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Subject: Bentonite

Bentonite

From: David Thickett <david.thickett<-a>
Date: Monday, July 28, 2008
Ruhi Baharad <conschem [at] imj__org__il> writes

>We have began testing bentonite/montmorillonite (Wisepac, Desi Pak,
>Wisesorb) to reduce relative humidity in display cases at the Israel
>Museum in Jerusalem. ...

Although I have never used the clays you mention, technical info for
Desi-Pak can be found at <URL:http://www.sphinxadsorbents.com>.

The initial RH achieved will depend on amount of clay per volume of
case, initial case RH (before clay was added) and the amount of
hygroscopic material within the case (dressing fabrics etc).  The
room RH and case air exchange rate will have an effect, but probably
not a large one.

The lifetime (to reach a particular RH), will then depend on amount
of clay per volume of case, the air exchange rate of the case and
the RH profile in the room.

The oven drying will depend on loading of bags in the oven, its
ventilation rate (the ventilation grill should be set as far open as
possible) and the temperature distributions across it.  The
performance is quite dependant on the oven type and is best
determined by running several 'dryings' at different time periods
and measuring RH of the bags.

The amount of clay essentially depends on the performance of the
case (air exchange rate), room RH profile, what RH you want to stay
below and how often you're prepared to change the clay.  Weintraub
gives some formula in his excellent Journal of American Institute of
Conservation paper, this is modified in Tetrault's book 'Airborne
Pollutants in Museums, Galleries and Archives'.  The 'MH' value can
be taken from the technical info if the RH is below 15-20%, as
there's no hysteresis.  If you have room RH data it is more accurate
to calculate the internal RH of the case, which can be easily done
with excel.  I have spread sheets which I'm happy to share which do
this. For details, see

    D. Thickett, P. Fletcher, A. Calver and S. Lambarth.
    The effect of air tightness on RH buffering and control,
    in Museum Microclimates, T. Padfield and K. Borchersen, eds. LP
    Nielsen Bogtryk, Hvidovre, (2007), 245-252

David Thickett
Senior Conservation Scientist
Collections Conservation Team
English Heritage
1 Waterhouse Square
138 Holborn
London EC1N 2ST
+44 207973 3322
Fax: +44 207973 3209


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Received on Monday, 28 July, 2008

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