A research team led by Dr. L. Robinet (Synchrotron Soleil, Saint Aubin, France) has recently published an interesting paper describing how the blue pigment, smalt, has faded in many famous paintings such as "The Heavenly and Earthly Trinities (The Pedroso Murillo)" by Bartolome Esteban Perez Murillo. The experiment was basically X-ray absorption spectroscopy near the Co K edge. The samples were tiny pieces taken from the original paintings archived in the National Gallery, London and the Louvre Museum, Paris. It was found that in intense blue particles the cobalt is predominantly present as Co2+ in tetrahedral coordination, while in colorless altered smalt the Co2+ coordination number in the glass structure is increased, and there is a transformation from tetrahedral towards octahedral coordination. This change correlates with the alkali content as well, indicating that it is caused by leaching of potassium cations, which act as charge-compensators and stabilize the tetrahedral coordination of the cobalt ions that is responsible for the blue color. For more information, see the paper, "Investigation of the discoloration of smalt pigment in historic paintings by Co K-edge micro X-ray absorption spectroscopy", L. Robinet et al., Anal. Chem., Article ASAP (DOI: 10.1021/ac200184f Publication Date (Web) May 9, 2011).
