The last half of the 20th century saw the widespread acceptance and application of environmental guidelines designed to protect museum collections. In recent decades, these guidelines—which delineate standards for temperature, humidity, lighting, and other environmental factors—have provided unprecedented stability for the environments of many museums, reducing the danger of damage to objects in their collections. Guidelines exist to enforce consensus, codify experience, and distill large amounts of technical information into general practice, reproducible simplicity, and rational institutional policy. Yet for guidelines to remain vital and useful, they must be periodically scrutinized against evolving knowledge and changing practices. Without regular examination, guidelines suffer from a creeping obsolescence.
Over the past decade, most preventive conservation guidelines—including those that apply to relative humidity (RH), temperature, and air pollution exposure limits—have been reviewed and, gratifyingly, have held up surprisingly well. These periodic reviews have resulted in increased flexibility in building operations and maintenance and in exhibitions display. At the same time, however, they often require greater attention to detail. Single and simple measurements are replaced with precise monitoring and record keeping.
If one chooses to operate near the limits of acceptable environmental standards—or is compelled to operate in this manner because of location in a historical building—the probability of damage to objects increases, making safe control a demanding occupation. Nevertheless, some researchers have developed a theoretical basis for suggesting that the risks can be managed consciously. For instance, at one time the limits for RH within a museum’s environment were very narrow and ensured that no RH-induced damage was possible—at a significant financial cost, of course. However, theoretical and experimental groundwork has indicated that if the drift is slow enough, reasonable safety might be possible over a much wider RH range, so that a slow seasonal oscillation could replace rigid limits. In other words, while fast change is bad, slow change may not be so bad—it may even be acceptable.
Another area where risks can probably be more effectively managed is museum lighting. It has long been clearly understood that, over time, uncontrolled lighting leads to damage, including fading on objects. As a result, the means to control light through restrictive exposures was sought. Not too long ago, the ”law” on illumination levels was strict and absolute. These exposure levels were deduced from a paucity of data, starting from about 1930, by several authors whose judgments were later reinforced by subsequent research. In 1961, when Garry Thomson, then scientific advisor at the National Gallery in London, suggested the now well-known illumination limits of 50/150/300 lux for objects of varying light sensitivity, he was actually averaging the recommendations of earlier researchers. In fact, 50 lux—a dark environment indeed—will still fade light-sensitive colorants and effect other color changes if given ample time. Although present at the start, the question of how long 50 lux could be tolerated was not given the same emphasis it gets today. With current ideas of risk management gaining greater interest, conservators and curators have had to think about how much damage over how long a period of time is acceptable. Thus, preventive conservation lighting standards have undergone a slow revolution.
Changes in thinking are also a result of changing technologies. One only needs to recall that fluorescent lamps had been commercially available for just 15 years before they were first suggested—in a 1953 International Council of Museums publication—as possible low-heat alternatives to incandescent lighting in some limited applications. No doubt they had been used before that time. Today we have many more lamp and fixture designs for track lighting, fiberoptics, and, perhaps soon, novel light-emitting diode (LED) alternatives.
Making lighting safer for sensitive artifacts at constant illumination has been the subject of recent study. In a demonstration project published in 2010 in the Journal of the American Institute for Conservation, Christopher Cuttle, now at the University of Auckland, used several 50-watt quartz-halogen lamps, filtered to the color-matching functions of the human eye, to approximate the color matching of unfiltered lighting. This research resulted in a major shift in envisioning museum lighting. Using three bands of colored light instead of one monochromatic light source reduced energy at certain wavelengths not essential for reasonable human visual color matching. As part of Cuttle’s research, an assessment by 16 observers noted that the differences between standard quartz-halogen lighting at 50 lux and three-band filtered quartz-halogen at the same illumination were slight, yet the three-band lighting significantly reduced the energy delivered to the surface of the object. This type of lighting will probably inflict less photochemical damage at equal illumination and duration.
Cuttle’s promising research helped precipitate a two-day experts meeting on museum lighting at the Getty Center (see Conservation, vol.18, no.1). The meeting, hosted by the GCI in October 2002, addressed a series of questions involving the lighting of old master drawings. Participants came from Canada, England, New Zealand, and the United States. They included conservators, conservation scientists, curators, and lighting engineers.
