THE OZONE FADING OF TRADITIONAL CHINESE PLANT DYES
YUN YE, LYNN G. SALMON, & GLEN R. CASS
5 5. CONCLUSIONS
Exposure to atmospheric ozone at the concentrations seen in some urban atmospheres will lead to the fading of many traditional Chinese colorants that are derived from plant materials. Colorants applied as a thin wash on the surface of paper are particularly vulnerable, while silk samples dyed by traditional procedures also will fade but at a slower rate. Samples of the dyes jiang huang (turmeric), zi cao (gromwell), and zi ding cao (violet) applied on paper faded by ΔE > 10 when exposed to 0.40 ppm ozone at 25°C, 50% RH for 22 weeks. Nearly all other colorant samples on paper changed by ΔE > 2 over the course of the same experiment. Silk samples dyed with zi cao (gromwell), su mu (sappan wood), huang zhi zi (gardenia), zi ding cao (violet), and jianghuang (turmeric) also changed color by an amount ΔE > 2.
The ozone concentration used in these experiments is at the upper end of the range of concentrations actually observed in urban atmospheres and is about 10-fold higher than the average ozone concentration present even in remote areas around the world. At these lower global background concentrations it would take 10 times longer (i.e., 220 weeks) of exposure to outdoor air to achieve the same degree of fading as observed in these experiments. However, since much of the color change measured in this work occurred rapidly during the first weeks of the experiment, we suggest that newly excavated dyed silk samples and samples containing Chinese natural organic colorants on other substrates should not be left exposed directly to the atmosphere (even in rural areas) for weeks at a time. Properly designed enclosures (storage boxes or cases) can scavenge ozone from the surrounding atmosphere and prevent its reentry into the storage system, thereby protecting the colored objects from premature fading due to ozone exposure.