Tracking the ravages of time in paintings.

• Position: Pigments

The discoloration of some pigments used in Impressionist paintings has been observed since the 19th century. Belgian researchers now have investigated how the colors of chrome yellow pigments change on both the micro- and nanometer scales. In the journal Angewandte Chemie, they reveal the likely mechanism for the reaction that slowly, but surely, changes the bright yellows of Vincent van Gogh's "Sunflowers" to an unsightly brown.

Many 19th-century paintings, such as Van Gogh's "Alyscamps" contain synthetic chrome yellow pigments, primarily lead chromates. These yellow paints clearly turn brown when sunlight reduces chromium(IV) compounds present in the upper layers of paint to chromium(III). The degree to which the yellow paints discolor depends on the chemical composition and the crystal structure of the pigments. The most significant color changes are observed for chrome yellow pigments with a high sulfate content.

In order to develop useful techniques for the protection and restoration of paintings, it is important to understand the precise mechanisms that occur as the pigments darken. To solve this problem, Haiyan Tan, He Tian, and a team from the University of Antwerp examined samples from a 100-year-old tube of oil paint from the estate of the Flemish Fauvist painter Rik Wouters.

Their investigation revealed a variety of particles whose cores and shells differ in composition--clearly the early stages of an aging process. The researchers used scanning transmission electron microscopy and various spectroscopic methods to analyze the particles and observe how they change when their aging is accelerated with UV light. The results allowed them to...