A partially water-sensitive oil painting by Karel Appel: Technical Investigation and a tailored approach to surface cleaning

Oil paintings from the 1950s to the 1960s by Dutch artist Karel Appel (1921-2006) present a number of degradation phenomena, including water sensitivity, which complicate conservation treatments. This study describes the technical examination, diagnosis of degradation phenomena, and subsequent surface cleaning treatment of the painting Stéphane Lupasco and Michel Tapié (1956) in the collection of the Stedelijk Museum Amsterdam. A technical examination of the painting material combining organic (gas chromatography-mass spectrometry [GC-MS] and electrospray ionization mass spectrometry [ESI-MS]) and inorganic (scanning electron microscopy with energy dispersive x-ray spectrometry [SEM-EDX]) analyses as well as pH and conductivity measurements investigated reasons for the water sensitivity of the mostly well-bound tube oil paints. Highly water-sensitive paints were found to have either low (acidic) pH levels and/or high conductivity readings. While the low pH may be caused by high concentrations of oxidized, carboxylic, (di) acid-containing degradation products from the oil medium, the high conductivity can be explained by the presence of inorganic, water-soluble salts. The diverse surface features of the paints required the application of different cleaning techniques. In addition to traditional swab rolling, materials such as agar gels and moistened makeup sponges were deployed successfully.