Thin-film study on the oxidation of linseed oil in the presence of selected copper pigments

In the context of investigating the thermal degradation and yellowing of painting media by oxygen, the effect of the green copper compounds, malachite (basic copper carbonate), verdigris (basic copper acetate), and copper abietate, on the oxidation of linseed oil films was studied. To this end, an experimental methodology suitable for in situ spectroscopic investigation has been introduced. Spin-coating is used for the preparation of thin oil films of controlled thickness (1.0-1.4 mu m) on quartz, glass, or silicon wafer substrates, enabling UV-absorption and IR spectroscopic studies to follow the course of thermal oxidation. The study is supported by chromatographic analysis of oxidative degradation products obtained at different processing conditions. A differentiation in oxidation of linseed oil is confirmed in the presence of the different copper pigments. On the other hand, light bleaching of yellow films is accompanied by degradation of the medium. On the basis of these results, the factors influencing the mechanism of oxygen-mediated polymerization, yellowing, and degradation of artist's oils in the presence of copper-based pigments are discussed.