Volume 15, Number 1, Jan. 1992, pp.19-21

Humidity Control in Cases: Buffered Silica Gel versus Saturated Salt Solutions

Silica Gel Versus Saturated Salt Solution Mechanics¹

Both buffered silica gel and saturated salt solutions can be used to regulate relative humidity in display cases, but these two methods for climate control work on entirely different principles.

To use silica gel, one must designate an acceptable range of RH. Buffered silica gel does not maintain a target RH but instead retards movement away from the RH to which the silica gel is initially conditioned. The RH in a case buffered by silica gel will drift within the preplanned range, in the direction of ambient conditions.

Saturated salt solutions, on the other hand, will supply water vapor to maintain a target RH as long as there is even a small amount of liquid in the tub of solution,² and under humid conditions, they will absorb moisture from the air as long as there is some undissolved salt in the tub.

Silica gel should be seen as a compromiser between new and old RH conditions; saturated salt solutions do not compromise.

Moisture Capacity of Silica Gel vs. Saturated Salt Solutions

We can compare the humidity-control performance of the two buffers by going through the steps involved in buffering a hypothetical display case³. For argument's sake, we will say that a 40%-60% RH range within the case is acceptable, with 50% being the target. In practice, this would mean that reconditioning of the silica gel would not be done until one of the extremes was reached.

A quantity of silica gel will sorb; that is, absorb or desorb, a known amount of water within a particular RH range. Regular density (RD) silica gel can sorb water amounting to approximately 3.5% of its dry weight within the 40%-50% RH range. This is computed by taking the average M-value⁴ over this range and multiplying it by 10 to get the approximate water-sorbing capacity over the range. In the 50%-60% RH range, RD silica gel can sorb approximately 2% of its weight in water. Note that RD silica gel is only about 60% as effective (2 percent/3.5 percent) at buffering RH above our midpoint of 50% compared to its buffering capability in the RH range below the midpoint. This will be important to consider during humid weather.

For our comparison, it is helpful to then convert this percentage to grams of available water per liter of silica gel. We can then directly compare that volume to our saturated salt solutions. Dry RD silica gel weighs 680 grams per liter⁵. Two percent of 680 grams is 13.6 grams. Therefore, between 50% and 60% RH, approximately 13.6 grams of water can be sorbed by each liter of silica gel that we expose within the display case. Similarly, in the 40% to 50% RH range, one liter of RD silica gel will sorb about 23.8 grams of water (3.5% of 680 grams). So, each liter of RD silica gel can provide a total of 37.4 grams of sorbable moisture. It should be remembered that while buffered silica gel contains more water, that water will only move into the air when the ambient RH goes beyond our extremes of 40% and 60% RH. It is irrelevant within this range.

Saturated salts can desorb 100% of their water volume. This could equal 1,000 ml per liter of true solution kept precisely at the saturation point. In practice, we must over-saturate the solution. Salt solids are needed to absorb atmospheric moisture when RH in the case goes above the target level. For argument, we will arbitrarily suppose that each liter of over-saturated salt solution may contain 750 ml of water and 250 ml of solid salt crystals. Let's select magnesium nitrate (MgNO3.6H2O) as the salt because it will maintain a 54% RH target at 70 degrees F⁶ and is a relatively stable salt.

A 250-ml volume of crystalline magnesium nitrate weighs 410 grams. This weight of solid can absorb approximately 160 grams of moisture during humid conditions⁷. So, our example can sorb 910 grams (160 grams + 750 grams) of moisture per liter of saturated magnesium nitrate solution. Unlike silica gel, all this water is available to correct the RH conditions in our case. As the water evaporates, the remaining salt crystals increasingly become a "sponge" capable of absorbing excess humidity.

Comparing the quantity of water that can be sorbed by a liter volume of each of the two buffering systems (910 grams divided by 37.4 grams), we can estimate the relative performance of the two types of buffers over the 40%-60% RH range. If equal volumes of saturated salt solution and RD silica gel were placed in a case, the salt solution would provide approximately 24 times more water for buffering over the 40% to 60% RH range than the silica gel. This translates to very minimal maintenance compared to silica gel.

