How does Craft Magic work?

[jules]

@darkwing

If you work on a carving and then leave it - as you "Must avoid distraction and maintain concentration whenever he is working on the object, just as if he was casting a spell, and like casting a spell the character can only work on a single object at a time" - so if you partly finish something, then try and cast a spell, it would surely count as "casting another spell" under the concentration rules on ArM5 p82, requiring a Sta + Concentration + stress die roll of 15 or more. Therefore, unless you are a genius at concentration you make casting any other spells while you have a partly finished object extremely difficult.

You must maintain concentration whenever you're working on the object, which allows for interruptions during which you are not maintaining concentration Otherwise, the heater shield exemple would be thoroughly impossible, since it takes 9 days.

The restriction that prevents you from casting another spell is that "like casting a spell, the character can only work on a single object at a time".
A magus can cast another spell while maintaining another through concentration, but casting a spell during a pause in Crafting Magic is like casting a spell in the middle of casting another, which apparently was clarified by David Chart as impossible, rather than requiring a high Concentration.