4.2.4 Creating microclimates in cabinets and cases

A microclimate is any variation from the prevailing temperature and relative humidity of the surrounding environment. It may be either negative or positive in its effects and may occur unsolicited or be artificially induced and maintained.

It may, at times, be necessary to create a microclimate within the larger building environment. This could be occasioned by the nature of the materials, the necessity of protecting valuable items, or by the desire to remove them from a controlled environment and exhibit them in one that is uncontrolled. Microfilm, maps and documents stored in file cabinets are obvious candidates for microclimates in high humidity environments. While the incidence of mold growth can be reduced in the collection as a whole through improved circulation, the closed metal cabinets designed for the storage of microfilm, maps and documents tend to retain moisture, especially if they are not frequently used. By artificially lowering their interior relative humidity a beneficial microclimate can be maintained.

A reduced humidity microclimate can be establised in a closed storage cabinet through the use of dessicants which abssorb moisture from the air. There are a number of products which can be used as dessicants. Two of the most readily available are silica gel (which is available in various grades) and is widely used in the U.S. and Europe, and Nikka pellets (also called Kaken Gel) which is used in Japan and the Far East. Nikka pellets have been found to be more effective than silica gel at humidities above 60%.¹² Silica gel often includes a color indicator which turns from blue to pink as moisture is absorbed and indicates when the material has reached its maximum absorption and must be reconditioned.

Before use, the dessicant must be conditioned to 0% relative humidity. This is done by heating the material in an oven to drive off moisture. The pellets or crystals can be reconditioned and used many times without losing their absorption capacity. After conditioning, the dessicant may be placed in the cabinets, either in trays in the base, or in small cloth bags in individual drawers. If the dessicant used is not a color indicator, a hygrometer or indicator strips must be placed in the cabinet to indicate when reconditioning is necessary. Once the cabinet reaches the desired humidity, and an equilibrium has been reached, the dessicant will require reconditioning less often. If the cabinets are used frequently, reconditioning may continue to be required at frequent intervals. The larger the quantity of dessicant used, the longer the microclimate can be maintained before reconditioning is necessary.

There is a great deal of literature available on the creation of microclimates, much of it dealing with the installation of exhibit cases and the packing and shipping of works of art, but virtually all of it is relevant to the control of environments in closed storage cases or other fixed locations. Stolow's recent publication¹³ contains information on state-of-the-art microclimates.