Estimating the Amount of Silica Gel Buffer Needed

The display case buffer should not need servicing more than once a year. In a northern temperature climate, within a non- humidified building, this means that we should provide enough buffer to prevent desiccation during the winter.

The ideal, air-tight case will not have to be serviced at all, even if only a small amount of silica gel is used. But museum display furniture is never air tight. So we must attempt to predict the leakiness of our cases in order to anticipate our servicing needs, whether using silica gel or saturated salt solutions.

Let's make our hypothetical case a large, upright exhibit case with exterior dimensions of 2 feet in depth, 8 feet in length, and 8 feet in height. Such a case may hold an air volume of about 120 cubic feet or 3.4 cubic meters. Saturated air at 20 degrees C contains about 17.12 grams of water per cubic meter. Air at 50% RH contains 8.6 grams per cubic meter. Our hypothetical case will therefore hold 29.2 grams of water vapor at 50% RH. If this air were completely replaced by 0% RH air, first the case air, and then its buffer, would lose 29.2 grams of water. In the more likely situation, the incoming air contains some water vapor and the buffer will lose its water more slowly. For example, when the incoming air is at 20% RH, the buffer will lose approximately 17.6 grams of water per air change for the above case dimensions⁸. Let's say that we have a 3-month period of dry ambient conditions due to indoor heating; e.g., December through February in a northern temperate climate. A tight case of this size may experience 1 air change per day. This would equal 90 days of losing approximately 17.6 grams of water, for a total of 1,584 grams of water over the whole period. Knowing that we will also have dry conditions before and after the three worst months, we might now add half-again as much to the total. Our dry season moisture loss might be 2,376 grams per case. We know that each liter of silica gel will provide 23.5 grams of water between 50% and 40% RH. So, to make sure that RH does not fall below 40% (to get through the winter without servicing the silica gel), in this size case we will need at least 100 liters of regular-density silica gel. 100 liters is a lot. If we were to pour this into our case bottom (which measures 2 ft. x 8 ft.), it would be nearly 3 inches thick. Buffering in the upper half of the RH range may require even more silica gel, because the silica gel is only 60% as efficient at buffering in humidities above 50%.

A large case like this actually can be expected to undergo more than one air change per day, so this estimate should be taken as optimistic. In actuality, gaskets fail, cracks develop in the construction, and cases periodically are opened by staff.

Estimating the Amount of Saturated Salt Solution Needed

If each liter of saturated salt solution contains 750 ml of water (750 grams), and our dry season need is 2,376 grams, then we might need 3.2 liters of salt solution to regulate RH in a very tight 2 ft. x 8 ft. x 8 ft. case. This would require approximately 10 kilograms of crystalline magnesium nitrate. To cover for the uncertainty in the case leakage rate, we might add half-again as much saturated salt solution. This would bring the total to 4.8 liters of solution per case and require 15.0 kilograms of solid salt.

Whatever quantity is used, it is important to record the starting weight of the saturated salt solution containers. We can then reweigh and calculate the quantity of water lost over time and thus determine more accurately the dry season needs for that case. While our goal is to avoid any servicing of the buffer, it should be understood that servicing desiccated saturated salt is as easy as adding water. The salt crystals formed during the winter increase the buffer's dehumidifying capacity in the summer months. Ideally, we will not consider changes to the quantity of buffer until a full twelve-month cycle has transpired.

Air Circulation

With all humidity buffering systems, we are confronted with the problem of moving the conditioned air to the artifacts on display. The saturated salt solution will actually maintain the RH perfectly only within the tub. That tub air will leak out through the Goretex or Tyvek cover into the air space below the exhibition deck. This below-deck air will then leak through the deck to the exhibition space. Each barrier that the air passes through slows down the response of the buffer to a change in case RH and reduces its RH control accuracy. So, after the opening and closing of an exhibit case, the re-equilibration of its air may take a long time, perhaps several days. Also, the humidity of the air above the deck will fluctuate more widely as the leakiness of the case increases. Whenever the case is opened, the RH balance will be upset. If possible, the below-deck saturated salt chamber should be accessible without opening the above-deck exhibit area.

