The Ozone Fading of Traditional Chinese Plant Dyes

Silk samples dyed with 12 traditional Chinese plant dyes were examined to measure their rate of fading upon exposure to atmospheric ozone. Samples of the same colorants extracted directly from the dried plant materials and applied to watercolor paper also were tested for their sensitivity to ozone-induced fading. The samples were exposed in a chamber to an atmosphere containing 0.40 parts per million ozone at 25°C ± 1°C and 50% RH, in the absence of light, for 22 weeks. Colorant-on-paper samples produced from the dyes Jiang huang (turmeric), zi cao (gromwell), and zi ding cao (violet) proved to be particularly reactive toward ozone and faded by ∆E>10 over the course of the experiment. Ju zi pi (tangerine peel extract) on paper changed color by ∆E>5, with an increase in reflectance below a wavelength of 530 nm and a decrease above 530 nm. All but one of the remaining colorant samples on paper changed by ∆E>2 as a result of ozone exposure. Color changes observed among the dyed silk samples generally were smaller than for the colorant-on-paper systems, due to their darker initial depth of shade and hence higher colorant concentration per sample, to their greater saturation of colorant throughout the cloth, and to the effects of mordanting. Colorant-on-silk samples showing color changes of ∆E>2 over the course of the experiment include zi cao (gromwell), su mu (sappan wood), huang zhi zi (gardenia), Jiang huang (turmeric), and zi ding cao (violet).