All silica gels are hygroscopic, and respond to the relative humidity (RH) of the surrounding air in the same way as most organic materials such as paper, textiles and wood. Like organic materials, the amount of moisture in silica gel will increase as the RH rises, and will decrease when the RH falls. Unlike organic materials that expand and contract with changes in moisture content, silica gel remains stable. Also, unlike organic materials, silica gel adsorbs and desorbs much larger amounts of moisture when the RH changes under normal conditions.
All exhibition cases have some degree of leakage. If the RH in the air surrounding the case is not the same as the RH in the case, moisture will be transferred between the case and the surrounding air. As the RH in the case increases or decreases, the moisture within the hygroscopic materials of the case will gain or lose moisture in order to remain in equilibrium with the RH of the surrounding air. Since the objective is to prevent a change in the moisture content of museum objects, we add silica gel as a supplement to the case. Since silica gel, if used properly, gains or loses a much greater quantity of moisture than the organic materials on exhibition, the presence of silica gel slows down the rate at which the hygroscopic museum objects gain or lose moisture. This is referred to as the silica gel's buffering capacity.