Metal Soaps in Art: from first observations towards a complete understanding
‘Special colloquium within the framework of our 70 YEAR ANNIVERSARY’
Old master paintings as well as modern and contemporary art are subjected to changes from the moment they have been made. Discoloration, increased transparency and darkening, crumbling of the paint, paint delamination and loss: these are just some of the degradation phenomena encountered on oil paintings. They affect the paintings’ appearance and stability, and as a consequence the conservation is often considered problematic. A fundamental understanding of the driving forces behind these ageing processes is important, how fast they take place, and how they are influenced by paint formulation, climate and conservation treatments from the past and present. This presentation focusses on the wide-spread degradation phenomena that is related to pigment-oil binder interactions, which are metal soaps. Metal soaps are the reaction product between metal ions, originating from pigments and/or driers, and fatty acids deriving from the organic binder. Metal soap related defects are observed in paintings by amongst others Rembrandt van Rijn, Johannes Vermeer and Piet Mondrian. Approximately 70% of paintings in museum collections are affected by metal soap-related degradation phenomena.
The metal soap phenomena are studied in miniscule paint samples that are subjected to synchrotron-based techniques as well as state-of-the-art lab-based techniques. The studies are combined with experiments on specially designed models of mature oil paint to unravel the chemical pathway. It was found that metal ions from pigments often migrate into the binding medium of an oil paint by binding to carboxylate groups on the polymer structure, forming an ionomeric structure. The release of fatty acids from the binding medium over time was found to be a trigger for widespread metal soap crystallization. The gained knowledge is relevant to conservators and art historians for a better understanding of paint degradation phenomena in order to develop and tailor more appropriate conservation treatments and to obtain a better understanding of the techniques employed by the artists involved.
Katrien Keune did her PhD in Chemistry at AMOLF in the group of Jaap Boon. Currently she is head of Science at the Rijksmuseum, a sub-department of Conservation & Science responsible for scientific analyses of objects in the Rijksmuseum collection. She also holds an appointment as Associate Professor of Chemistry at the University of Amsterdam (HIMS-UvA). Her expertise lies in aging and degradation studies of pigments and oil paintings at the micro- and molecular level, especially related to pigment-binding medium interactions.