The performance of any buffer will be improved by increasing its evaporative surface area. Some salts degrade over time to such an extent that they can emit measurable quantities of gaseous pollutants. This potential has caused conservators to dismiss outright the concept of using any salt solution within a tight case. If the control principle and amount of available water were the same, then one should never chose salt solutions over silica gel. But because saturated salts offer great advantages over silica gel, they deserve further study.
Many salts are quite stable in the crystalline state.
As a further backup, metal coupon testing can be performed prior to installation. At any rate, because of the uncertainty surrounding the permanence of saturated salt solutions, metal coupons should be installed in the finished case and periodically monitored. Two sets of metal coupons should be used per case. One should be placed within the below-deck conditioning space and another within the exhibit space. With these simple precautions, the potential liability of a particular saturated salt solution as a pollution source can be evaluated while being used.