A new method of Synchrotron
radiation based X-ray fluorescence elemental mapping has allowed the revealing
and reconstruction of a portrait underneath a portrait, it has also allowed us
to gain more insight into the genius that was Vincent van Gogh. Previous
techniques of conventional X-ray radiation transmission radiography (XRR) and
infrared inflectography (IRR) helped to initially recognize a faint head under
the surface painting of van Gogh’s Patch
of Grass. However, due to the limitations of these methods, the revealed
face could not be identified and the new method of Synchrotron radiation based
XRF was implemented. XRF works based on the principle of the photoelectric
effect: a core level electron is promoted out of the atom. The painting was
subjected to an X-ray bundle from a synchrotron radiation source, and the
fluorescence of the layers of paint was measured. This technique is
advantageous because the measured fluorescence is unique to each element as the
intensity of the fluorescence is proportional to the number of excited atoms. This
allows each atom in the paint pigments to be identified and charted. Additionally,
the high-energy fluorescence of the smaller elements will not be masked by that
of the larger elements. X-ray absorption near edge
structure (XANES) was performed to determine which molecule the antimony was
part of and therefore which pigment was used for the underlying portrait of the
unknown face. XANES spectra are dependent on the multiple scattering resonances
of the excited photoelectron at the atomic absorption site. The K-edge spectra from the XANES was
compared to that for Naples yellow (Pb2Sb2O7)
and antimony white, (Sb2O3) both which contain antimony
in the molecular formula for the pigment. An analysis with μ-XANES at the Sb LIII
and LI edges indicate a spectrum of Naples yellow. Another
series of XRF analyses were conducted on heads in paintings by van Gogh, which
verified the presence of Hg, Zn, Pb, and Sb. Elemental distribution maps were
constructed to give approximations to the colors van Gogh used.
It is estimated that about 1/3 of van Gogh's paintings are paintings underneath a painting.
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