4 DISCUSSION

THE REACTION OF OZONE with organic materials has been widely studied17 and has found application in various processes ranging from organic syntheses to water sterilization. It has been determined that, for the most part, ozone attacks organic molecules at carbon-carbon double bonds, both in olefinic (straight carbon chain) and, more slowly, in aromatic (benzene ring) structures. This reaction oxidizes the organic molecule, ultimately cleaving the carbon atom framework at the site of initial ozone attack. Detailed studies of the mechanism of the ozone reaction with alizarin lakes18 and with indigo19 are presented elsewhere that illustrate the chain of events involved. Since these double bonds often form part of the chromophore in organic colorants, their destruction from ozone reaction usually leads to a loss of color. The observed sensitivity of the natural colorants in this study can be rationalized on the basis of this accepted scheme of ozone reaction.

However, the pertinent issue is not whether a particular colorant will react with ozone—if the colorant contains carbon-carbon double bonds, it will react—but rather, how quickly a colorant system will fade upon exposure to ozone at concentrations found in museum environments. The observed ozone fading of a colorant system is actually a very complicated process governed by a variety of factors. First among these is the chemical reactivity of the colorant towards gaseous ozone. This property depends on the molecular structure of the colorant, particularly on the number and types of chemical bonds and atomic sites vulnerable to ozone attack. Often the chemical reactivity of a colorant can be altered by processing during manufacture (with included additives or particle coatings, or by variations in particle shape or size); during application (through interaction with mordants or substrates, through control of the colorant concentration or degree of aggregation, and through the use of binders or coatings); or during the aging of a museum piece (which could experience extensive oxidation or other chemical degradation in reactions with light, the atmosphere, and pollutants). Finally, the color change produced upon ozone reaction of the colorant system is related in a complicated way to the colorant particle size, shape, concentration, and optical properties, as well as to the physical structure of the colorant system.

The thorough investigation of all these factors on the ozone fading behavior of museum pieces is beyond the scope of the present study. However, to the extent that these factors could be controlled during sample preparation, the physical structure of the colorant systems should be uniform enough to allow meaningful comparisons of the ozone sensitivity of the colorants. In addition, some colorant samples were available from several sources (see Table 1), and these materials, representing different manufacturers, formulations, or eras, were tested and compared. In nearly every case, different samples of a colorant demonstrated only slight variations in ozone reactivity, suggesting that the formulation, particle size distribution, and aging history probably played relatively minor roles in determining the ozone resistance of the colorant systems in this study. The sole exception to this trend was observed in the sample of “Burnt Madder Lake,” which, unlike the other madder lakes, was practically inert towards ozone (ΔE < 1 after 12 weeks). The reason for this lack of reactivity is unclear, but it could be the result of prior oxidation of the pigment in a charring process,9 which might render the product less susceptible to further oxidation by ozone.

The results of this exposure test are summarized in Table 4, in which the most ozone-sensitive colorants on paper have been identified on the basis of their (average) rates of fading and the extent to which they had faded after the 12-week ozone reaction. Since the ozone exposure was relatively mild and of short duration (representing an ozone dose equivalent to only a few years inside a museum), it is possible to differentiate only among the most reactive colorant systems. The time dependence of the ozone fading, shown in Figure 4, was qualitatively the same for all the reactive colorant systems. In these cases, rapid fading began almost immediately upon exposure to ozone, with the largest color changes (as measured by ΔE) occurring during the first week. Subsequent weeks of ozone exposure produced successively smaller color changes. By the end of 12 weeks, most of the colorants are fading at a rate that if extrapolated to longer exposure times would still not alter the relative rank ordering among the colorants tested based on total color change (ΔE) observed in response to a given ozone exposure. Because of this ordering of the time dependence of the ozone fading, the colorant systems can be grouped according to the extent of fading after 12 weeks into rather arbitrary categories of Very Reactive (ΔE > 9 after 12 weeks at 0.40 ppm ozone), Reactive (ΔE = 3 − 8 after 12 weeks), and borderline cases (ΔE = 1 − 3 after 12 weeks but with a monotonic increase in ΔE value over time). The remaining colorant systems showed no definite color change after the 12-week exposure and are termed “Unreactive” towards this ozone dose.

Table 4 Classification of the natural colorant/paper systems according to their observed fading during the 12-week exposure to 0.40 ppm ozone at 72�F, 50% RH, in the absence of light. (See text for the criteria upon which these categories were based.)

The interpretation of the observed fading rates in terms of the chemical reaction kinetics requires knowledge of the relation between the measured spectral reflectances and the concentrations of the colorants in these reactive systems. This connection has been made previously in the study of acrylic paint layers13 and glazes20 using a computer color-matching formula based on a simplified Kubelka-Munk analysis. However, the application of such an approximate theoretical treatment based on a model of a thick homogeneous paint layer, having smooth boundaries and whose color changes only from reduction of the colorant concentration, is inappropriate for the analysis of the ozone fading of the colorant systems in this study. The optical absorption and scattering properties of these samples, comprised of pigment particles embedded to some depth in the paper, are very complex compared to those of a thick, well-characterized paint layer. Further, by its nature the decolorization of the samples by ozone reaction involves alteration of the pigment particles themselves (not just their concentration), possibly leaving the particles coated with colorless ozone reaction products (which would alter their optical properties), or distorting and eventually decreasing the pigment particle size distribution. These effects will in turn change the optical response of the colorant system in a very complicated way throughout the course of the ozone exposure. Clearly, a more complete theoretical formalism and more thorough characterization of the colorant systems are needed before the chemical kinetic details of the ozone reaction can be inferred from color measurements alone.

For now, then, the ozone fading results are presented to illustrate the effect of prolonged ozone exposure on the appearance of these colorant systems. The apparent slowing of the observed ozone fading of these freshly prepared colorant samples suggests the possibility of very different ozone fading kinetics for older works of art, which may have experienced substantial oxidation due to atmospheric or photo-oxidation over the years and in which the rapid initial reaction (or reactions) may already have been completed. Exposure testing of aged samples is necessary to explore these variables. Nevertheless, there is compelling evidence prompting the routine protection of works of art containing natural colorants from prolonged exposure to atmospheric ozone.