Adopted from Trends in microclimate control of museum display cases by Jerry Shiner
An early active microclimate enclosure-In 1938, a well-sealed display case with mechanical humidity control was built by Bill Young to create dryer than ambient conditions for an Egyptian bust in the Boston Museum of Fine Arts. The gallery could not be controlled to the low humidity necessary to protect the object. Young’s showcase used an electric pump (an advance then possible due to the nearly ubiquitous availability of electric power) to move air from the display case past an absorbent compound. With the simple addition of a motorized mechanism to control air flow to a substance that was usually used as a passive buffer, humidity control could now be effectively provided in a sealed display case with a much larger volume than a picture frame. The system was elegantly simple: The entire dehumidifying device was hidden in the plinth below the transparent glass case. Air was drawn from the case by a manually controlled electric pump and then forced through a bed of calcium chloride, removing much of the air’s moisture before the air was re-injected into the upper display case. An interesting feature of the system was a gasometer, which moderated changes in barometric pressure. A similar method (diaphragm bags) is still used on some very tightly sealed cases.
In some circumstances the materials and design of an enclosure create a self-buffering microclimate - the moisture exchange between air trapped in the enclosure and hygroscopic materials remains balanced. This is more common where the ratio of the volume of air to the buffering objects is relatively small and is often apparent in very small enclosures (e.g. a well-sealed picture frames) in smaller display cases with generous amounts of cloth and wood surfaces, and occasionally in larger enclosures too (e.g. plaster walled dioramas filled with stuffed animals and other moisture-holding materials).