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Subject: Saturated salt solutions to control humidity

Saturated salt solutions to control humidity

From: Graham Sussex <sussggmj>
Date: Monday, June 18, 2007
Stefanie Pfeifer <stefanie_pfeifer [at] gmx__de> writes

>I am writing my diploma thesis in conservation / restoration of
>photographs in Berlin and am interested in the method of using
>saturated salt solutions to control humidity. I got the information,
>that some objects (photographs in this case) treated with
>"salt-conditioning" deteriorate more heavily in accelerated aging
>tests. This information refers first of all to the use of potassium
>nitrite as the salt, but also to potassium dichromate. Might anyone
have similar experiences?

I have no experience of degradation of photographs used with
saturated salt solutions although I have used saturated salts to
control RH in corrosion tests.

I found that it required good stirring of the salt solution and a
well maintained air flow to give a reasonably consistent RH around
the chamber.

How much would the RH have to change (from the 45% RH at 20 deg. C
for saturated nitrite) to cause problems for the photographs?  Was
the degradation specific to those salts compared to others?

I am not sure I would like to use either salt anyway as dichromate
is toxic and nitrite can be a good food for bugs to form
nitrate--although maybe not in concentrated solutions. Incidentally,
the RH above a saturated nitrate solution is about 94% at 20 deg. C.

Graham Sussex

                  Conservation DistList Instance 21:12
                 Distributed: Wednesday, June 20, 2007
                       Message Id: cdl-21-12-004
Received on Monday, 18 June, 2007

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