Embellishments to the Case Design

To a limited extent, the case design can assist with moving the conditioned air. Heat from exhibit lights can inadvertently increase air circulation within a case. Air circulation should be first proven to be a critical problem before any kind of fan is installed. This introduces electricity and the problems of mechanical system failures, even though the mechanical device is only a fan. While a fan may not be necessary to achieve uniform relative humidity in the case, it will definitely increase the response time of the buffer.

An embellishment is to line the below-deck air space with cotton fabric or batting. Cellulosic fabric will quickly lose or gain small amounts of moisture as needed to shorten the response time of the buffer. It is a way of increasing the evaporative surface area near the small tubs in the below-deck area. The performance of any buffer will be improved by increasing its evaporative surface area.

Pollution and Saturated Salt Solutions

Some salts degrade over time to such an extent that they can emit measurable quantities of gaseous pollutants. This potential has caused conservators to dismiss outright the concept of using any salt solution within a tight case. If the control principle and amount of available water were the same, then one should never chose salt solutions over silica gel. But because saturated salts offer great advantages over silica gel, they deserve further study. Many salts are quite stable in the crystalline state. These can be selected and combined with air pollutant scavengers (e.g., buffered tissue) incorporated into the below-deck design if acidic breakdown gasses are even remotely possible. As a further backup, metal coupon testing can be performed prior to installation. At any rate, because of the uncertainty surrounding the permanence of saturated salt solutions, metal coupons should be installed in the finished case and periodically monitored. Two sets of metal coupons should be used per case. One should be placed within the below-deck conditioning space and another within the exhibit space. With these simple precautions, the potential liability of a particular saturated salt solution as a pollution source can be evaluated while being used.

Acknowledgement

The author would like to thank Jane Drake Piechota and Scott Fulton for their helpful comments and criticisms. He wishes to thank Elizabeth Welsh for her tireless editing.

Footnotes

1. This article compares two methods of buffering relative humidity within large exhibit cases. Mechanical climate control systems are not discussed. With proper staffing and monitoring, such systems may also provide reliable RH control.

2. The "tub" here refers to the containers described by Julie Creahan: "Controlling Relative Humidity with Saturated Calcium Nitrate Solutions," WAAC Newsletter, Vol. 13, No. 1, January 1991. This user-friendly container design eliminates hazards commonly associated with saturated salt solutions, such as spillage and salt creep.

3. All calculations in this paper are estimates based on the usual conditions found by the museum conservator. These conditions include a high degree of uncertainty about the exact performance of exhibit case hardware, about the extent of variation of cyclical micro-environments, and about the accuracy of predicting how cases will respond to changes in buffering. In the face of such uncertainty, the conservator is still compelled to act. The calculations are offered as part of the process of critical thinking needed to work through the buffering problem. As such, they have a high margin of error but are not necessarily erroneous. Day-to-day actions taken within the museum environment can never achieve laboratory precision.

4. An M-value is an experimentally determined approximation of the weight of water that can be sorbed by 1 kg of silica gel (or another buffer) when the RH changes 1% in a specified relative humidity range. See S. Weintraub: "Studies on the Behavior of RH within an Exhibition Case. Part I," ICOM Committee for Conservation, 6th Triennial Meeting, Ottawa, 1981.

5. This value is for the commonly available desiccating silica gel grade 03, mesh size on 8. There are many other forms and grades of silica gel, including Artsorb(tm). The Artsorb(tm) form holds up to 5 times as much water but is also 6-8 times more expensive per kg than RD silica gel.

6. Saturated salt solutions maintain slightly different relative humidities at different temperatures. At 76 degrees F, magnesium nitrate will maintain 52% RH. At 65 degrees F, it will maintain 56% RH.

7. More crystals or less water can be added if high humidity is expected to be the more critical case condition. This would tip the balance in favor of providing a better drying buffer at the expense of a humidifying buffer. It will not appreciably change the total quantity of sorbable water per liter of over-saturated salt solution.

8. These calculations assume that the exhibit case is in an unhumidified, heated room during severe winter conditions.

Dennis Piechota is a conservator of objects and textiles. With his wife, Jane Drake Piechota, he consults for museums on environmental conditions and on the movement of museum collections, as well as providing collection treatment.