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.