Some vanish, some don't. Here are the relevant spells (which are in
the rules):
Create magical barrier [845]
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Usage: USE 845 [aura]
Required aura: one to eight points
Required item:
Time to cast: ten days
Erects a magical barrier which prevents anyone from entering the
location. The barrier will lose one point of aura at the end of
each turn, and will dissipate when it has no aura remaining.
For example, USE 845 1 will create a barrier over the location
which lasts until the end of the turn. USE 845 2 will create
a barrier which lasts until the end of the next turn.
Multiple casts of 845 are additive.
A barrier which achieves eight points of aura becomes permanent.
Permanent barriers do not decay at the end of each month. They
may only be removed by a magician.
Aura defaults to one.
Remove magical barrier [846]
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Usage: USE 846 [aura]
Required aura: one to four points, as given
Required item:
Time to cast: seven days
Attempt to remove a magical barrier from the location in the
given direction. If direction "0" is given, a barrier over
the current location will be removed.
The chance that the barrier will be successfully removed is
proportional to the aura invested in the attempt:
aura chance of success
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1 10%
2 25%
3 50%
4 75%