Pigment samples are taken from Seurat's Sunday Afternoon on the Island of La Grand Jatte in areas representing three separate stages of production of the painting, from an earlier Seurat painting entitled The Bathers at Asnières, and from a painting that served as one of the preliminary sketches for La Grand Jatte. The main goal is to determine which of the pigments is unstable and responsible for color changes which were noted within a few years of the painting's completion. Methods used to identify the pigments present are polarized light microscopy, microprobe analysis, and x-ray diffraction. By his later stages of work the artist is seen to elimin ate iron oxides and blacks and add in zinc yellow, and to decrease the number of components mixed together to create a color. Zinc yellow is found to be the unstable pigment responsible for the color changes. See also AATA 21-1380 for reference to entire proceedings.