Local History

Light

This article originally appeared in Exchange, a newsletter published by the Wisconsin Historical Society. (Volume 23, Number 6, November/December 1981) It is the second in a series of articles titled Conservation Corner. The series features information about maintaining an adequate environment for the storage and exhibition of historical collections, employing proper collection care techniques, and recording and cataloging historical collections. This article was written by Tom McKay, retired local history coordinator for the Wisconsin Historical Society. 

In providing a proper environment for museum collections, no element is inherently more contradictory than light. With a few exceptions, the preservation of most museum collections would be enhanced if they were not exposed to light at all. Yet, it is obvious that a museum object which never sees light is of no use at all. Caught in this contradiction, museums must strive not to eliminate but to control the exposure of collections to light.

The three most common sources of light in museums are daylight, fluorescent tubes, and incandescent bulbs. While all three damage museum collections, they are listed in the order of the danger they present. In attempting to minimize their detrimental effects, a museum must be concerned with the light source, its intensity, and the length of an object's exposure to light. Remember that exposure to intense light for a short period or exposure to lower light levels for a long period can be equally damaging.

The primary detrimental effects of light are photochemical deterioration — fading is a common example — and heat build-up. Photochemical deterioration will occur in any organic material including wood, paper, fabric, dyes, leather, etc. These materials comprise the largest portion of most local history collections. Photochemical deterioration causes colors to fade, varnish and paint to check, wood to bleach, fabric to "sun rot"; the list could go on. Ultraviolet light is the most active agent of such damage. However, ultraviolet light is not, as is often perceived, the only cause of photochemical deterioration. Think of light as refracted through a prism. The segments of the spectrum from the green down through the blue and violet also cause photochemical deterioration. From the other end of the spectrum — yellow, red, and infrared — comes the strongest heat build-up. Heat build-up causes dimensional change and stress, accelerates chemical reactions, and softens finishes such as wax and varnish.

Considering the litany of damage above, control of light in the museum environment may seem hopelessly depressing. Nonetheless, some basic information and procedures can decrease light damage enormously. Daylight is the most dangerous light source because it is the most intense. On a sunny day, daylight is 200 times more intense than the illumination from artificial lights in a well-lit room. Fluorescent light is a concern because it yields high amounts of ultraviolet light in comparison to other artificial light sources. The most immediate danger of incandescent light is heat build-up.

First and foremost, beware of daylight. Direct sunlight is the worst culprit, but even northlight or daylight filtered through ultraviolet shields will cause more photochemical deterioration than unshielded fluorescent lights. Daylight is that intense. Heat build-up in objects in direct sunlight can climb far in excess of 100 degrees Fahrenheit. The only defense against daylight is to block it out. Many local history museums cannot simply block up all their windows because they occupy historic buildings. In such cases, start by doing what the historic occupants did: close blinds and drapes to reduce direct sunlight. As a further step, close all blinds, drapes or shades when the museum is closed or when a particular room is not in use. If you have police patrols that prefer to see in windows, make covers for cases, furniture, and mannequins to block sunlight when the museum is not in use. Cloth covers should be made of white sheets, white flannel or unbleached muslin to avoid heat build-up.

Blocking the intensity of daylight is followed in priority by limiting the length of exposure to all light sources. Temporary exhibits are an ally. Objects that are not currently on exhibit can be kept in darkened storage rooms and cabinets. Changing the objects on exhibit will extend their life accordingly. If you have a sunny and cheerful room, put your office, not your exhibits, there. Plan long-term exhibits in the areas with the lowest amount of daylight. Never put wood pulp paper, fabrics, colored prints or watercolors on permanent exhibit.

The next priority is to manage artificial light sources carefully. To avoid heat build-up, place objects no closer to light sources than is necessary for adequate illumination. If incandescent bulbs must be placed in cases, vent the tops of the cases. Plastic sleeves are available for fluorescent tubes to filter ultraviolet light. If the museum can afford the investment, ultraviolet shielding plastic or varnish can also be applied to windows. Check with your nearest glass distributor or try one of the following sources.

Remember that ultraviolet shielding materials gradually lose their effectiveness and require replacement. The lowest practical levels of light are achieved when light is directed toward objects rather than the areas where visitors stand. Finally, remember to turn off all lights when the museum is closed or an area is not in use.

For a museum to function, light is a necessary part of the environment. At the same time, it is an insidious enemy. The damage it causes is imperceptible on a day-to-day basis but is dramatic and nearly always irreversible if it continues unabated. It is another element of the museum's environment that requires regular procedures and constant vigilance to extend the life of the museum's collections.