In March, 2020, and then again in November, the Oregon Historical Society (OHS), like many similar institutions around the world, closed its doors to the public to slow the spread of the COVID-19 virus. Those closures forced OHS’s exhibit production team to temporarily set aside some of our exhibit-related projects, which was disappointing. That pause, however, allowed us to refocus our energies on long-term goals, such as reviewing OHS’s display case inventory and improving production methods to further align with museum best practices. In 2020 and 2021, we used the time typically spent planning for, building out, and installing exhibitions, to build what we call climate cases — object display cases that help control relative humidity (RH).
Museum displays always involve compromises; OHS’s mission is to both make our state’s history accessible and to preserve the collections that inspire curiosity about all the people, places, and events that have shaped Oregon. Whenever an object is displayed in a gallery for any length of time there is a risk of damage from the surrounding elements: light, temperature, humidity, and sometimes even the construction materials surrounding the object in a case. Over the years, conservators, curators, and collection and exhibition staff have devised myriad methods and protocols to minimize these risks to the greatest extent possible. One of these protocols is to modify traditional display-case building methods to better protect encased objects within.
The type of case most museum visitors are familiar with is a simple pedestal case: a structure sitting on the floor, usually made of wood or metal, topped by a deck on which objects sit, encased by a glass or plexiglass vitrine. This type of case provides security for the objects — protecting them from dirt, touch and theft — but they are still exposed to the atmosphere of the surrounding gallery, which may or may not be suitable. Further protecting objects sometimes requires separating the interior of the case from the gallery environment outside. Hence, the climate case.
When properly designed and monitored, a climate case creates and maintains a microclimate where relative humidity (RH) can be set to an optimal range for the objects displayed inside. Different materials are affected by humidity in different ways and to different degrees. For example, high RH can cause corrosion in metals, while low RH can make paper or ivory or wood brittle and prone to cracking. Fluctuating RH, in and of itself, can cause damage due to swelling and contraction. So, when designing this type of case, museum professionals aim to establish the appropriate RH level for its varying contents and, once achieved, to stabilize that level over time.
There are many ways to do this, both active (mechanical) and passive (using buffering materials that absorb and release moisture), and there are many factors that determine which approach to use. Generally, an active method is appropriate for large enclosures that require RH to be maintained over a long period of time. OHS’s display cases tend to be smaller, and we follow a passive approach employing the most common buffer used in museums, silica gel. While readers are probably familiar with silica as a desiccant or drying agent, when conditioned properly it can also be used to release moisture into an environment and hold the appropriate level of RH for a set amount of time. When the gel is no longer effective, it can be removed from the case and re-conditioned.
In order to maintain this microclimate, the interior of the case must be completely sealed using a vapor-proof protective barrier, both in the chamber beneath the deck, where the silica gel cartridges are kept, and within the vitrine above, where the artifacts are visible. This prevents air leakage and the surrounding case materials absorbing moisture. The vapor-proof barrier also has the added benefit of preventing the case materials off-gassing harmful pollutants, which could be trapped inside and potentially damage the objects. The various barriers that can be used range from epoxy-based paints to varathanes to applied films. We use a combination of a material called Marvelseal, applied to the climate chamber, and an appropriate primer and sealer for the visible areas above.
Once sealed, the key factor in designing a climate case is to allow for the exchange of air between the climate chamber and the interior of the vitrine. In a normal case, the display deck sits flush on the structure below. In a climate case, however, we provide a large opening in a subdeck and raise the main deck above that, leaving a substantial gap for circulation. This allows the chamber’s RH environment to creep up into the vitrine and, after a certain amount of time, to reach equilibrium throughout. At this point, the microclimate stabilizes for as long as the silica gel remains effective. An outside hatch is provided in the body of the case to allow for replacement of the silica gel cartridges without disturbing the artifacts above.
When designed and constructed properly, a climate case provides the flexibility to create appropriate RH environments for the range of objects OHS displays in its exhibits. In combination with the many other strategies OHS employs to mitigate the harmful effects of the environment, the ability to draw from a substantial inventory of these new and improved cases further allows us to unite the two vital strands of our mission: education and preservation. As OHS has reopened to the public on March 6, 2021, with limited weekend hours, the exhibit production team will once again shift its focus to designing and installing exhibitions, and hopefully readers will soon be able to explore the galleries with a better understanding of what we do to protect our collection. Be sure to look for a case with a hatch and then observe the artifacts inside closely. Why do these particular objects require a climate case? What are they made of? What kind of climate might they require?
We’ve included a video in this post to provide more details about the actual building of a climate case. If readers want to delve into the science of the matter, I recommend the links from the National Park Service and the American Institute for Conservation listed in the sources list at the end of the post.
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