The Salon du Tapis d'Orient is a moderated discussion group in the manner of the 19th century salon devoted to oriental rugs and textiles and all aspects of their appreciation. Please include your full name and e-mail address in your posting.
by Sara Wolf
Most rug and textile collectors have had experiences with museums and have developed views and theories
about them. They also often have questions about some of the standards rug museums have and the strictures they
impose. Conservation is one area in which such questions often arise. It struck me that instead of simply commiserating
with one another about what we often see as arbitrary, even seemingly benighted standards, that we might ask a
professional conservator to talk to us about conservation measures and the rationales on which they are based.
Such an exchange might in fact provide a bit of light rather than just the usual heat. So I asked Sara Wolf, who
holds the Margaret Wing Dodge Chair in Conservation at Washington's Textile Museum if she would be willing to host
a salon in which she engaged in such an exchange with us. Sara, has generously, perhaps bravely, agreed and she
is our host for this salon on rug conservation. Her opening essay focuses on light standards but she is prepared
to deal with other issues of rug conservation that might interest us.
R. John Howe
John Howe has asked me to explain some of the apparently arbitrary behavior of museums with regard to things
like light levels. I should begin by saying that regardless of the explanation, not everyone is going to be happy
with the answer. In fact, the simple answer for this topic is, "you have to draw the line somewhere,"
and that is, by its very nature, an arbitrary point. However, believe it or not, the location of the line is, in
fact, carefully thought out, and by no means capricious. And now for the science behind the "line."
For a chemical reaction (i.e., deterioration) to occur, some amount of energy must be supplied. In the display environment, this can be heat or light. The shorter the wavelength of the light, the greater the energy supplied, and the more readily the chemical reaction takes place. We're all familiar with the effects of the shorter (more energetic) wavelengths of the light spectrum on our skin. It is the ultraviolet range of the spectrum that also causes the fastest color change in textiles. However, the visible violet through red light ranges also causes damage, albeit more slowly.
We are most familiar with light deterioration in the form of color change. What happens when light interacts on a molecular level with any organic material is that the light energy eventually causes a break somewhere in the molecular structure. Since the color you perceive is dependent upon a specific molecular structure, we see that "broken" structure as color change. That change is not only fading, but sometimes darkening, and in other instances, yellowing. In addition, the light energy is not only interacting with the molecules of colors (dyes), but with the fibers themselves, causing weakening and embrittlement.
So, all light is bad, and the shorter wavelengths (ultraviolet) are worse. How do we cope? We control the light sources to eliminate the shorter part of the spectrum, and limit exposure to slow the rate of change. What is most difficult, is that virtually every material reacts somewhat differently to light exposure. We have more to consider than a simple fiber and a dye-stuff. There are mordants, impurities, dirt, air pollution, chemical treatments, the impact of use, insects, and on and on that can influence the rate of deterioration of an individual object, and thus, that object's ability to withstand the impact of light.
In 1978, Gary Thompson, Scientific Advisor at The National Gallery in London, published The Museum Environment. Having conducted an exhaustive review of the literature on light deterioration, he developed a set of standards for light exposure. For the most sensitive materials (of which textiles, paper, watercolor, natural history specimens, wallpaper and dyed leather are included), he set the maximum exposure at 50 lux, or 5 footcandles. This number is an average of what might be considered appropriate for a range of materials. Further, the length of light exposure needs to be considered so that deterioration is minimized.
Because the 50 lux level is an average, it certainly could be considered arbitrary. However, because there is no way to test each individual object to develop a deterioration index, it is the line that most museums have chosen to draw. At this level, the objects should be completely visible, labels easy to read, and the color rendering quite good. It should be, but often is not. For that part of the discussion we get into quality of light bulbs, placement of lighting, and the colors surrounding the objects you are looking at. Lighting design is as much a science as an art, and to be done well, it is often quite expensive. So, if you can't see something well in an exhibit, it is apt to be as much a function of the lighting design as the actual light level.
There has been a good deal of research done on museum lighting in the past 10 years, and on the rate of fading of different dyes. Research done by Deborah Bede when she was the conservator at the Museum of Fine Arts in Boston, showed that even the light exposure recommended by the museum field is too much for many textiles. She took readings with a colorimeter (measures very small changes in color) throughout the course of a 3-month textile exhibition, and found that nearly every textile and color measured underwent some kind of change. Granted, some were very small changes, but alert us to the fact that even with standards, we are not eliminating deterioration.
Since change is a factor of both light levels (brightness, range of spectrum) and duration of exposure, one might reasonably ask if exhibitions could be of shorter duration with a higher light exposure. It sounds like a reasonable solution, but light level and duration don't work together in perfect proportion. And, there also is the factor of the time it takes to prepare an exhibition and install it (which naturally determines, in part, how long it stays up on the walls).
The bottom line is that light causes irreversible damage, and because a museum has an obligation to the preservation of objects over centuries, policies must be implemented that balance this responsibility with the equally important requirement to make those objects available and accessible to the current viewing public.