The Raman spectra of approximately 20 reference samples of cobalt-based green and violet artists' pigments of various provenance (present day manufactory, historical reference pigments and samples from a contemporary artist's studio) were acquired to assist in the identification of unknown cobalt-based pigments in works by Pablo Picasso (1881–1973) and Jasper Johns (b. 1930). Specifically, Raman spectra were obtained for various cobalt titanate greens (Co2TiO4), highlighting variability in peak positions because of ionic substitutions in the spinel structures of such pigments. The Raman spectra of the cobalt violet pigments magnesium cobalt arsenate (MgxCo3-x(AsO4)2) and hydrated forms, ammonium cobalt phosphate hydrate (NH4CoPO4*H2O), and anhydrous cobalt phosphate, Co3(PO4)2 were also recorded; some of which are presented here for the first time. Fourier Transform Infrared spectroscopy (FTIR), x-ray fluorescence spectrometry (XRF) and x-ray diffraction (XRD) were employed to confirm the composition of the reference materials. An optimal technique to characterize microscopic cross sections from the studied artworks is Raman microscopy, which can also provide valuable information on the hydration states of the examined phases. The multi-analytical methodology allowed the identification of the unknown green pigment used by Jasper Johns as cobalt titanate green (C.I. PG 50); representing the first documented occurrence of this pigment in the palette of a renowned contemporary artist. Both magnesium cobalt arsenate (in anhydrous and hydrated forms) and anhydrous cobalt phosphate were identified in the Picasso painting.
This research advances the knowledge of 20th century synthetic inorganic pigments used by artists and documents their usage in actual works of art. Copyright © 2012 John Wiley & Sons, Ltd.