FURTHER STUDIES ON TRANSPARENT GLAZE FADING: CHEMICAL AND APPEARANCE KINETICS
PAUL M. WHITMORE, & CATHERINE BAILE
In this paper a simple model that describes the essential features of the photochemical reaction in a transparent glaze has been developed and tested. The rate of colorant loss is assumed proportional to the total amount of light absorbed, and this rate predicts the observed concentration loss that is linear with exposure dose for highly absorbing glazes, progressively slowing to an exponential loss with exposure dose for partially absorbing glazes. This model also predicts that only the glaze absorbance determines its fading rate and not the colorant concentration or distribution. Thus film thickness and prior fading should not affect the observed colorant loss rate. In experiments on PR66 glazes, this predicted behavior was observed: the colorant loss was found to be dependent on the total amount of colorant in the glaze layer and not on its thickness or colorant concentration. However, previously faded glazes seemed to react more slowly to light exposure than fresh glazes having the same concentrations. The reasons for this behavior are unknown, but it has been observed in studies of textile dye fading. It is not clear that this is a general phenomenon, but because of its implications for the preservation of artifacts, it merits further investigation.
In contrast to the photochemical reaction kinetics, the color of the fading glaze does not show steady evolution but rather a distinct series of stages. The fading of the highly absorbing glazes produces small steady color changes that, because they are caused by the optical properties away from the main absorption peak, may not resemble “fading”: hue or chroma changes may be the most easily perceived. Once fading has progressed so that the reflectance of the absorbed wavelengths is in the middle range, the second stage of fading produces much larger rates of color changes that will tend to derive from the chroma loss and value increase more typical of “fading” behavior. After most of the color has been lost, the appearance changes will slow. This trend suggests that the glazes having intermediate reflectance minima can be considered the most vulnerable to light exposure because of their comparatively high rate of color change. Alternatively, the highly absorbing glazes, which retain the desired high chroma but are on the verge of rapid second-stage fading, may also be considered at great risk of having their appearance seriously compromised by further light exposure.
It is worth noting that the chemical kinetic and appearance kinetic descriptions of the glaze fading offer different views of the “fading rate,” and in fact this term can only have meaning when the measure of fading has been described. Measured by the colorant loss, the fading is seen to be steady (i.e., linear with exposure). As the colorant loss becomes exponential with time, the loss can be accurately described as becoming progressively slower. However, a chemist may choose to describe the photochemistry as a local first-order process that is slowed by the filter effect in the highly absorbing glaze and approaches the reaction rate of the isolated molecules as the glaze becomes very transparent. This view would describe the increase of the first-order reaction rate constant (the slope of the semilogarithmic plot of fig. 3b) as the speeding up of the overall first-order reaction. In contrast to these chemical descriptions, one could accurately describe the fading of a highly absorbing glaze in terms of its appearance changes, which progress slowly in a dark glaze, then reach a point when the color changes speed up, then finally slow down as the glaze becomes completely faded. Each of these descriptions of the “fading rate” is correct and useful, but clearly confusion results unless one specifies the measure being used to track the process. While the most common usage in conservation is to refer to the appearance changes as the most relevant and important measure of the course of fading, it has been demonstrated here that there is no simple connection between the appearance changes and the chemical changes.
Finally, it is appropriate to note the obvious limitations of applying this description to glazes that occur on paintings. The model described here presents the simplest picture of the fading of a transparent glaze. Glazes that do not conform to the assumptions of this model, such as green copper resinate glazes that turn brown on light exposure, are likely not to conform to the general trends described here. Faded glazes composed of mixtures of colorants (or of natural colorants that are themselves mixtures) may have greater resistance to light than equivalent “fresh” glazes simply by virtue of their having already lost the more fugitive components of the mixture. The glaze fading model was developed not to attempt accurate description of all possible glaze applications. Rather, it is presented as a starting point for estimating the likely fading of a paint about which there may be little other information. The trends discussed should be considered a guide that may help in developing appropriate, prudent exhibition conditions and as an aid in directing efforts to monitor the condition and possible sensitivity of such paints.