Saturated salt solutions to control humidity

Oliver Tietze <oliver.tietze [at] gmx__net> writes

>A small church near Leipzig has got an altarpiece from 1520. It is
>in very bad condition due to the rapidly changing climate during the
>service. In order to prevent more damage a showcase has to be built.
>The RH in the church in winter is around 60% and the temperature
>around 2 deg. C, rising up to 15 deg. C over 12 hours, and slowly
>decreasing over 2 day after the service and up to 22 deg. C in
>Summer.
>
>Now I am looking for a buffer material for the showcase. A saturated
>solution of calcium nitrate could be it. ...

Speaking from my own lab experience, there are other problems with
saturated salt solutions. Probably the biggest problem is that after
several cycles of evaporation and condensation in the salt solution
dish, the salts tend to climb over the edge of the dish and down the
other side leading to potential chemical contamination of the case.

If your primary problem is not humidity leakage in and out of the
case and only temperature driven humidity changes, then I would
agree with others that additional organic material should buffer the
changes pretty well. We've been running lab tests here with archival
materials (books or paper or photographs) in a (vapor) sealed
container cycling the temperature in the environmental room around
the uninsulated box. So far even with a relatively low density of
material, the roughly 24 cubic foot (0.7 cubic meter) container is
holding humidity pretty constant.

Douglas Nishimura
Image Permanence Institute
Rochester Institute of Technology


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                  Conservation DistList Instance 25:16
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