Silica gel can be used to remove moisture from the air, or as a humidity-buffering agent to maintain or buffer the relative humidity (RH) to a constant level. In either case, silica gel may need to be conditioned before use or before reusing.
Silica gel comes in two forms: indicating and non-indicating. Both can be dried in a regular oven, not a microwave, at 250°F (120°C). The amount of time needed to dry gel can vary, but if spread out in a single layer on a baking sheet, one hour should be a sufficient amount of time to thoroughly dry gel. Some silica gel comes in containers that may not be oven-safe. Unless otherwise indicated on the packaging, remove the silica gel from the container for drying and replace once dried.
The simplest way to condition silica gel to a higher RH is to enclose it in a sealed container or zip lock bag with a source of water—a dish of water or a soaked sponge—making sure it doesn’t come in contact with the water. Leave for one day to equilibrate. To test the RH of silica gel, enclose a sample of the conditioned silica gel in a sealed container or plastic bag with a hygrometer for one day and then take a reading.
A technical bulletin on silica gel is available for purchase or view the Art Preservation Service's "Methods for Reconditioning Silica Gel" (PDF).