The text:
Traditions arcane
Cast 10 minutes (somatic, verbal)
Range touch; Targets 1 non-magical object up to 20 cubic feet in volume and up to 80 Bulk
Duration 1 day
You shrink the target to roughly the size of a coin. This reduces it to negligible Bulk. You can Dismiss the spell, and the spell ends if you toss the object onto a solid surface. The object can't be used to attack or cause damage during the process of it returning to normal size. If there isn't room for the object to return to normal size when the spell ends, the spell's duration continues until the object is in a location large enough to accommodate its normal size.
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The shrunk item becomes a coin-sized object for a day. If there isn't enough room, it will not expand even if the duration ends.
What happens if I make or buy a small wooden box, and put the shrunk item in it? Would it remain shrunk until I open the box?