> 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 accomodate 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 callibration 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 why you haven't queried the conservation staff
> at the Henry Ford Museum.
>
> Jack C. Thompson
> Thompson Conservation Lab
> Portland, OR
> http://www.teleport.com/~tcl/index..html
>
>
> ...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)....
> The Canadian Conservation Institute recommends 20kg/meter cubed (a lot!).
> I have cases with volumes ranging from 25 to 75 square feet, so I would
> like to know if we can use less than this CCI recommendation, but still
> buffer the RH effectively. Any ideas? ....
>
> Clara Deck,  Henry Ford Museum & Greenfield Village, Dearborn MI, 48121