Silica gel is a safe, inexpensive, and useful product to help regulate relative humidity (RH) in an enclosed space. Such a microenvironment is the best tool available for stabilizing corroding metals. Conservation product suppliers, such as Gaylord, Talas, and Conservation Resources, sell several varieties of silica gel.
Silica gel serves two primary purposes for museum collections:
1. RH reduction: Certain materials, especially metals, require environments with low RH in order to preserve them for perpetuity. Whereas 50% RH is the optimal level for a variety of artifacts, closer to 35% is usually best for metals. In order to stabilize corroding metals, they need to be stored in an airtight container; plastic (polypropylene or polyethylene) tubs are often the most convenient for creating microenvironments. With the right amount of silica gel included in the container, RH levels can drop to 15% or less–the necessary range for metal stabilization. This article can help you estimate the amount of silica gel you’ll need for a particular space.
For a good discussion of metals care, see http://www.nps.gov/museum/publications/MHI/Appendix%20O.pdf
Orange-indicating silica gel will be orange in its conditioned form (ready to adsorb moisture) and then turn either dark green or white when it becomes saturated. It will then be time to recondition the gel. Conservation Resources’ guidelines for reconditioning orange-indicating gel call for baking it in a glass pan in a low-heat oven (220-230 degrees F) for at least an hour. A little indicating gel can be mixed with a larger amount of conditioned regular gel as a cost-saving measure. Use a humidity strip or a hygrometer to monitor the humidity within the microenvironment and adjust the amount of silica gel according to the target RH level.
2. RH stabilization: Canisters of silica gel will offset seasonal RH fluctuations as well as more rapid RH-altering events, such as power loss. The gel beads will trap moisture at times of high ambient RH and then release moisture when the RH gets below optimal levels (around 50% for most artifacts). When using silica gel to stabilize, humidity strips or hygrometers are still useful to make sure RH is near target levels and adjust the amount of gel accordingly. With the right amount of gel in place, it should only need to be changed or re-conditioned once each year. The color-indicating feature is less important for stabilization purposes.
For more on preservation environments and monitoring devices, see http://www.nedcc.org/resources/leaflets/2The_Environment/02TemperatureAndHumidity.php