FURTHER STUDIES ON TRANSPARENT GLAZE FADING: CHEMICAL AND APPEARANCE KINETICS
PAUL M. WHITMORE, & CATHERINE BAILE
The fading of colors due to exposure to light is an inevitable consequence of exhibition, and minimizing those fading changes has become a central concern in preservation efforts. Because lighting judgments carry this risk of irreversible color change, it is reasonable to seek a deeper understanding of the process of light fading. With that knowledge, one might be able to recognize paint applications unusually sensitive to light exposure and be better equipped to sensitively monitor some attribute that signals the onset of rapid color change. Several efforts have been made to increase our understanding of the color and photochemistry in fading paints. These studies include the work by Johnston-Feller and Feller concerning the fading of alizarin crimson tints (Johnston-Feller et al. 1984; Feller et al. 1986; Johnston-Feller 1986) and of transparent glazes (Johnston-Feller and Bailie 1982; Johnston-Feller 1986) and the work of Thomson on the stratification of colorant in a fading glaze (Thomson 1965).
These seminal contributions highlight the important physical or chemical principles that govern the fading of paints. While their focus was on specific processes or colorants, these studies defined many of the major trends and fading behaviors that can provide guidance in predicting the course of fading for a paint system. There will, of course, be exceptions to such generalizations, but precise prediction of the light sensitivity of any given paint application is an impractical goal. Instead, making judgments based on general behavior is a more realistic starting point for a preservation strategy.
In this article we describe the optics and photochemistry of transparent glazes in order to outline the general course of fading in these paints (or, by analogy, in other transparent colored layers such as printing inks or plastics). We begin with a discussion of the optics of glazes, which not only impart their special appearance but also control the fading of the colorant within the layer. The combination of simple optical properties and simple photochemical reaction kinetics allows us to predict the colorant loss in a fading glaze. We then show that the colorant loss observed in the visible light fading of a glaze formulated with an organic red pigment, Pigment Red (PR) 66, is approximately that predicted by the simple model. This understanding of the photochemical reaction kinetics forms the basis for our description of the types and rates of appearance changes that will occur as the colorant is lost in a fading glaze. Just as the colorant loss is not generally steady with time, the color changes, particularly the loss of the high chroma that is the most distinctive quality of many glaze applications, are not steady with time. These ideas are summarized in terms of the color changes that are expected for different depths of shade of a glaze formulation, the relative light sensitivities of those formulations, and the change in light sensitivity of a single glaze through the course of its fading. Finally, we seek to interpret these results in terms that can guide attempts to monitor the fading of glaze applications on art objects, highlighting the key color or spectral features that might be of most value in tracking the course of fading.
It is essential at the outset to distinguish the measures used in this article to characterize the course and rate of the glaze “fading.” The term “fading” is usually taken to mean the light-induced alteration in the color of a material. While this process is generally associated with an increase in lightness (Munsell value), it has been shown that this color change is not necessarily the only one that can occur in fading transparent glazes (Johnston-Feller and Bailie 1982). The section devoted to what we call the “appearance kinetics” describes glaze fading in terms of these color changes and rates of color changes. Alternatively, the process of “fading” can also refer to the underlying photochemical reaction that leads to colorant loss. The section on chemical kinetics describes fading in terms of this reaction and characterizes it by the rate of colorant loss or, for processes that follow simple mathematical forms, by the so-called rate constants for the reaction. These rate constants are simply the parameters that uniquely define the mathematical form, for example, the slope of a line relating concentration to exposure. While each of these measures—the appearance changes, the concentration loss, and the rate constants for specific mathematical forms—is a valuable means of describing the fading process, it will be clear that one's impression of the course and rate of the process will depend on which of these measures is used. This point will again be made after the fading process has been examined from these different perspectives.