Aluminium X-ray absorption near-edge spectroscopy analysis of discoloured ultramarine blue in 20th century oil paintings

XANES was used at the aluminium K-edge to investigate the coordination of the Al atoms in the pigment framework. Using the focused synchrotron beam available at the PHOENIX beamline from the Swiss Light Source, scans with high spatial resolution (5 μm) and with a relatively low (2 μm) depth interaction could be performed, enabling analysis of the discoloured lines on samples collected from one of the paintings. The comparison of spectra from healthy (blue) and discoloured (white) areas, supported a degradation mechanism involving the removal of aluminium atoms from tetrahedral coordination in the ultramarine framework, which results in the formation of six-coordinate aluminium compounds separate from the pigment structure. This causes a permanent opening of the cage framework, enabling the reaction of the chromophore and subsequent colour loss.