Orpiment, a yellow arsenic sulphide pigment (As2S3), is among the many pigments used by old masters that suffer from photo degradation. They fade under the influence of light which causes them to break down into non-colouring or false-colouring components. For orpiment, earlier research showed that light causes it to degrade into white arsenolite (As2O3). The new study that has just been published in the Journal of the American Chemical Society uncovers a new degradation pathway in which light plays no role.
First author is Fréderique Broers, a PhD student supervised by Prof. Katrien Keune (head of Science at the Rijksmuseum and professor at the University of Amsterdam), Dr Florian Meirer (Utrecht University), and Prof. Koen Janssens (University of Antwerp). The study provides valuable information for conservators that are treating paints that contain orpiment pigment. It is also important that they are aware that toxic arsenic species can be present throughout the whole painting.
Degradation within weeks
In her study, Broers set out to explain the abundant occurrence of a particular type of arsenic components (so-called As(V) species) that always accompany arsenolite in orpiment degradation. These components are found throughout all layers of oil paintings, from support to varnish. Performing advanced chemical studies using synchrotron radiation at the Stanford Synchrotron Radiation Lightsource (SLAC National Accelerator Laboratory, United States), Broers and co-workers were able to establish that orpiment degrades as a result of contact with a medium such as linseed oil or egg tempera.
The chemical details are that the orpiment is partially dissolved, which gives rise to the formation of As(III)-OH species. Subsequent oxidation leads to the formation of the As(V) species that migrate throughout the medium. This degradation mechanism takes place within weeks and does not need any light exposure.
It was also established that arsenolite species, the prime product of light-induced degradation, can further degrade to As(V)-species. The As(V)-species can react with available metal ions to form new crystalline species both at the surface or within the paint layers. As an example, the combined use of orpiment and lead white pigment, which is present in many seventeenth century paintings, can lead to the formation of lead arsenates. This can lead to cracking and delamination, or to a visual change.
This research is part of the research project 3D Understanding of Degradation Products in Paintings at the Netherlands Institute for Conservation+Art+Science+ (NICAS)
Fréderique T.H. Broers, Koen Janssens, Johanna Nelson Weker, Samuel M. Webb, Apurva Mehta, Florian Meirer and Katrien Keune: Two Pathways for the Degradation of Orpiment Pigment (As2S3) Found in Paintings. J. Am. Chem. Soc., April 14, 2023. DOI: 10.1021/jacs.2c12271