THE DEVELOPMENT OF THE HUMIDITY CONTROL MODULE AT FIELD MUSEUM
THE MODULE in the Webber Resource Center has been in operation now for more than four years. Since the switch to the steam evaporator system in May 1988, the relative humidity in its two cases has been consistently maintained at 50 � 2%, excluding the periods with mechanical problems mentioned above. Including these periods, the module has maintained an average of 48% RH. Although there has been some fluctuation within the cases, it has not been as great as that in the hall itself during the same period (fig. 9). All of the artifacts in the cases are in extremely good condition, and since they have been in this exhibit no signs of dimensional instability have been observed that might result from humidity fluctuations. Following the installation of deionizers to the modules in the Egyptian exhibit in June 1989, the desired levels of 42 � 2% have been maintained.
Comparison of ambient and north case relative humidity levels in Webber REsource Center
While the above description may suggest that a break-in period is necessary, the modifications to the module described in this paper have eliminated the necessity for such a period. In August 1989 and September 1990, five modified modules were installed in Field Museum's new Pacific exhibit. These modules have maintained the relative humidity in the cases at the set 50% level from the time they were installed. Nevertheless, for the first time user, a break-in period is recommended.
It is also clear that if the water supply is hard, it will be necessary to use deionizers with the humidity modules.
In summary, we are very pleased with the performance to date of the humidity module. The staff of the Division of Conservation believes it has found a workable solution to the problem of displaying moisture-sensitive materials in uncontrolled exhibition areas and is making a long-term commitment to the use of more modules.