Volume 13, Number 1, Jan. 1991, pp.17-18, drawing

Controlling Relative Humidity with Saturated Calcium Nitrate Solutions

The Seattle Art Museum is presently housed in a landmark Art Deco building constructed in 1933. After completion of a new downtown facility next winter, the old building will continue to be used for display and storage of the sizable Asian art collection and other portions of the museum's permanent collection. The HVAC system is antiquated, however, and has no air cooling capacity. Moisture can be added to--but not removed from--the air. This seriously limits our ability to control relative humidity (RH) levels in galleries and art storage.

In order to provide a stable environment for objects that need special RH levels or are sensitive to changes in relative humidity, our efforts have gone to control of RH using silica gel in the micro-environments of storage and exhibition cases. We have found silica gel to be quite effective in situations that require low RH, but when elevated RH is the goal, silica gel can be problematic. The gel needs to be preconditioned to the desired RH, a time-consuming procedure that requires a significant amount of space and is hard to control. Large quantities of gel are required, and the cases need constant monitoring and frequent gel changes, particularly if the desired RH is significantly higher than the ambient air. Our frustrations with silica gel have led us to explore other possibilities for humidity control.

In his book The Museum Environment¹, Garry Thomson mentions saturated salts as one possible method of maintaining a constant RH. A specific example of their use was described in a paper presented by E. E. Astrup at the 1987 Triennial Meeting of the ICOM Committee for Conservation². She described the use of calcium nitrate solutions to control humidity in exhibition cases at the Viking Ship Museum in Oslo, Norway. At this museum, which experiences widely fluctuating RH over a year's time of from 25% to 85%, calcium nitrate in saturated solution has been used for over thirty years to maintain RH levels at 48%-62% within permanent exhibition cases.

A simple explanation of the principles involved in maintaining RH with saturated salts is as follows. If a container of water is placed in a closed chamber, the water will evaporate until 100% RH is reached. Any salt added to the water will reduce the achievable RH, dependent upon the number of particles dissolvable in the water; i.e., the solubility of that salt. Each salt, if mixed to the saturation point in water, maintains an associated characteristic RH (at a given temperature) in the microenvironment around it. If air of low relative humidity is introduced into the chamber, water molecules will evaporate from the saturated solution until the characteristic RH is achieved. Conversely, if air of high relative humidity is introduced, the solution will absorb water molecules from the air until that RH is reached.

At the humidity range we were interested in maintaining, 55%-60% RH at 70 degrees F, two appropriate salts were listed in the Merck Index³: sodium bromide and calcium nitrate. Another salt, sodium nitrate, was just above that range. We consulted with John Twilley, Senior Research Chemist at the Los Angeles County Museum of Art, to determine whether either of these salts could present risks to art objects in a closed case. Since bromides have the potential to develop halogens at the vapor stage, sodium bromide was eliminated. We decided upon calcium nitrate as our test salt; it was in the correct humidity range and had been used successfully by the Viking Ship Museum for over thirty years with no ill effect to objects. Simple experiments with calcium nitrate solution are ongoing at the Seattle Art Museum to determine whether volatiles are escaping into the test case. Results will be reported in a future WAAC Newsletter.

1. "Servin Saver"
2. Salt Solution
3. cut-away part of lid
4. Gore-tex attached to outer lid with double-faced mylar tape.

One problem associated with saturated salt solutions in cases is that the salts have a tendency to crystallize and creep up and over the sides of the containers. Poly(vinyl chloride) (PVC) containers can prevent this from occurring, but we decided to avoid them since PVC can evolve hydrochloric acid in elevated humidities. Instead, we developed the following method of containing salts, suggested by conservator Steve Weintraub⁴. We cut away the raised center of the lid of a tightly sealing Rubbermaid "Servin' Saver" plastic food storage container and covered it with Gore-tex fabric, which allows water vapor to pass freely in and out of the container while preventing the salts from creeping out. To maximize the surface area of the solution in relation to the solution volume, we selected low, rectangular containers. Care must be taken when attaching the Gore-tex to the container lid. We attached the Gore-tex on the outside of the lid with double-faced Mylar tape, then turned the lid over and put a continuous bead of hot glue along the rim of the hole. The Rubbermaid container is made from a random copolymer polypropylene, the lid is low density polyethylene, and the Gore-tex is PTFE (polytetrafluoroethylene); all three are inert. We have had no problem with creep with this method.

