MICRO-FTIR AND MICRO-RAMAN STUDY OF PAINTS USED BY SAM FRANCIS

Raman microscopy and Fourier-transform infrared (FTIR)analyses were both utilized in a recent study of the painting materials used by the American artist Sam Francis (1923-94), in particular a collection of sixty-three pots of custom-made, pre-mixed paints that were found in his Santa Monica studio after his death. Although other analytical techniques were also used in this study, this paper reports on the performance of FTIR and Raman microscopy, with a particular emphasis on their relative ability to detect synthetic organic pigments. These pigments are often hard to detect in paint samples due to their very small particle size, and the fact that only minimal quantities are needed in some paint formulation to produce extremely vivid colours. In general, Raman microscopy was found to be more successful in detecting all pigments, both organic and inorganic. Sixteen different organic pigments were identified by Raman microscopy in thirty-five of the paint samples, including those from the azo, phthalocyanine, quinacridone, disazo, diarylide, dioxazine, indanthrone and perinone families. In contrast, FTIR only detected organic pigments successfully in eighteen of the paint samples, and in most of the cases where FTIR failed it was due to the strong and broad absorptions of the fillers. The inorganic pigments identified by Raman included natural and synthetic pigments such as hematite, goethite, magnetite, cobalt phosphate, cobalt titanate, ultramarine, amorphous material such as graphite but also baryte and calcite fillers. FTIR was also effective in detecting fillers, but very few of the inorganic pigments. However, FTIR appeared much better suited to the detection of the binder, primarily an acrylic emulsion, which typically gave very strong and distinctive peaks, compared to the fairly weak and broad peaks visible with Raman microscopy. The two techniques appeared very complementary and the use of both was required to gather a complete understanding of Francis’ paints composition.