Deterioration in Abstract Expressionist Paintings: Analysis of Zinc Oxide Paint Layers in Works from the Collection of the Hirshhorn Museum and Sculpture Garden, Smithsonian Institution

Abstract

ABSTRACTA recent visual survey of Abstract Expressionist-era paintings in the collection of the Hirshhorn Museum and Sculpture Garden (HMSG), Smithsonian Institution revealed a particular type of paint layer separation. Earlier work by the authors showed that zinc oxide in oil paint is a contributing factor to the problem. Ten samples from five Abstract Expressionist-era paintings as well as twenty-three samples eight years or older from the Smithsonian Institution’s (SI) Materials Study Collection were analyzed by pyrolysis – gas chromatography – mass spectrometry (Py-GC-MS), and unexpectedly significant amounts of oleic (cis-octadecenoic) acid were detected in samples containing high proportions of zinc oxide (25 % or greater by weight). In a typical fully cured oil paint, the oleic acid is oxidized to azelaic (nonanedioic) acid. Although the formation of zinc soaps in oil paints is well-known, the detection of zinc oleate in paints by Py-GC-MS has never been described. The close-packing of the oleate chains in the plate-like structure of zinc oleate prevents the oxidation of the cis-double bond, and therefore prevents the formation of azelaic acid. The detection of zinc oleate in paintings is an indication that the paint layers are at risk for future separation.