THE OZONE FADING OF TRADITIONAL CHINESE PLANT DYES
YUN YE, LYNN G. SALMON, & GLEN R. CASS
1 1. INTRODUCTION
Ozone is naturally present in the remote, background troposphere, where the global background ozone concentrations range from 20 to 40 parts per billion (Seinfeld and Pandis 1998). In polluted urban air, peak daily ozone levels typically range from 100 to 400 ppb (Seinfeld and Pandis 1998). Previous studies have shown that this chemically active pollutant can cause damage to organic materials. Ozone exposure can lead to cracking of natural rubber products (Newton 1945), erosion of the binders used in paints (Campbell et al. 1974), loss of tensile strength in textiles (Kerr et al. 1969), and fading of commercial anthraquinone-based synthetic textile dyes (Salvin 1969). Over the past decade, a series of experiments has shown that many of the organic colorants used by artists are particularly susceptible to ozone-induced fading. The ozone resistance of modern artists' watercolors, both organic and inorganic, has been explored (Shaver et al. 1983; Drisko et al. 1986). Further study of the traditional organic watercolor pigments used in Western art that are derived from plant and insect sources shows that many of these pigments fade rapidly in the presence of ozone at the concentrations found in today's urban atmosphere (Whitmore et al. 1987). Natural colorants that are particularly susceptible to ozone fading include madder lakes, indigos, and curcumin. The ozone-induced fading of traditional Japanese colorants applied on paper and on silk cloth and used in traditional woodblock prints has also been studied (Whitmore and Cass 1988). The traditional Chinese natural colorants remain to be examined.
As is well known, China is the birthplace of sericulture. The earliest excavated silk is a group of ribbons, threads, and woven fragments, all dyed red, dated to 3000 b.c. (Kuhn 1988; Scott 1993). In recent years, thousands of silk artifacts, including silk fragments, robes, textile paintings, and tapestries, most of which are beautifully dyed, have been unearthed by archaeologists in China (Watt 1997). To protect these historic relics from oxidation and decomposition after exposure to air and light, specifications need to be established for proper storage and display. Among the environmental factors that should be considered is the risk of ozone-induced fading of the traditional Chinese colorants. The purpose of this article is to quantify the rate of ozone fading of traditional Chinese plant dyes applied on silk (and, for purposes of comparison, on paper) in order to identify ozone-sensitive colorants for which special precautions may be warranted.