The solubility of calcium nitrate is approximately 38 g/100 g of water. We added the solids to the water a little at a time and mixed vigorously. When the solution reached the saturation point, salt crystals were present at the bottom of the beaker. A shallow layer of the salt solution was then poured into the Rubbermaid containers, which were placed in the recess normally used for silica gel in an exhibition case. We used 400 ml of salt solution to stabilize the humidity in 1.5 cubic feet of air in our test case. Our tests did not, however, determine the minimum amount of solution required. Holes had been drilled in the art deck to allow air flow between the gel area (now holding calcium nitrate solution) and the area beneath the plexiglass bonnet. Two recording hygrothermographs were set up: one within the case and one adjacent to it which measured the ambient air. Since our test case was not a truly airtight chamber, we expected to see some fluctuation in humidity.

We monitored the test case and the surrounding area for two months. During the first week, the RH inside the test case climbed steadily from 52% to 58%. Whenever the case was opened to change the hygrothermograph chart, the RH would drop, but it would reach and maintain 59%-60% RH by the end of the week. The relative humidity in the room ranged from 49%-65% during the test period. The hygrothermograph chart within the case exhibited a consistent straight line with gradual increases, in comparison to the chart recording ambient air which showed abrupt changes in RH.

To substantiate the data collected in the test case, we decided to do an additional test using two identical cases displaying non-reactive objects on exhibit. When we started the test, the cases, which were situated under lights, were at 42% RH. For one month, we regulated one case with calcium nitrate solution and the other with humidified silica gel. The case conditioned with calcium nitrate quickly responded and climbed to 58% RH by the second week. It was able to hold a 58%-60% RH for the month-long duration of the test. The test case containing silica gel was less successful. We added as many trays of gel as the case would hold, but were unable to raise the RH higher than 48%. Poor case design was partially responsible, as the gel area was too small, but this type of problem is not uncommon when attempting to modify cases to accommodate silica gel. Huge quantities are required to achieve desired results.

In conclusion, in both test situations we were able to maintain RH between 55% and 60% in exhibition cases by using calcium nitrate in a saturated solution. The problem of crystallized salts creeping out of the solution container was controlled effectively by the use of tightly closing containers and Gore- tex. In addition, salt solutions involve less set-up time and less maintenance than silica gel (the Viking Ship Museum adds water to the salt solutions once per year).

We are very optimistic about the potential use of saturated solutions of salts to control RH in museum cases. We intend to continue testing them before using them with sensitive objects. The following salts, listed with their constant humidity at 70 degrees F, might be useful in museum cases: potassium carbonate, 44%; sodium dichromate, 52%; sodium nitrate, 66%; and magnesium nitrate, 56% (at 68° F)⁵

We would be very interested to hear about experiences others have had with salts and especially to hear any reservations you may have about their use in exhibition cases.

Throughout the experiment we had the invaluable assistance of Seattle conservator Patricia Tuttle-Leavengood and the technical advice of John Twilley, Senior Research Chemist at the Los Angeles County Museum of Art.

References:

1. Thomson, G.: The Museum Environment, London: Butterworths, 1986.

2. Astrup, E. E.: "Is It Worthwhile Re-Looking at Salt Solutions as Buffers for Humidity Control of Showcases," ICOM Committee For Conservation, 8th Triennial Meeting. K. Grimstad, ed. Pp. 853- 858, Sydney, 1987.

Astrup, Eva E. and Kristin E. Hovin Stub: "Saturated Salt Solutions for Humidity Control of Showcases--Conditions for a Successful System," ICOM Committee for Cons., 8th Triennial Mtg., K. Grimstad, ed., Dresden, 1990.

3. Merck Index, 10th Edition. Martha Windholz, ed. Pp. misc-103. Rathway, NJ, 1983.

4. Weintraub, Steven: personal communication.

5. Handbook of Physics and Chemistry, "Table of Constant RH Solutions." CRC Press.