Study of the chemical composition and the mechanical behaviour of 20th century commercial artists' oil paints containing manganese-based pigments

The present study deals with 20th century manufactured artists' oil paints containing raw and burnt umber pigments, this is, natural earth pigments resulting from the combination of iron and manganese oxides. Manganese, in particular, is known to be a primary drier and to have a siccative effect on oil paint films. This research aims to show the diversity of formulations behind apparently same commercial names as well as to understand how the content of manganese, the presence of modern lipidic media and the hydrolysis mechanisms can promote significant differences in the expected mechanical properties of oil paint films, thus conditioning their long-term performance. Several manufactured artists' oil paint films containing manganese were selected. Dried films from raw and burnt umber oil paints by Winsor&Newton® (UK), Grumbacher® (USA), Gamblin® (USA) and Speedball® (USA) were studied and information about their chemical composition and mechanical behaviour is here presented. In addition to the identification and the study of the inorganic and organic components present in each formulation through LM, SEM-EDX, FTIR-ATR, XRD, GC-MS analysis, tensile tests were run and stress–strain curves were obtained. Together with evident hue differences, the obtained results showed significant differences in the chemical composition and the mechanical behaviour of the oil paint films.