Early Viridian Pigment Composition CHARACTERIZATION OF A (HYDRATED) CHROMIUM OXIDE BORATE PIGMENT

Abstract

Pigment analyses on more than 90 paintings dating from between 1885 and 1943 (A. Jawlensky, W. Kandinsky, F. Hodler and C. Amiet) have revealed that the majority of samples with chromium oxide hydrate green contain a spectroscopically conspicuous by-product. With the aim of tracking down the origin of this component, the so-called Guignet green, a variation of viridian produced by calcination, was systematically synthesized under varying conditions. The resulting products were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, energy dispersive spectrometry, electron microprobe microanalysis with wavelength dispersive spectrometry, X-ray diffraction, thermogravimetric analysis, secondary electron and backscattered electron imaging by scanning electron microscopy, polarized light microscopy and colorimetry. The composition of pigment products varies with the calcination conditions. Higher temperatures and/or longer calcination times lead to an insoluble borate matrix incorporating chromium oxide particles. The result is an amorphous chromium oxide borate Cr₂O₃·(xH₂O)–Cr₃BO₆ polymorph with low water content. Interpreting chromium oxide hydrate green samples within this context, it turns out that calcination conditions in the past were highly variable, leaving behind considerable amounts of borates in the final product. Current data covering more than 60 paintings by Jawlensky suggest that the use of a specific production batch can be resolved in time. Identification of borates in chromium oxide hydrate green samples from paintings may thus provide information about the origin of the paint product used.