Recently, I asked whether a Crystal Ball could be used to cast Scrying many times by different people. The answer was that yes, I could, but it may take ten minutes of casting before another person can attune to it. But I'm wondering now whether the crystal ball must be held the whole time to use Scrying, a 10-minute spell.
The Crystal Ball allows me to "Cast the Scrying spell" while touching it. The long casting time rules say that you must "spend your action each turn" to keep casting a spell. I'd guess that, since I can't cast Scrying when I'm not touching the Crystal Ball, that means I fail on any turn I stop touching it, since I can't spend my action.
How does this work? If you can explain why this works with magic items in general, bonus points.
Best Answer
You must touch the crystal ball for 10 minutes to cast Scrying
Scrying has a casting time of 10 minutes. You are considered to be casting the spell for that entire 10 minutes, from the moment you start casting it until the moment you finish (or until the casting is interrupted). The crystal ball says:
Since the crystal ball only allows you to cast Scrying while touching it, you must be touching it continuously for the entire 10 minute casting time in order to cast the spell. If at any time you are no longer touching the crystal ball, you lose the ability to cast Scrying, and hence the casting is interrupted immediately, in the same way it would be interrupted if your concentration was broken.
More generally, if you must fulfill any condition in order to cast a spell, that condition must be fulfilled continuously for the entire casting time, or else the casting fails as soon as the condition is no longer fulfilled.