Recently, a European international research group led by Professor K. Janssens (Antwerp University, Belgium) has succeeded in solving the scientific mechanism of color darkening in the paintings of Vincent van Gogh. Some readers may remember a previous news article, "Synchrotron XRF revealed Van Gogh's hidden painting", No.5, Vol. 37 (2008), which explained how synchrotron X-ray spectroscopy and imaging are powerful tools in the analysis of such paintings. In the present work, the research group discusses the change in color from yellow to dark brown in two Van Gogh paintings, Bank of the Seine (1887) and View of Arles with Irises (1888). They also systematically studied the aging process of artificial samples using pigments. The chrome yellow pigment is chemically lead chromate (PbCrO4), which may include some amount of PbSO4 and/or PbO. During their research based on X-ray micro-spectroscopy, it was found that part of the material is transformed into hydrated chromium oxide (Cr2O3・2H2O), which is known as viridian, i.e., a blue-green pigment under sunlight or UV light irradiation. They also noted the formation of other Cr(III) compounds. Their conclusion was that the color change is due to the reduction from Cr(VI) to Cr(III) on the surface of the paintings, and the formation of a thin layer containing Cr(III). This would be the reason for the brownish color. Most of the experiments were done at beamline ID21 at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). For more information, see the papers, "Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 1. Artificially Aged Model Samples" and "2. Original Paint Layer Samples", L. Monico et al., Anal. Chem., 83 1214-1231 (2011).
