Silica gel

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).

At English Heritage we have had great success using silica gel to
maintain low relative humidity even in low-quality display cases. We
have lined traditional wood and glass cases with Moistop, a
polyethylene/aluminium foil laminate, and have managed to keep the
RH below 20% for more than a year, using approximately 20 kg of
silica gel per cubic metre, when the external RH was well over 50%.
If you just want to stabilise the RH at some higher value then you
need less conditioned silica gel.

As Jack Thompson says, the RH to aim for depends on the objects
concerned, and you would want to display objects with compatible
requirements in the same case.

It is important to line the case with a single sheet of Moistop,
carefully folding the corners and bringing the edges up and over the
rim of the case.  It's rather like wrapping a parcel from the
inside!  We stuck self-adhesive EPDM weatherstrip around the top of
the case, and sealed the glass into the lid with clear silicone
sealant.  We concealed the silica gel under a Correx baseboard, and
placed an ACR datalogger inside to record the conditions.

I assume when you say 25 to 75 square feet you meant cubic feet?

Barry Knight
English Heritage
London

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                  Conservation DistList Instance 10:29
                Distributed: Friday, September 20, 1996
                       Message Id: cdl-10-29-002
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