Silica gel has been used to control relative humidity in exhibition cases for several decades. One of the major drawbacks that has limited the use of silica gel for this purpose is the difficulty involved in reconditioning it. Traditionally, the entire load of silica gel is removed from the case and placed in a chamber or room held at the proper relative humidity. Conditioning the gel requires time and good air flow.
It is possible to recondition silica gel within the exhibition case without having to remove it. This can be accomplished by adding water or dry silica gel, referred to here as desiccating gel, to the case. If the water or dry gel is placed in the silica gel compartment of the case, the evaporating water or dessicating gel will initially affect the relative humidity in the silica gel compartment. In order to gradually and safely recondition the permanent supply of silica gel and the overall exhibition case humidity, the silica gel should be spread thinly within the compartment to maximize surface area, and the surface area of the water or desiccating gel must be designed to limit its speed of evaporation or adsorption. In this way, the increase or decrease in RH brought about by the water or desiccating gel will principally effect the silica gel compartment and its contents.
Because of the much larger surface area of the permanent silica gel supply, this gel will quickly adsorb or desorb the change in moisture brought about by the addition of the conditioning water or gel. As a result, the RH within the display portion of the exhibition case will gradually increase or decrease based on the extent that the permanent silica gel is conditioned, and will not have a rapid change in RH that might otherwise occur with the uncontrolled and rapid evaporation of water or adsorption from the desiccating gel.
In order to determine how much water or desiccating gel to add, it is necessary to calculate approximately how much water must be added or removed from the case in order to recondition it to a desired RH level. By knowing the amount of permanent silica gel in the case, it is easy to calculate how much water must be added or subtracted to recondition the gel, based on the moisture content/relative humidity relationship of the gel. If there is a great deal of organic material in the case, additional water or desiccating gel can be used to compensate for the moisture content requirements of these materials. Sample calculations are discussed to illustrate this method.