG judgement call whether it can do this several times at once. I would rule this out IMCs: at the same time a device can maintain concentration once and only once on each effect instilled with that modification.

Then the rule about an item concentrating becomes balanced for enchanted devices with uses per day or many charges.
By dropping effect Duration from Sun to Concentration, and having the device maintain concentration for its wielder,
- you decrease the effect magnitude by one, then raise the effect level by 5,
- you commit to only have the effect in operation once at the same time, but need not bother with effects expiring at the time you renew the Parma Magica.

For items with a single charge or just a few charges, there is then still a little exploit possible, for which I give an example below.

A one-shot amulet of (ArM4 p. 132) 'Stir the Slumbering Tree', R: Touch, D: Concentration, T: Ind, device maintaining concentration, has only an effect total of 20, but attached to a tree provides the owner with a Permanent tree servant.

Considering that this amulet took some vis and lab work and is rather easy to dispel, and comparing that to the lab work needed to research the spell MuHe(Me) 25 'Stir the Slumbering Tree' proper, which allows to create many such Permanent tree servants for 5 pawns of Vis each, I would let this exploit stand, however.
The player has invested some thoughts into it, has chosen a good approach to solve his problem, and has used, not broken the rules. His character has indeed acted like a Magus.

Maybe there are worse exploits of this kind, but I have not yet seen a really dangerous one.

Kind regards,

Berengar
(Edited to cover devices with more than one instilled effect, too)

The entire Ars Magica magic system often relies on or needs story guide discretion. When something seems broken or unweildy, or is likely to disrupt the game, then overrule it.

Magic item creation can be rorty. However, it does take a long time to make a magic item and a lot of vis, so perhaps it should be.

~Alex