Volume 13, Number 2, May 1991, p.11

Update and Feedback: Controlling Relative Humidity with Saturated Calcium Nitrate Solutions

Follow-Up Report: Calcium Nitrate Solutions for RH Control

In the January 1991 WAAC Newsletter (Volume 13, Number 1, pp.17-18), I reported the results of an experiment conducted at the Seattle Art Museum to control RH in exhibition cases with the use of saturated calcium nitrate solutions. We were able to maintain an RH level between 55% and 60% during our 2-month test period. We solved the problem of salt "creep" with the use of tightly sealing Rubbermaid containers with Gore-tex windows.

We have continued to observe our test case for an additional six months. During that time, the case maintained an RH level of 60- 62% while the ambient RH fluctuated between 45% and 58%. We did nothing to the salts during this time. It should be noted that no salts escaped from their container.

A further issue was raised by John Twilley of the Los Angeles County Museum of Art. He was concerned with the theoretical possibility of the evolution of acidic volatiles within the enclosed case due to chemical reactions within the saturated solution. He suggested that an open container of sodium bicarbonate adjacent to the salt-solution container could act as a "sink" to capture these theoretically possible vapors. To test for this possibility, we constructed 2 airtight chambers. In one, we put the container of saturated calcium nitrate solution and a beaker of deionized water (pH 7). In the other, we placed the saturated salt solution, deionized water (pH 7), and a small open container of sodium bicarbonate. A change in the pH of the water in the test containers would indicate the evolution of acidic vapors in the chamber. The second test chamber would measure the effectiveness of bicarbonate of soda to counter any acidic build- up. We tested the pH of the deionized water on a weekly basis for one month and found there was no change in pH in either test chamber. Since we found no evidence of escaping acidic vapors, it appears that the bicarbonate of soda is an unnecessary precaution.

We intend to continue to investigate the use of saturated salts for relative humidity control in exhibition cases, including the testing of additional salts listed in the January 1991 WAAC Newsletter article. We would appreciate any comments or suggestions you may have regarding salts in museum cases.

Comments from Garry Thomson

I remember Sue Sack showing me a case in the Brooklyn Museum which she had stabilized with magnesium rather than calcium nitrate for an RH rather lower than yours. I think it had been running for a year or so when I saw it. Sue avoided salt creep by using stainless steel containers. And there's plenty of experience around the world.

You point out the main difficulty--salt creeping over the lip of the container. Tim Padfield acquired extensive experience while working at the Victoria and Albert Museum. He used a semi- permeable membrane like yours, but made, I think, of silicone.

A minor difficulty, more a worry, is the danger of metal corrosion if the solution got spilled or, in spite of the membrane, some crystals found themselves in the case.

Though you'll probably find it's all right, the conditioned air might not find its way to all parts of the case. It's worth checking this by distributing 2 or 3 RH probes within the case. But one wants to avoid small fans or other mechanical devices.

Remarks from Eric F. Hansen

The use of a saturated solution of calcium nitrate to maintain the atmosphere in a closed case at ca. 60% RH appears to be safe and workable. Several issues should be considered, however, when planning to use this method of humidity control.

First, although calcium nitrate solutions appear to be safe under these conditions, the same conclusions should not be applied automatically to all salts that can be used to control RH at various levels. For instance, solutions of bromide salts used in a laboratory experiment to control the RH turned bright red after several months, which may mean that bromine gas is being produced; this would be undesirable in an exhibit case. If other salts are considered for other RH levels, they each should be tested.

Second, is 60% RH an ideal relative humidity for the artifacts in the case? For most materials, this RH is considered too high; it can accelerate the deterioration of organic materials and promote microbiological activity.

Third, if the artifacts in an exhibit case whose humidity is being regulated by salt solutions are likely to off-gas degradation by-products in an enclosed space (a mummy, for instance), consideration should be given to the possibility of chemical reactions between the substances the artifact is off-gassing and the humidity-regulating salts.