A major motivation for the scientific study of artworks is to understand their states of preservation and ongoing degradation mechanisms. This enables preservation strategies to be developed for irreplaceable works. Intensely-hued cadmium sulphide (C
dS) yellow pigments are of particular interest because these are key to the palettes of many important late 19th and early 20th century masters, including Vincent Van Gogh, Pablo Picasso, Henri Matisse, and Edvard Munch. As these paintings age, their cadmium yellow paints are undergoing severe fading, flaking, and discolouration. These effects are associated with photodegradation, the light-facilitated reactions of CdS with oxygen, moisture, and even the paint binding medium. The use of common optical and X-ray methods to characterize the physical state of the pigment is challenging due to the mixing of the various components of the paint at length scales smaller than their resolution. Here, we present an atomic-scale structural and chemical analysis of the CdS pigment in Edvard Munchs The Scream (c. 1910, Munch Museet), enabled by new electron microscope detector technologies. We show that the CdS pigment consists of clusters of defective nanoparticles ~5-10 nm in diameter. It is known from the modern use of such particles in photocatalysis that they are inherently vulnerable to photodegradation. Chlorine doping and a polytype crystal structure further enhance the sensitivity of the CdS pigment to photodegradation. In addition to The Scream, we have also observed this inherently unstable pigment structure in Henri Matisses Flower Piece (1906, Barnes Foundation). The fundamental understanding of the pigments nanoscale structures and impurities described here can now be used to predict which paintings are most at risk of photooxidation, and guide the most effective preservation strategies for iconic masterpieces.
