Enlightening yellow in art

The Scream by Munch, Flowers in a blue vase by Van Gogh or Joy of Life by Matisse, all have something in common: their cadmium yellow pigment. Throughout the years, this colour has faded into a whitish tone and, in some instances, crusts of the paint have arisen, as well as changes in the morphological properties of the paint, such as flaking or crumbling. Conservators and researchers have come to the rescue though, and they are currently using synchrotron techniques to study in depth these sulphide pigments and to find a solution to preserve them in the long run.

“This research has allowed us to make some progress. However, it is very difficult for us to pinpoint to what causes the yellow to go white as we don’t have all the information about how or where the paintings have been kept since they were done in the 19^th century”, explains Letizia Monico, scientist from the University of Perugia and the CNR-ISTM. Indeed, limited knowledge of the environmental conditions (e.g., humidity, light, temperature…) in which paintings were stored or displayed over extended periods of time and the heterogeneous chemical composition of paint layers (often rendered more complex by later restoration interventions) hamper a thorough understanding of the overall degradation process.

Marine Cotte (left) and Letizia Monico discuss about the samples for the experiment. Right picture: Monico prepares the samples. Credits: C. Argoud.

Proceeding in the same manner as a detective would, Monico and her collaborators took a very hands-on approach, by “reconstructing the scene”: they created oil mock-up paints from commercial-based pigments with the same chemical composition as those from the masterpieces. Subsequently, they introduced them into a chamber where the samples were artificially aged with UVA-Visible light (mimicking museum indoor lighting environments) and in conditions of 90% of humidity. “Most of the literature points out to light as the main cause of degradation of the paint, but we wanted to see what happens from the beginning of the life of the piece of art”, explains Monico.

The team took the samples before and after the ageing process to two synchrotrons: ESRF and DESY. “You need a synchrotron if you want to track the oxidation of the sulphur in the cadmium yellow”, says Monico. They combined X-ray diffraction with X-ray fluorescence and soon enough the results were there. “Monico and her team are long-term users on my beamline”, Marine Cotte, beamline responsible on ID21, where experiments took place and co-author of the study, explains, “and throughout the years we have understood their needs and managed to provide them with answers”, she adds.

Another moment during the preparation of the experiment. Credits: C. Argoud.

They gained precious insight into the photocorrosion process and photocatalytic activity of cadmium yellow pigments in the oil binder. The combination of humidity and light proved to be key in the degradation of the pigments. They also noticed that the cadmium yellow pigments that contained zinc is more prone to degradation than the pigments that don’t have this element. 

The findings will help museums to create the best environment for masterpieces for future generations. For now, though, the team is focusing on studying the effect that additives in historical paintings have in the pigment. “It will be an additional piece of the puzzle. Ultimately we want to follow the mechanism of alteration of pigments and what factors trigger them”, concludes Monico.

Reference:

Monico, L. et al, Chem. Eur. J. 10.1002/chem.201801503. 

Text by Montserrat Capellas Espuny

Top image: Some of the mock-up paints, prepared by Letizia Monico. Credits: C. Argoud.