MONITORING THE FADING AND STAINING OF COLOR PHOTOGRAPHIC PRINTS
PEOPLE OFTEN WANT TO display original color photographic prints in a museum, gallery, or archive; unfortunately, however, such display exposes the prints to light which will cause gradual image deterioration in the form of fading and changes in overall color balance. In addition, many types of color prints are subject to stain formation. Such stains are most readily visible in the lighter areas of a print and are usually yellow; with a number of common color print materials, stain formation over a period of time may be the most obvious type of image deterioration. Compared with most other types of artistic media, color photographs generally fade and/or stain fairly rapidly when displayed, and many types of color prints are quite unstable even when kept in the dark.
Changes which take place in dark storage can usually be arrested only by placing the photographs in low-temperature and low-humidity storage. At normal room temperatures, dark storage changes (often referred to as dark-fading)1 will continue whether or not a print is on display. Thus, when a print is displayed, the total change that takes place is some combination of dark-fading staining and light-fading/staining.
Light-fading is caused by both visible light and ultraviolet radiation. For most types of color prints displayed in normal museum display conditions, image deterioration caused by ultraviolet radiation is much less significant than changes caused by visible light and storage at room temperature.
Museum display conditions for color photographs vary widely from one museum to another and even within a given institution. Typically, color prints are illuminated with tungsten lamps of about 2,800–3,200°K; intensity on the print surface is about 130–325 lux (12–30 footcandles) for about 10 hours per day. The light is normally filtered by the glass sheet used in framing the prints. Low illumination levels of about 50 lux (5 footcandles) have been suggested for display of color photographs; however, the author believes that this level is too low for proper viewing of most color photographs. Shorter display times at higher illumination intensities are preferable. With many common types of color photographs, such as Kodak Ektacolor 74 RC chromogenic2 prints (often incorrectly referred to as “Type C” prints), dark-storage reactions may predominate when the prints are displayed under low-level tungsten light; the use of extremely low-level tungsten illumination on the order of 50 lux (5 footcandles) may result in little if any gain in print life.
High-intensity accelerated light-fading tests often produce data which do not give an accurate indication of actual long-term fading and staining characteristics,3 and long-term dark-storage data are not presently available for the wide variety of existing color print materials or for the wide range of dark-storage and display conditions in museums and galleries. It is therefore difficult to predict with any certainty the rate at which changes may take place for a given print material. Currently available color print materials manifest a wide range of differences in display and dark-storage fading characteristics.4
To determine the changes that take place in a color print over a period of months or years, it is necessary to measure periodically the color and optical density of a print directly. Alternatively, the changes can be measured indirectly with a “fading monitor” made of the same type of color print material as the photograph in question and subjected to the same light, temperature, and relative humidity conditions as the color photograph. Measurements are made with an accurate electronic color densitometer designed for photographic use. The quantitative data thus obtained indicate at which point in time slight—but visually significant—changes have taken place so that the user knows when to retire the original print to cold storage in the dark, substituting copy prints for study and display purposes. Table I shows the quantitative limits of acceptable change suggested by the author.
Because the relationship between dark-fading/staining and light-fading/staining of the dye sets used in the many types of color photographic materials is complex, and for a variety of other reasons, measurement of the total light exposure received by a print using the British Wool Standards, NBS Fading Papers, integrating photometers, and so forth will generally not indicate accurately the degree of deterioration of a color photograph during long-term display and/or dark storage.
The color print montoring procedures described below can also be used to measure changes in monochrome photographs such as silver-gelatin and silver-albumen prints.5 Likewise, yellowing and other forms of deterioration in works of art on paper and in other paper objects can be monitored during long-term display and storage. The measurement techniques described here are also applicable in documenting changes in objects after conservation treatments. Changes in watercolors and paintings can be measured using similar procedures, but because of the wide variety of colorants used in paints, a spectrophotometer may have to be employed6 in addition to, or in place of, a color photographic densitometer to ensure accuracy. Color photographic images are composed of cyan, magenta, and yellow colors which have spectral absorption peaks within a fairly narrow range; photographic densitometers are designed to measure colors with these spectral characteristics. While the concept of predetermined limits of change is directly applicable to paper objects, watercolors, paintings, and so forth, the limits selected will probably differ from those suggested here for color photographs. Still, it is important to measure and record the visual characteristics of all objects of these types quantitatively so that any changes in future years can be determined with reasonable accuracy.
Basic to any monitoring system is an instrument calibration procedure which assures the continued accuracy and comparability of measurements even after old equipment has been replaced by new instruments of different design. Maintaining system accuracy for hundreds or thousands of years in the future will require careful planning as well as the very long-term preservation of color photographic calibration standards in an unchanged state through the use of humidity-controlled cold storage. Preserved color photographic calibration standards are thought likely to provide more accurate densitometer calibration than would be possible using porcelain plaques or stable pigment standards, which have spectral characteristics different from those of the cyan, magenta, and yellow dyes used in color photographs.