Relative humidity control in microenvironments

In Conservation DistList Instance: 10:6 Caroline Gilderson-Duwe
posted the following enquiry:

>How can I safely, accurately and consistently increase the humidity in our
>display cases? ... I am not aware of a product or process to increase rH
>levels.

There is indeed a cheap and effective way of setting steady relative
humidities, whether these be higher or lower than the ambient RH.
Saturated aqueous solutions of salts have extremely stable
equilibrium RHs; these do vary somewhat with temperature, but the
variation is rarely significant (and, of course, controlling
temperature in a small case is comparatively straightforward).  RH
can be controlled by selecting the appropriate salt (or mixture of
salts), making up a saturated solution, and placing it in the case
in an open container (such as a deep tray).  It may take a little
time to reach thermal and moisture equilibrium.

Tables of the equilibrium RH of various salts are widely available;
for instance, in the ASHRAE Handbook of Fundamentals (published by
the American Society of Heating, Refrigeration and Air Conditioning
Engineers, and updated every few years).  As an example,  Arnold &
Zehnder give the following RHeq figures:

    Magnesium Nitrate
        57.4% at 10C
        55.9% at 15C
        54.4% at 20C

    Sodium Chloride
        75.7% at 10C
        75.6% at 15C
        75.5% at 20C

    Potassium Nitrate
        96.0% at 10C
        95.4% at 15C
        94.6% at 20C

("Monitoring wall paintings affected by soluble salts," in The
Conservation of Wall Paintings, Proc. Symp. organised by the
Courtauld Institute of Art and the Getty Conservation Institute,
London 1987;  Ed S. Cather, GCI 1991)

It is important to ensure that losses through evaporation are made
up, but that the solution remains saturated. I hope this is of some
help to you,

Robyn Pender
Conservation of Wall Painting Department
Courtauld Institute of Art
Somerset House
Strand
London WC2R 0RN

                                  ***
                  Conservation DistList Instance 10:9
                   Distributed: Friday, July 12, 1996
                        Message Id: cdl-10-9-003
                                  ***