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.