From the discussions at this meeting, it was evident that there were eight major lighting strategies that could improve the display lifetime of works of art on paper, such as old master drawings. Four of these strategies constitute the core of existing guidelines: reduce illumination levels of existing sources; interrupt illumination through the use of switches and motion detectors; remove ultraviolet and infrared radiation; and spread out exhibition display periods over many years, using assumptions about the most fugitive component to set total exposure amounts. (Monitoring actual color change on artifacts could be added to this core group, but it is quite rarely carried out in practice.)
Beyond those four ideas are four other strategies that have thus far received less attention. These strategies include using new light sources such as LEDs with intrinsic three-band character; using filters designed to emulate the three-band concept on existing lamp architecture; investigating further the benefits of anoxic environments on reduced photochemical potential; and increasing the use of risk management methodologies with radiometric rather than photometric monitoring techniques. With input from the Getty Center meeting participants, the GCI decided to pursue these four possibilities as a set of activities that together define a research program.
Shepherding new light sources, such as LEDs, to destinations in museums, libraries, or galleries—along with testing visitor response to new lighting—will be increasingly valuable. Well-designed visitor testing has benefits that include not only the evaluation of aesthetic appropriateness of a new light but also a chance to test sensitive issues like age-reduced viewer visual acuity at low illumination levels. The GCI will begin research and testing in this area at the end of 2004.
A second activity for the research program capitalizes on the fact that the human eye is a poor judge of the relative energy of two equally bright but different sources. While the same object equally illuminated by daylight and incandescent light fades at different rates, the less destructive source may be as acceptable for viewing as the more destructive one. Thus, the strategy of retrofitting hardware like track-installed, quartz-halogen fixtures or fiberoptic illuminators to provide acceptable color rendering for the human eye at reduced overall irradiation (energy) can be pursued on two fronts. One front is to assemble filter packets from available products that achieve the desired goal; the other is to design a single glass filter and manufacture it. The former approach has the benefit of lower initial research costs and the potential for off-the-shelf filters with fewer long-term manufacturing support uncertainties. The latter approach can be more energy efficient and provide a closer match to the spectral reflectance characteristics of illuminated artifacts. The GCI is researching both fronts—one with the Los Angeles County Museum of Art (LACMA), and the other under contract to the University of Texas, El Paso. It is anticipated that for both projects, external groups will verify that the filters achieve a reduction in light damage, all else being equal.
The third strategy to achieve safer, longer display lifetime is to examine oxygen-free microenclosures, assessing their benefits and liabilities. Most, but not all, photochemically damaging processes involve oxygen in one of two fundamental ways. Remove oxygen and those paths are theoretically blocked—and the absorbed energy is dissipated by a safer route. Unfortunately, oxygen is not always needed for photochemically damaging processes, and some important colorants used in artworks have been shown to be susceptible to change even in the absence of oxygen. Such anoxic light-induced change is termed photoreduction, and its extent in museum artifacts is not known. Nor is it known to what extent those photoreducing components can be detected in advance in individual objects, which can then be excluded from such environments. Clearly a large screening study is in order. Also needed are techniques to make the construction of atmosphere-controlled encapsulations practical and inexpensive at the level of individually framed works. Some of these techniques have been worked through at Tate Conservation Department in Britain, with support from the Liverhulme Trust. The GCI is in discussion with other institutions regarding systematic materials screening under anoxic atmospheres.
Finally, considering altering the emission spectra of exhibition lighting or adopting new light sources altogether suggests that it is time to improve the basic manner in which light monitoring is carried out. In the past, conservators have been content with measuring lux or footcandles. For a variety of good reasons, this was an acceptable practice. But better management demands better tools. When the spectrum of an incandescent lamp is altered, measuring illumination based upon the human eye’s sensitivity loses relevance. It would be best to measure the incident energy for the same perceived level of brightness. Energy units are not new in conservation—specifications based on the number of microwatts per lumen of allowable ultraviolet light have been around for as long as footcandles. But in the absence of a need, or a desire, to measure energy directly, rebuttable presumptions about energy levels have replaced direct measurements. The GCI and LACMA are pursuing this research into monitoring.
With all of these research objectives in mind, the GCI, along with its partners, hopes ultimately to provide museums and libraries greater flexibility in extending the display lifetimes of their light-sensitive artifacts. This achievement, in turn, will better facilitate all the functions of modern museums, whose stewardship calls on them to preserve, display, and educate.
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