Smalt is a potash glass with a blue colour given by cobalt, common as an artists' pigment between the 16th and 18th centuries. It often deteriorates, losing its colour and causing dramatic changes in the appearance of paintings. The changes in structure and elemental composition accompanying the loss of colour in the glass were examined in samples from paintings in the National Gallery, London and the Louvre using synchrotron micro Xray absorption spectroscopy, synchrotron reflectance FTIR microspectroscopy, Raman microspectroscopy and SEM-EDX. The leaching of potassium was found to be responsible for concurrent structural modifications in the glass: increase in polymerisation of the silicate network and change in cobalt coordination from tetrahedral towards octahedral, leading to colour change in the pigment. These results also form valuable reference data for examinations with lab-based techniques to distinguish between smalt that is degraded or a pale grey variety deliberately chosen by the artist. résumé Le smalt est un verre potassique dont la couleur bleue est due au cobalt, communément employé comme pigment par les artistes du xvi e siècle au xviii e siècle. Il se détériore souvent, en se décolorant et en entraînant des modifications radicales dans l'apparence des tableaux. Les changements de structure et de composition élémentaire qui accompagnent la décoloration du verre ont été examinés sur des échantillons provenant de tableaux du Louvre et de la National Gallery, à Londres, par microspectroscopie d'absorption X par