Cobalt pigments and the spectrum of colours they create

Pigment characterisation on modern and contemporary paintings and other works of art is crucial for finding solutions to conservation and restoration issues. A variety of non-destructive analytical techniques, specifically elemental and molecular spectroscopies, are commonly employed for this purpose, and libraries of reference spectra are used for identification. Although cobalt was isolated by the Swedish chemist Georg Brandt in 1735, a wide range of cobalt compounds have been used since early antiquity for paints, and to impart a rich blue colour to glass, glazes, and ceramics. Smalt was the earliest widely used cobalt-containing pigment, becoming in the 17th century a distinctive blue pigment in baroque style painting. Eventually, cobalt featured heavily in the development of some of the early synthetic pigments. In the 19th century, cobalt pigments, often combined with other elements to produce a variety of colours beyond blue [1-3], became significantly intense and stable, prompting a large number of artists to experiment with them until the present day. In this research, a number of swatches of historical and modern cobalt-based pigments applied in different binders (oil, casein, acrylic resin and polyvinyl acetate) and produced by different artist’s paint manufacturers between the second half of the 20th century and the early 2000s were selected. The swatches were used as a tool to evaluate and test a range of spectroscopic methodologies for pigment identification as well as to expand the spectral database of the Conservation and Art Technological Studies laboratory (CATS) of the National Gallery of Denmark (SMK). The paints were initially examined by XRF spectroscopy to verify the presence of cobalt and accessory elements, and subsequently by reflectance spectroscopy to characterise their molecular properties. While most of the measurements provided a good match between the suppliers’ colour descriptions and the analytical results, the chemical compositions of some colours were revealed either to contrast with the details in their labels or to be different from one supplier to another. Whereas the former group includes exclusively violet paints, the latter group is mostly composed of blue and green tints, whose composition can vary greatly from one manufacturer to another and can produce very heterogeneous results. Because the data yielded by XRF and reflectance spectroscopies was not sufficient to fully understand the multifaceted nature of such pigments, additional analyses were carried out by micro-Raman and external reflection-FTIR spectroscopies, to obtain supplementary information. While only few pigments could be further characterised by their Raman spectra, the FTIR analyses provided a knowledge base for the evaluation of the effect of different binders on the pigments’ spectral features. This research aimed at investigating cobalt-based colours with a range of spectroscopic techniques, and the obtained results confirmed the effectiveness of complementary analyses for non‐destructive identification of pigments in different binders. Also, this work highlights the importance of building a spectral database of cobalt-based paints from the second half of the 20th century that will be of significant value as a reference for other researchers in the field of heritage science.