1 INTRODUCTION

CONTROLLING RELATIVE HUMIDITY in exhibit halls at Field Museum of Natural History, Chicago, is difficult. The present building opened in 1921. Like other large museums of this period, Field Museum contains an expansive central hall of approximately one million cubic feet stretching the entire length and height of the building. Large exhibition halls open off this main hall on two levels. All of these public areas are interconnected; only 2 of the 34 exhibit halls are discrete spaces.

Over the past decade, various climate control systems have been installed in different parts of the museum. While they have made the building much more comfortable for staff and visitors, they have had minimal effect on the well-being of the collections on display. The existing climate control systems cannot effectively control the exhibit areas of the museum to the exacting standards required for the protection of moisture-sensitive objects. For example, in the Webber Resource Center, a study hall on the first floor of the museum, the yearly fluctuation of relative humidity ranges between 23% and 77% (fig. 1). The weekly fluctuation is minimally 4% and has been as great as 25%. Relative humidity levels in other halls, while not exactly the same as in the Webber Resource Center, show similar weekly and yearly fluctuations. Such fluctuations in relative humidity have contributed to the warping, cracking, and splitting of many artifacts on display throughout the museum.

Fig. 1. Ambient relative humidity in Webber Resource Center. Each black rectangle shows the weekly fluctuation in the relative humidity.

In 1986, Field Museum embarked on a major renovation of its exhibition areas. The majority of these projects involve the Department of Anthropology's collections of ethnographic and archaeological materials, a large proportion of which are composed of moisture-sensitive materials, including leather, skin, wood, bone, ivory, and fibers of all kinds. These projects involve moving artifacts from the stable environment of climate-controlled storerooms to a fluctuating environment in exhibit halls.