THE OZONE FADING OF TRADITIONAL CHINESE PLANT DYES
YUN YE, LYNN G. SALMON, & GLEN R. CASS
3 3. EXPERIMENT
Silk cloth samples, dyed with su mu, jiang xiang, hong cha ye, wu bei zi, zi cao, ban lan gen, huang zhi zi, ju zi pi, huang bai, zi ding cao, jiang huang, and da qing ye, were kindly provided by the Suzhou Silk Museum. Technical identifications of the plant materials used to make the dyes and the main components of the plant dyes are given in table 2. At the Suzhou Silk Museum, the dyes were reported to have been mordanted only with sodium chloride and acetic acid. Chemical analysis of dissolved dyed silk samples by inductively coupled plasma mass spectrometry in our laboratory showed the presence of trace amounts of calcium and iron, which may have entered into the mordanting process unintentionally. A variety of mordants are found in ancient Chinese silk, including iron, alum, copper, etc. (Zhao 1985). Information as to whether the silk was degummed was not provided. The dried plant materials from which the dyes for the paper samples were made were also obtained from the Suzhou Silk Museum unless otherwise noted.
For each silk cloth sample, a small piece measuring approximately 2 x 5 cm, was cut off and sewn onto a patch of watercolor paper for easy handling. For purposes of comparison, samples of the unmordanted dyes on watercolor paper were prepared in our laboratory. Since the plant materials received were already dried, it was not easy to extract the colorants by traditional techniques. Soxhlet extraction was employed with methanol as the volatile solvent. After extraction, the solution was concentrated using a rotary evaporator before being air-brushed (Iwata HP-A) onto a 2.5 x 5 cm piece of watercolor paper (cut from Arches 140 lb. hot-pressed paper) by a technique introduced in previous work in this laboratory (Whitmore et al. 1987). An effort was made to produce samples on paper that had a reflectance of approximately 30–40% at the wavelength of maximum absorption. It has been found that with initial depths of shade in this range, the color change is sensitive to changes in the colorant concentration (Johnston-Feller et al. 1984). The colorant-on-silk samples were used as received from the Suzhou Silk Museum; they were generally dyed to a deeper shade than the colorant-on-paper samples. In contrast to the paper samples, where the colorant was applied to the surface of the paper, the silk samples were saturated with colorant throughout the depth of the cloth.
The ozone exposure apparatus used in these experiments is described in detail elsewhere (Whitmore et al. 1987). Briefly, air at a flow rate of 2 lpm was purified by drawing it through a bed of activated carbon, a bed of Purafil, and a high-efficiency particle filter, followed by humidification to 50% RH. Next, the air flow was drawn through three commercial ozone generators (Ultra Violet Products SOG-2) operated in parallel. The ozonated air then was admitted to a light-tight stainless steel and glass exposure chamber. Both silk and paper samples were mounted on anodized aluminum panels, which were hung in the exposure chamber adjacent to the chamber walls, with the samples facing the interior of the box. A uniform ozone concentration was maintained within the exposure chamber by use of a magnetically coupled stirring system. The ozone concentration inside the chamber was measured continuously with a UV photometric ozone monitor (Dasibi Model 1003-PC), which was attached to a strip chart recorder. The samples were exposed to 0.40 ± 0.005 parts per million ozone at 25°C ± 1°C, at 50% RH, in the absence of light, for 22 weeks. Unexposed control samples of both the colorants on silk and the colorants on paper were maintained for purposes of comparison to the ozone-exposed samples.
The fading rate of the colorant-on-silk and colorant-on-paper systems can be characterized by the change over time of their visible reflectance spectra. The diffuse reflectance spectra of these samples were measured with a Diano Match Scan II reflectance spectrophotometer, before and at intervals during the ozone exposure experiment.
Two frequently used color notations, the CIE tristimulus values (X, Y, Z), and the CIE chromaticity coordinates and luminous reflectance (x, y, Y), calculated for CIE Illuminant C, are employed to provide information about the visual appearance of the material under the specified standard illuminant. The total perceived color difference between the samples before and after exposure to ozone can be conveniently represented by a single quantity, ΔE, which is calculated for CIE Illuminant C using the CIE 1976 L∗a∗b∗ formula (Billmeyer and Saltzman 1981).