THE KINETICS OF FADING: OPAQUE PAINT FILMS PIGMENTED WITH ALIZARIN LAKE AND TITANIUM DIOXIDE
Ruth Johnston-Feller, Robert L. Feller, Catherine W. Bailie, & Mary Curran
ABSTRACT—The physical-chemical principles that govern the fading of pigments have been difficult to discover solely by visual observation of changes in color. The authors' use of precise spectrophotometric reflectance measurements and modern color-matching computations based on the Kubelka-Munk theory have made it possible to demonstrate that the decrease in concentration of alizarin lake (fading) during exposure in a Fade-ometer� follows first-order kinetics. The traditional artists' colorant, alizarin lake, was used as a model of a fugitive organic pigment in mixture with a scattering white pigment, titanium dioxide, applied in a poly(vinyl acetate)-based paint at complete hiding. The experimental results indicate that the red-colored alizarin lake tends to be converted to a yellow form which in turn fades to a colorless condition, also by first-order kinetics. The specific rate of fading is shown to be essentially constant over a wide range of initial concentrations of colorant and of pigment volume concentration.