Understanding Oil-Paint Brittleness

When artists working with oil paints want to add white to their palettes, many turn to paints pigmented with zinc oxide, which is cheaper and less toxic than paints pigmented with lead carbonate. But much to the horror of artists and art conservators, zinc white, unlike lead white, often turns brittle and cracks within just a few years. Stuart G. Croll and Malia Zee of North Dakota State University, working with Marion F. Mecklenburg of the Smithsonian Institution’s Museum Conservation Institute, are trying to understand the physicochemical process that keeps lead white paint tough but makes zinc white paint brittle. “There are huge differences in mechanical properties and solubility between oil paints made with different pigments, which pose problems in art conservation and restoration,” Croll told C&EN. He believes that the key difference lies within the metal soaps that are produced as moisture permeates the paint over time. Some of these soaps, Croll thinks, are liquid-crystalline ionomers—polymers with both neutral and ionic repeating units. In some cases, metals or ions cross-link these polymers, and when there’s an excess of cross-linking, the paint becomes brittle.