Saturated salt solutions to control humidity

Douglas W. Nishimura <dwnpph [at] rit__edu> writes

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

I'd like to speak *in favor* of what we unfortunately call
'saturated salts'. It is important to describe a saturated solution
of the actual compound used (e.g., saturated magnesium nitrate).

For about twenty years I have used saturated magnesium nitrate or
saturated calcium nitrate to passively maintain mid-range RH in
closed exhibit and storage cases. The method is an adaptation of
Julie Creahan's 1991 method of using enclosed plastic tubs to safely
isolate the nitrate slurry from the rest of the case. This solved
the problems of accidental spillage and creep.

For access to her original article and the ensuing discussion see:

    <URL:http://cool.conservation-us.org/
        waac/wn/wn13/wn13-2/wn13-206.html>

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

Following Julie's design my tubs are fitted with large Gore-Tex or
Tyvek ports in their lids to allow vapor exchange. It's important to
understand that these tubs are never opened in the exhibit case.
Indeed in my environment (Boston, MA area) the lids are only
occasionally opened once sealed. There is no danger of salt crystals
moving out of the sealed tubs.

For maintenance, the tubs are re-weighed once every one to three
years (depending on the case history and its tightness), that weight
is compared to the original gross weight, and a small amount of
water is added to bring it back to its original weight. The lid is
replaced and the tub is returned to its case. Our in-house winters
are on-balance drier and longer than our summers are wetter. In our
heated buildings the time spent below the mid-range RH is greater
than that above, leading to a slow desiccation of the tubs and
avoiding under-saturation of the nitrate solution. YMMV

One added benefit of saturated nitrates is that the water weight
record you create when topping off the tubs gives you a clear record
of the annual moisture budget of each case in milliliters H2O/air
volume/year. This is very useful when designing new cases in
general, diagnosing case tightness and predicting future maintenance
needs.

Regarding the slow response of saturated nitrates, the Gore-texed
tub configuration slows that response even further! In dynamic
environments they should be thought of only as large controlled
moisture reservoirs- not responders. Instead, exhibit cloth case
linings and sometimes archival paper and silica gel impregnated
paper underlinings serve as the quick responders. The saturated
nitrate tubs should be thought of as passively maintaining the EMC
of these fast responders which in turn maintain the RH of the air
volume around the exhibited objects.

Over the years I have not been able to make metal test coupons
corrode due to saturated nitrate humidification. I have also not
found in practice any tendency to lose control due to water
absorption in periods of high humidity. As long as solid nitrates
exist in the tub control is maintained without the need to mix or
crush the nitrate.

I first moved to saturated nitrates as a way of decreasing the
maintenance failure rate I saw with all attempts to create a stable
mid-range in-case humidity. I think that remains the critical
problem, more than whatever RH stabilizing technique we select.

Dennis Piechota
Conservator
Fiske Center for Archaeological Research
UMass Boston
Office: 617-287-6829


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                  Conservation DistList Instance 25:18
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