Quantitative characterisation of pigment mixtures used in art by fibre-optics diffuse-reflectance spectroscopy

Abstract

Fibre-optics diffuse reflectance spectroscopy (FORS) was used to characterise pigment mixtures in paints used in art. Measurements are non invasive, without any contact with the sample. The experimental device is portable, therefore measurements can be performed in situ, without moving the work of art under investigation from its conservation place. The protocol was validated thanks to modern gouache paints: 10 pure gouaches were used as references and 27 binary mixtures of these pure gouaches were studied. Reflectance spectra are processed using the Kubelka–Munk theory in order to get scattering and absorption coefficients of the references. Assuming a linear dependance of these optical properties with the pigment volume concentration (PVC) of the components of paint layers, the protocol enables qualitative as well as quantitative interpretation of the reflectance spectra measured on binary mixtures of references. Indeed, for most cases, numerical processing of FORS-measurements performed on a mixture leads to the identification of its components. Besides, once the components are identified, it is possible to compute their respective proportions with an accuracy of 5%.