Although I do not know where the walk-in vault to which you refer was located
I surmise that it was in a hot/dry region.
Walk-in vaults are not very well sealed with regards to the environment and
if the air outside the vault is dry, it should not come as a surprise that
the air inside the vault is also dry.
Fire-proof safes/file cabinets are better sealed against ambient conditions.
The moisture came from the paper & file folders.
The institution in question is located in a dry region, in a building which
does not have good environmental control.
During the night it is cold (outside and, to a slightly lesser extent, inside);
most of the work involving files going in and out of the fire-proof cabinets
happened during the early hours of business, when the %RH was highest.
During the day the temperature rises, outside and inside the building and the
%RH decreases. Except inside the fire-proof cabinets. But they are not
completely unaffected.
Temperature does increase inside the fire-proof cabinets and that forces
moisture out of the paper and into the air inside the drawers, increasing
the %RH inside the file cabinet, and because there is insulation against
establishing quick equilibrium with the outside environment the cabinet
possesses a higher moisture content than the room in which it is located.
There is also the matter of partial pressure differentials but I'm not
going to go into that.
Jack
>Date: Tue, 22 Nov 2005 14:54:56 -0700
>From: "Gabriel Austin" <gabrielaustin@earthlink.net>
>To: <exlibris@library.berkeley.edu>
>Subject: RE: fireproof safes for rare books
>How did the moisture get in? I worked at a library that had a walk-in
>vault. We had to keep a filled bucket of water in the vault. The water
>evaporated in about a week's time
>Gabriel A
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