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The museum visitor perceives displayed objects as part of an unfolding exhibition narrative, and most viewers do not consider the individual object beyond its immediate presence in the current exhibition. A museum is formed around a collection of objects and exists to teach with this collection by exhibiting it; exhibition is an expected fact of museum life. But in the life of an individual object, which is usually stored in a controlled environment, an exhibition is an adventure, something far out of the ordinary and an event fraught with potential danger. It is the job of the conservator, working with the exhibit team, to mitigate the risks inherent in exhibition. To do this, it is first important to identify the main environmental agents involved in object deterioration.
Temperature and Relative Humidity
Temperature and relative humidity are intertwined (as the temperature rises, relative humidity falls), and these linked variables present unique challenges for the exhibit team. The majority of the Museums objects are stored in a carefully controlled climate. The temperature fluctuates by less than 5 degrees (from a set point of 70 degrees Fahrenheit). Similarly, the relative humidity never varies by more than 5 percent (from a set point of 50 percent). But relative humidity and temperature fluctuate frequently in the exhibit galleries as a result of spring and summer rainstorms, sudden heat waves, winter heating, even mopping the floor. Last August the relative humidity in the galleries jumped suddenly, changing by as much as 35 percent. This far exceeds the acceptable +/- 5 percent variation.
Why is this a concern? Because dramatic changes in relative humidity cause dimensional change in artifacts--all of which have some water vapor present in their structure. Sudden drops or jumps in relative humidity can cause warping, splitting, cracking, and checking (formation of a checkerboard crack pattern) in wooden artifacts. Paint layers can separate from substrates as the two retain and release water differently. Seemingly stable ceramics may show sudden salt crystal growth on their surfaces, perhaps even pushing away glaze, as humidity rises, then falls. Unstable alkaline components in glass will dissolve in high humidity, causing the glass to weep. Corrosion on bronze and iron will also accelerate with rising humidity, as chloride ions on the metal surface react with water vapor to form hydrochloric acid, which eats into the metal, causing deep pits and unsightly corrosion.
Temperature, apart from its relation to relative humidity, is somewhat less of a concern for the conservator. High temperatures can accelerate some deterioration, since higher temperatures tend to speed most chemical reactions. Low temperatures are usually quite good for most objects, often contributing favorably to long-term preservation. The difficulty that comes with low temperatures is the increased relative humidity. Cool temperatures in which water is not removed from the air (think about basements) create a wonderful environment for molds and fungi to flourish.
While objects remain in a controlled storage environment, the air is carefully filtered to remove pollutants. In exhibit galleries with or without this filtering capability, pollution can come from many sources. The most familiar is probably exhaust from automobiles, which generates sulfur dioxide. Sulfur dioxide, in and of itself not very harmful, quickly combines with oxygen and water vapor in air to form sulfuric acid, which damages all sorts of artifact materials: limestone, marble, the plaster used in frescoes, paper, cotton, linen, silk, wool, leather, parchment, iron, and silver. But pollution can also come from within the museum, specifically from materials used to create the exhibition and the exhibit cases themselves. Many woods, used to create exhibit cases and structures within the exhibit gallery, release organic acids that cause corrosion on metals. Iron is especially sensitive, but bronze and other metals are also damaged. Many paints also release organic acids as they dry or cure. Fabrics, used to cover exhibit case decks, can release volatile sulfides if they contain wool or silk or if they are dyed with sulfur-based dyes. Volatile sulfides cause quick corrosion of silver and lead artifacts.
The light that allows us to view museum objects can also damage them. Most light damage to artifacts results from one particular type of light--ultraviolet. Both ultraviolet and visible light cause fading of many pigments and dyes, and for this reason it is a good idea to keep the levels of both low in the exhibit gallery. But ultraviolet light also speeds many degradation reactions, contributing to the breakdown and embrittlement of fibers and of cellulosic materials like wood and paper. It can also damage leather and parchment. Light also generates heat, which is expressed as infrared radiation. Lamps that emit a high proportion of infrared light, and thus generate a lot of heat, are dangerous to use inside closed exhibit cases for this reason.
Treating the Objects
The variables described above are the main agents of risk to objects when they leave a controlled storage environment. Fortunately, the conservator and the exhibit team can do many things to limit potential damage to artifacts when they go on exhibit. The artifacts themselves may be treated to make them more stable and better able to withstand the stress of exhibition, and the exhibit environment can be modified in many ways to make it safer for exhibited objects.
When this statue of a female servant (KM 88195), shown here before and after conservation, was examined before its installation in Individual and Society in Ancient Egypt, a thick layer of dark brown wax was found on its surface. The wax, which was greasy and smelled a bit like shoe polish, was probably applied by an antiquities dealer to mimic either the look of a darker, more exotic wood or the oxidation and aging of wood. After consultation with curators Janet Richards and Terry Wilfong, the wax was carefully removed, primarily by Lori Khatchadourian, assisted by Candace Mui.
For instance, some of the bronze figurines in the Kelseys current special exhibition, Individual and Society in Ancient Egypt, had visible corrosion problems. These bronzes were tested to determine whether their corrosion products contained chloride ions, which would create the potential for further corrosion to occur in a humid environment. If the corrosion was found to contain chloride, the bronzes were cleaned to remove this corrosion and treated with a corrosion inhibitor. A small silver cup (KM 3727) in the exhibition was cleaned to remove a thick layer of silver sulfide tarnish, and once the silver surface was polished to an acceptable level, it was coated with an acrylic resin to help protect the surface from further tarnish during exhibition.
Manipulating the Environment
The exhibit environment can also be manipulated in many ways to make it safer. Temperature at the Kelsey is controlled by the universitywide heating system in the winter, and, in our special exhibition gallery, with a window-unit air conditioner in the summer. Relative humidity control is a greater challenge for the Kelsey, but, when needed, stand-alone humidifiers or dehumidifiers can be used. Additionally it is possible to create micro-climates inside exhibit cases. Silica gel can be conditioned to a specific relative humidity and placed within the case. The case must be well sealed so that air does not leak out or in, but once this is accomplished, the silica gel will release or retain water so that the case interior always maintains the intended relative humidity. Pollutants can also be mitigated in creative ways. Special papers, coated with activated carbon and/or what are called molecular sieves can be placed inside cases to remove pollutants and volatile acids. Woods used in case and exhibit construction can be selected based on how much acid they are known to release. Woods used in exhibit cases can also be sealed with vapor barriers to prevent volatile acids from entering the case environment. Tests can be performed on fabrics and other materials used inside cases to make sure that they do not release harmful chemicals. The safest latex paints can be chosen for the gallery and applied early, so that the paint has ample time to dry and cure before objects are installed. Light can also be controlled so that it presents the least risk possible. Years of research on pigment fading and light-induced deterioration have given the field of exhibit conservation useful standards to determine safe light levels for different materials. Exhibit light levels can be adjusted by direct dimming when available, the use of screens, or judicious pointing of the light fixtures. Heat from lamps can be vented as part of the fixture design, and cool lamps can be chosen for use inside cases.
The Kelsey Museum employs all of the methods described above to keep our artifacts safe while on exhibit. The entire exhibit team works together to choose good materials for exhibit construction and to employ innovative design ideas for protective cases and lighting. Conservation for an exhibition can be challenging, but it is always an interesting project and one that good teamwork makes pleasant and rewarding.