ABSTRACT
Oil paint develops cracks and eruptions as it ages. Chapter 6 presents current understanding of this aging process focusing on chemical changes in the oily binder. Linseed oil is the most widely used oil binder because it contains a high concentration of linolenic acid (C18:3) and linoleic acid (C18:2). These polyunsaturated fatty acids contain allylic carbon atoms and are easily autoxidized. Artists have long known that paintings gain mass as they absorb O2 from the air. Oil paintings must stand for several months to “cure” before they can be exhibited. The model of aging proposed by Boon et al. (1997) comprises hydrolytic and oxidative chemical changes to the triglyceride esters of the oily binder. Water from the atmosphere is responsible for hydrolysis of ester linkages. Absorbed oxygen reacts with allylic carbon atoms in the unsaturated sites forming peroxides and ultimately produces carboxylic acids due to oxidative scission of the carbon chains. Cross-links form between chains leading to the formation of a polymerized system. Carboxylic acid groups react with basic pigments to form metal soaps. Monometal soaps migrate in water channels and can erupt through the surface of the painting. Dimetal soaps can function as ionic cross-linkers. The aged oil paint system is known as ionomeric. An analytical scheme was developed to distinguish between carboxyl groups formed by hydrolysis and those formed by oxidative scission. FTIR and mass spectral data support the analytical scheme. Mechanisms are presented to justify the formation of various products of autoxidation of linseed oil, including aldehydes, carboxylic acids and covalent cross-links. Aging of oil paint with the formation of metal soaps is recognized to be important problem by the art conservation community.
