AbstractsPhysics

One of the causes leading to appearance changes of oil paintings is saponification of basic pigments due to a decrease of reflected light. This thesis contributes to a deeper understanding of appearance changes due to saponification of white pigments. Furthermore, effects on painting appearance, especially in terms of transparency of the paint, were studied from a chemical and optical point of view for other color change phenomena such as discoloration of lakes and other pigments. Two late nineteenth century paintings, from the artists J.E. Millais and L. Fildes, that have lost details in dark areas, were studied. Both were affected particularly by saponification of a white pigment, zinc oxide. Experiments with reconstructed paints readily demonstrated transparency changes of the paints by saponification. However, to become visible, the paint’s build-up and chemical and physical conditions are important factors. Fading of lakes or pigment decomposition leads to not only color changes but also a transparency increase due to the reduction of light absorption. In combination with the saponification of white pigments, this caused severe contrast changes in the F.H. Verster’s self-portrait (1921) and an early 19th century painting by Th. Rousseau. These case studies show that the light reflection from the paint, which can change due to chemical and physical changes in a paint system, is the key to understand the current appearance of paintings. Thus, changes on the molecular level in an aging paint may irreversibly alter the color balance and harmony configured by paint combinations intended by the artist.