Non-invasive spectroscopic analyses of M. E. Chevreul's colour plates: shedding new light on 19th century colour theory representation

During the early 19th century, colour's theories flourished. In this context, Michel-Euge`ne Chevreul (1786-1889) developed a three-dimensional hemispherical colour space, reproduced and used for several decades by manufacturers, scientists, artists, architects, gardeners and designers, among others, to easily identify and classify colours. Despite the major influence and variety of chromatic spaces created by M. E. Chevreul, they have been the subject of a few scientific studies until now. This study will provide new information on the evolution of the printing technique, the materiality and the visual appearance of six different colour circles printed in 1847, 1855, 1861 and 1864. A dedicated analytical methodology was implemented combining visible and near-infrared reflectance imaging, X-ray fluorescence, Raman, short-wave infrared fibre optic reflectance and mid-infrared Fourier transform spectroscopies. This material identification revealed the coexistence of newly synthetized inorganic pigments, such as lead chromate, Prussian blue or Emerald/Scheele green, with established organic and inorganic pigments, such as cochineal lake, vermilion or minium. This study revealed a temporal evolution of the printing technique that leads to an improved colour representation.