1 INTRODUCTION

Studies carried out in recent years have shown that several categories of artists' colorants fade, many of them substantially, when exposed in the dark to purified air containing parts per billion (ppb) levels of air pollutants. The air pollutants that have been studied to date include ozone (Shaver et al. 1983; Whitmore et al. 1987; Whitmore and Cass 1988), nitrogen dioxide (Whitmore and Cass 1989), nitric acid (Salmon et al. 1992; Grosjean et al. 1992), formaldehyde (Williams et al. 1992), and peroxyacetyl nitrate (CH3C(O)OONO2, hereafter PAN) (Williams et al. 1993).

The selection of these pollutants for study of their possible impact on colorants in museum collections is based on their abundance in ambient and indoor air, including museums (Hisham and Grosjean 1991a, 1991b). Formaldehyde is ubiquitous in indoor air (U.S. National Academy of Sciences 1981), and museums worldwide are no exception (Hatchfield and Carpenter 1986; Grosjean et al. 1990). Ozone, nitrogen dioxide, nitric acid, and PAN are oxidants produced in photochemical smog and have been identified as major pollutants in many urban areas of the world that experience this type of air pollution, including Athens, Rome, Rio de Janeiro, Cairo, and Mexico City, (Nasralla and Shakour 1981; Lalas et al. 1987; Tanner et al. 1988; G�sten et al. 1988; Tsani-Bazaca et al. 1988; Grosjean et al. 1990). Several of these urban areas also have a substantial amount of cultural property.

One major air pollutant, namely sulfur dioxide, has yet to be studied for its possible adverse effects on artists' colorants. With the exception of a few urban areas where ambient levels of SO2 are low due to stringent regulations on the sulfur content of fuels (e.g., California), SO2 is a major air pollutant, and often the most abundant pollutant, in many parts of the world including those with a high density of cultural property (Bennett et al. 1985; De Koning et al. 1986). Regions that experience high levels of SO2 include urban areas in Italy (Fassina 1978), India (Lal Gauri and Holdren 1981), Greece (Economopoulos 1987), and England (Hackney 1984). Although less documented, SO2 pollution is thought to be particularly severe in Eastern Europe (Ember 1990; Veldt 1991) and in urban and industrial regions of China (Zhao et al. 1988). Levels of 40–50 ppb SO2 have been recorded in museums (Hackney 1984; Brimblecombe 1990), and sulfur dioxide has been recognized as a potential threat to cultural property for a number of years (Thompson 1978; Baer and Banks 1985). In fact, more than a century ago, paintings at the National Gallery, London, were covered with glass for protection against damage by SO2 and soot (Eastlake et al. 1850). More recently, textile dyes have been observed to fade upon exposure to SO2(Beloin 1973; Hemphill et al. 1976).

In this article, we describe the methods and findings of an investigation focusing on color changes resulting from exposure of 34 artists' colorants to levels of sulfur dioxide that are relevant to museum air quality. The observed color changes are compared to those resulting from exposure of the same colorants to other major air pollutants and are discussed with respect to their implications for colorant-containing art objects in museum collections.