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 artists' 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).