THE OZONE FADING OF TRADITIONAL NATURAL ORGANIC COLORANTS ON PAPER
Paul M. Whitmore, Glen R. Cass, & James R. Druzik
RECENT STUDIES HAVE demonstrated that significant amounts of ozone (O3) present in the outdoor atmosphere can invade a museum environment.1,2 This chemically reactive gaseous air pollutant could prove hazardous to works of art and cultural property, for the damage to organic materials, particularly certain polymers,3 textiles,4 and textile dyes,5 from prolonged ozone exposure has been well documented. Little of this previous work, however, has been concerned specifically with the action of ozone on artists' materials; thus, the extent of the danger to museum collections and works of art has not been fully explored.
In an effort to identify those artists' materials which are vulnerable to ozone attack, our laboratory is currently engaged in a series of chamber exposure studies to determine the colorfastness of artists' pigments exposed to an atmosphere containing ozone. In the first exploratory experiment, reported elsewhere,1 a small number of modern watercolors on paper, representing a variety of organic and inorganic pigments, was exposed to ozone. Subsequently, a second experiment tested the ozone resistance of a more complete selection of modern artists' watercolors (from the Windsor & Newton line) containing organic pigments.6 These two investigations succeeded in targeting the synthetic alizarin lakes and a lake of Basic Violet 14 (a triphenylmethane dye) as being extremely reactive towards ozone.
Almost all the pigments used in modern artists' paints are man-made chemicals, developed in many cases as substitutes for rare, expensive, or unstable natural colorants of plant, animal, or mineral origin. Since works of art which pre-date the development of modern synthetic colorants were produced using these traditional pigments, the pollution resistance of natural coloring agents has a direct bearing on the preservation of museum collections.
This report describes a chamber exposure experiment performed to test the ozone sensitivity of a number of traditional natural colorants. Pigment samples were obtained from collections at the Harvard University Art Museums, the Balboa Art Conservation Center, and the Los Angeles County Museum of Art. The pigments were applied to watercolor paper, and color measurements were made instrumentally at intervals during the exposure test. The samples were exposed at 72°F, 50% RH, and in the absence of light, to 0.40 parts per million (ppm) ozone for 12 weeks. This average ozone concentration, although several times greater than that normally encountered in an indoor environment, was nevertheless within the range observed during a heavy smog episode in an outdoor metropolitan setting.7 The total ozone dose during the 12-week exposure was equivalent to about four years of exposure to outdoor air in Los Angeles, or to about eight years inside a typical air-conditioned building. The relative ozone sensitivity of the natural colorants was determined on the basis of the rate and severity of the color changes that occurred during this ozone exposure.