Saturated salt solutions to control humidity

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?

An alternative, but related, approach to controlling RH might be
worthy of considering for furthering your objectives specific to the
photographic conservation domain. From the posting, it is unclear as
to whether you will be specifically implementing the saturated salt
solution. approach and/or studying its effects on photographic
conservation and/or trying to achieve stable RH under experimental
conditions for your research. In either case hope the info below is
of use.

An alternative method for controlling RH, at least in the
short-term/experimental conditions, while avoiding/limiting
unwanted/unknown side effects of salts in saturated salt solutions,
involves the use of glycerin (i.e. glycerol)  water solutions as per
testing standard:  D5032-97(2003) Standard Practice for Maintaining
Constant Relative Humidity by Means of Aqueous Glycerin Solutions

   "1.1 This practice describes a method for obtaining constant
    relative humidity ranging from 30 to 98% at temperatures ranging
    from 0 to 70 deg. C in relatively small containers by means of
    an aqueous glycerin solution."

    <URL:http://www.astm.org/cgi-bin/
        SoftCart.exe/DATABASE.CART/REDLINE_PAGES/D5032.htm?E+mystore>

    **** Moderator's comments: The above URL has been wrapped for
    email. There should be no newline.

On the positive side, simply varying the solution concentration
according to the anticipated test temperature conditions (although T
variations in these conditions do not have a significant effect on
the RH obtained by these solutions), provides a flexible means of
attaining an RH specific to the conditions you require, not as
dictated by a range of specific and available salts. The more
glycerin added to a fixed water quantity (i.e. higher concentration
of glycerin solution), the lower the RH obtained.

On the negative side, calculating the appropriate concentration is
not a straightforward procedure and requires supplementary data
tables. Also note that since glycerin is a biological nutrient, it
is suggested that for longer use, copper sulphate in low
concentration (circa 0.1% w/w) is added to act as a
fungicide--possibly coming back to the original problem of side
effects, albeit in significantly lower concentration.  Possible side
effects on the test materials would need to be considered and
compared with those of alternative test systems (e.g. saturated salt
solutions), or indeed other biocides. Alternatively, biocides could
be avoided by periodically replacing/renewing solutions to limit
biological growth.

In our cases, the protocol was used twice with low-carbon steel test
coupons (to avoid their possible contamination and subsequent
corrosion from saturated salt solutions) in 9 litre (food storage)
containers: firstly, for the accelerated laboratory testing of
protective coatings (daily cycling, 8 and 16 hour respectively,
between ambient RH and T, and 90% RH while under 35 deg. C in an
oven) and; secondly, for creating specific corrosion product
morphologies on prepared surfaces by cycling between two containers
of varying RH (8 and 16 hour respectively between 60% RH at ambient
T (15-20 deg. C) and 90 %RH while under 35 deg. C in an oven).
Continuous datalogging confirmed the calculated %RH's were attained.

James Crawford
Metals conservator and researcher
Conservation Division
Heritage Malta
East Wing, Former Royal Naval Hospital
Bighi, Kalkara
CSP12 Malta


                                  ***
                  Conservation DistList Instance 21:13
                   Distributed: Friday, June 29, 2007
                       Message Id: cdl-21-13-005
                                  ***