pXRF and IR Fluorescence Imaging Studies of CdS Alteration in Paintings by Edvard Munch and Henri Matisse in Oslo, Copenhagen, and San Francisco

The identification of altered cadmium yellow paints in early modernist works is critical to their stabilization and to the long term preservation of the paintings in which they occur. The identification of incipient photo-alteration of these pigments, before there is visual evidence of their chemical degradation, is of particular concern. The alteration of these pigments causes chalking, flaking, fading, and darkening of the yellow paints, leading to irreversible changes in the physical and chemical structure of the paint layer, and dramatically altering the appearance of the work. Standoff, holistic methods for the identification of this phenomenon are desired to rapidly and efficiently survey the condition of the pigment across an entire work, and also to minimize invasive and destructive analyses wherever possible. Such methods are a particular need for collections with large holdings in Impressionist and early modernist works from the 1880s to the 1920s, where these cadmium yellow alterations are particularly problematic and a rapid surveying method for the collection is needed.
To address this challenge, four holistic methods were attempted (both alone and in concert): ultraviolet-induced visible fluorescence; ultraviolet-induced infrared fluorescence; multispectral imaging; and x-ray fluorescence. Questions addressed included: is the imaging method being tested comprehensive? Is it efficient at surveying an entire painting? Does it reveal the state of preservation of the pigment? Does it reliably discriminate among intact versus altered cadmium yellow pigments? To answer these questions, the methods were tested on Henri Matisse’s Le Bonheur de vivre (1905-6 The Barnes Foundation, Philadelphia) and oil sketches for this work in The Museum of Modern Art, San Francisco, and The Statens Museum for Kunst, Copenhagen. They were also tested on Edvard Munch’s The Scream (c. 1910, Munch Museum, Oslo). It was found that ultraviolet-induced visible fluorescence has the best ability to discriminate between altered and unaltered cadmium yellow paints (even before alteration is visible to the unaided eye), while multispectral imaging allows for the most efficient and comprehensive localization of the cadmium pigments in a work.