A macro-to-microscale investigation of the metal-oxalates phenomenon in Southern Netherlandish oil paintings from the 15th to the 17th centuries

For more than two decades, heritage science has attempted to describe the complex physicochemical mechanisms governing oil binder and pigment transformations as oil paintings age through time. Metal oxalates constitute one of the families of ionic species that have recently been identified as a new type of alteration products in numerous oil paintings. These coordination complexes are considered highly problematic as they can precipitate on the paint surface, forming a thick, opaque, highly insoluble crust, which radically modifies the visual properties of an artwork and alter its readability. However, unlike metal soaps, the mechanistic aspects of metal oxalate formation within oil paintings had been much less investigated at a fundamental level until they were systematically addressed by the BELSPO-funded MetOx project (Metal-oxalates in the 15th to 17th Century Southern Netherlandish oil paintings).