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

Abstract

A specific case of synthetic ultramarine degradation was observed in three oil paintings from the early 20th century. Pigment particles were found to have been discoloured, resulting in intricate patterns of white lines, approximately 10 to 30 microns wide, criss-crossing the paint surface. Colour in ultramarine pigments comes from the encapsulated sulphur radical anions, chromophores, inside the cage framework built from SiO44− and AlO45− tetrahedral units linked by the oxygen atoms. In healthy pigment, the charge is balanced by cations such as sodium and the framework stabilises the reactive radicals, preserving the colour. This discolouration mechanism was investigated using X-ray near edge spectroscopy (XANES).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 (5μm) and with a relatively low (2μm) 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.