Air Circulation
With all humidity buffering systems, we are confronted with the problem of moving the conditioned air to the artifacts on display. The saturated salt solution will actually maintain the RH perfectly only within the tub. That tub air will leak out through the Goretex or Tyvek cover into the air space below the exhibition deck. This below-deck air will then leak through the deck to the exhibition space. Each barrier that the air passes through slows down the response of the buffer to a change in case RH and reduces its RH control accuracy. So, after the opening and closing of an exhibit case, the re-equilibration of its air may take a long time, perhaps several days. Also, the humidity of the air above the deck will fluctuate more widely as the leakiness of the case increases. Whenever the case is opened, the RH balance will be upset. If possible, the below-deck saturated salt chamber should be accessible without opening the above-deck exhibit area.
Embellishments to the Case Design
To a limited extent, the case design can assist with moving the conditioned air. Heat from exhibit lights can inadvertently increase air circulation within a case. Air circulation should be first proven to be a critical problem before any kind of fan is installed. This introduces electricity and the problems of mechanical system failures, even though the mechanical device is only a fan. While a fan may not be necessary to achieve uniform relative humidity in the case, it will definitely increase the response time of the buffer.
An embellishment is to line the below-deck air space with cotton fabric or batting. Cellulosic fabric will quickly lose or gain small amounts of moisture as needed to shorten the response time of the buffer. It is a way of increasing the evaporative surface area near the small tubs in the below-deck area. The performance of any buffer will be improved by increasing its evaporative surface area.
Pollution and Saturated Salt Solutions
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. These can be selected and combined with air pollutant scavengers (e.g., buffered tissue) incorporated into the below-deck design if acidic breakdown gasses are even remotely possible. 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.