Imaging TOF-SIMS and NanoSIMS studies of Barite-Celestite particles in grounds from paintings by Van Gogh

Late in the 19^th century, Van Gogh like many painters would buy ready-made artistsÂ’ materials. For matters of dating and authenticity, it is interesting to distinguish the different formulations and production batches, and to identify the nature and the source of the materials used. This information is useful also to point to different manufacturers, and is of relevance for studies of other impressionists besides Van Gogh. One of the materials used as paint extender is barium sulphate. In its natural source, barite, barium can be replaced by strontium in a continuous solid solution series from barite (BaSO₄) to celestite (SrSO₄). In the process of making artificial barium sulphate most of the impurities are eliminated. Therefore, strontium can be used as an indication of natural barium sulphate in paints, and is potentially an interesting feature for discriminating different sources of the natural mineral. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and NanoSIMS are surface sensitive instruments that combine mass-spectrometric analysis with spatial resolution to give information on both the inorganic and organic fractions of a sample. In this paper we show their efficacy in the characterisation of the ground paint and in particular of barite-celestite in a painting by Van Gogh, Plaster Figure of a Female Torso (F216j, dated mid. June 1886). The barite-celestite particles have overlapping TOF-SIMS maps for sulphur, barium and strontium. NanoSIMS images show that larger particles (about 20 µm across) consist of separate phases of barite and celestite, while the smaller particles (few microns size) consist of a single phase, probably as a result of the grinding of the mineral. Isotopic ratios of strontium were also obtained in an attempt to trace the origin of the mineral.