Vincent van Gogh, one of the founding fathers of modern painting, was known to save canvas in a very particular way: He reused the canvases of abandoned paintings by covering them with layers of white and then painted over them. It is estimated that about a third of his paintings bear other artwork underneath. Recently, researchers collaborating from a range of institutions have developed a technique to reveal the hidden paintings in astonishing detail using X-ray fluorescence (XRF) elemental mapping (Dik et al., Anal. Chem. 2008, DOI 10.1021/ac800965g).
Older methods for investigating covered paintings involve X-ray radiation transmission radiography (XRR) and Infrared Reflectography (IRR), both of which can reveal hidden paintings, but without much detail.
Joris Dik of the Delft University of Technology in Holland and his co-workers have used non-destructive XRF elemental mapping, to investigate van Gogh's painting Patch of Grass. Previous analyses had shown that this painting covers the image of a female head, but the details were blurred. Dik and colleagues scanned a 300 cm2 area of the painting with a narrow beam (0.5 × 0.5 mm2) of quasi-monochromatic synchrotron radiation, using a counting time of 2 seconds per pixel. The whole scan took about two days, separately recording the fluorescence spectra for each pixel. The idea is to identify certain elements by their fluorescence fingerprint. In particular, the distribution of Hg, Sb and Zn reveals the presence of specific paints used at van Gogh's time, namely vermillion (Hg), Naples yellow (Sb) and white (Zn), so revealing a much more detailed image of a woman's head. To confirm these results, Dik and his colleagues examined another van Gogh paining, Head of a woman, with a portable XRF unit. As expected, they found Hg in red areas, such as lips, and Sb and Zn in light areas. Using this new technique, researchers might be able to uncover other hidden paintings, thus providing better understanding of the Artist's life and habits.