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Subject: Silica gel

Silica gel

From: Jack C. Thompson <tcl>
Date: Wednesday, August 28, 1996
Clara Margaret Deck <deck [at] umd__umich__edu> writes

>I would like to hear from anyone who has worked with silica gel as a
>buffering agent for sealed exhibit cases. More specifically, I would
>like to know what ratio of silica gel-to- case volume has proven
>effective in maintaining a reasonable RH range over time (say, three
>months).

Clara,

There is no, single, magic weight:volume ratio of silica gel-to-case
volume; there is, however, an elastic list of questions to be
answered when considering this solution to an exhibit or storage
issue.  The first question to be answered is whether or not an issue
or problem is being addressed.

If the artifacts in question have equilibrated to their environment,
they may be damaged by being placed in a strictly controlled
environment for, say, three months only to be returned to a
relatively uncontrolled storage environment which is very different
from the artificial exhibit environment.

If the decision is made to attempt to create a stable level of
relative humidity in an exhibit case, the next issue to address is
that of the case. An existing case, or a purpose-built case.  If the
existing case is to be retro-fitted, how tight is it, and how tight
can it be made.

A wood and glass case is well neigh impossible to make air tight,
unless the temperature in the display area is tightly controlled.
Worst case: sunlight gallery on a hot day with a lot of people in
the gallery.  Air inside (and outside) the case expands until it
finds (or creates) a vent and outgasses until the pressure inside
the case matches the room air.  At night, with the sun down,
curtains closed, and people gone, air in the room cools.  The
relative humidity, which now includes additional moisture from the
public (from perspiration and respiration) goes up.  Air inside the
exhibit case also cools off and a slight vacuum is formed.  The case
takes humid air in until the pressure inside the case once again
matches the ambient pressure.

After a few such cycles the silica gel inside the case can be
working hard to maintain an ever increasing relative humidity.

A purpose-built case made from anodized metal and glass can be made
very tight, but provision should be made for a hidden air bladder
which will expand and contract to accommodate air pressure changes.

Another issue to be considered is whether the case will be active or
static; i.e., will the air inside the case be driven by a fan or by
temperature changes.

This is important because a static case takes time to equilibrate,
and even then micro-climates are created.  The target %RH is most
likely to be attained in close proximity to the silica gel, and
wandering away from the target as the distance from the silica gel
increases.

In an active case, a fan (or fans) move all of the air in the case
through the silica gel.  Equilibrium is more quickly achieved and
maintained.

In either case, some provision should be made to monitor the
environment inside the case.  This can be accomplished simply and
inexpensively with good dial hygrometers (one near the bottom and
another near the top inside the case), so long as one person is
responsible for taking regular (3X/day...) readings to insure that
the cases are staying within established(?) limits.

Personally, I prefer electronic data loggers.  Their cost is
comparable to hygrothermographs, and they are less susceptible to
calibration errors.

The issue of target %RH is dependent on the nature of artifacts to
be exhibited, and it is important that artifacts with similar
relative humidity requirements be gathered together.  For instance,
an illuminated manuscript should probably not be included in a case
which contains a collection of crystals which might effloresce at a
relative humidity which is satisfactory for the manuscript.

There are always more issues to consider, but this is a beginning.

This response does not address the issue of why you state CCI's
recommendation of silica gel per cubic meter, yet only give the
square footage of your cases, nor asks if you've queried the
conservation staff at the Henry Ford Museum.

Jack

Jack C. Thompson
Thompson Conservation Laboratory
7549 N. Fenwick
Portland, OR  97217
503-735-3942 (voice/fax)

                                  ***
                  Conservation DistList Instance 10:23
                  Distributed: Friday, August 30, 1996
                       Message Id: cdl-10-23-004
                                  ***
Received on Wednesday, 28 August, 1996

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