1 INTRODUCTION

THE FADING OF THE PIGMENTS used in paints is ubiquitous. It is only a matter of time; the more fugitive the pigments used, the shorter the time. Some of the pigments used by the old masters were extremely fugitive—some very stable. What remains for current generations to enjoy is the result of differential rates of fading; what future generations will see of modern paintings will also be the result of the differential rates of fading of the pigments each artist has used in his time.

As a paint fades, the eye perceives a complex change in one or more of the psychological dimensions of color: hue, lightness, and saturation. These subjectively-sensed changes are related to decreases in the concentrations of the colored components in a highly complex way—our eyes are not physical measuring instruments. Hence, using visual comparisons alone, investigators have not been able to express the degree of fading quantitatively in terms of the changes in the concentration of the colored materials.

Pigments are based on chemical compounds and should, therefore, obey stoichiometric and kinetic laws of chemical reactions in the course of their deterioration induced by photochemical and thermal activity. Owing to the physical complexity of the components in paint films, however, the chemical mechanism for the fading of pigments in paint films has not generally been considered amenable to analysis according to the principles of chemical kinetics.

Today, computer color-matching techniques are widely used by manufacturers of colored materials, such as paints, plastics, and textiles, to calculate colorant formulations—the mixture of pigments necessary to make a desired color. We consider that these same techniques can also be used to determine a colorant formulation necessary to match faded materials in terms of the amounts of original pigments remaining following exposure. Thus, in the investigations to be described, color-matching techniques were used to determine the concentrations of pigments present at any given stage in fading.