X-ray fluorescence spectrometry (XRF) and micro-X-ray diffraction (μXRD) were used to analyze the composition of pigments on a pastel drawing, Special No. 32, by Georgia O’Keeffe. XRF analyses showed that, among other pigments present in the drawing, the red, orange, and yellow pigments may possibly be identified with lead- and chromium-based pigments: lead chromates, red and yellow lead oxides, and/or lead carbonates, plus calcium-based pastel fillers, such as whiting or gypsum. XRD examination of a sample removed from a dark mottled area of coral red pastel confirmed that this pigment layer, which is associated with a darkened appearance and high Pb:Cr ratios, matches the red lead oxide, minium (2PbO⋅PbO2).
