Understanding of the alteration mechanism of smalt used in oil paintings using combined Co and K K-edge XRF and XANES spectroscopy

The experiment focuses on the understanding of the alteration mechanism of smalt used in oil paintings between the 15-18th c. First, we want to quantify the state of alteration of smalt in historical and model samples in order to propose digital reconstructions of the original aspects of the paintings and second, we want to investigate the chemical interactions between the pigment and its environment to have a better understanding on the weathering causes. MicroXANES at the Co K-edge at ID21 has successfully been used to differentiate an altered smalt from a well-preserved one, but the penetration of X-rays at this energy prevents probing the state of weathering at the grain scale. We propose to combine submicronic XRF and high resolution microXANES mapping of smalt grains and their environment at the Co and K K-edges to relate its chemical state to the color of the grains and to follow the chemical transformations of K leached out of the grains