Optical and colorimetric characterization on binary mixtures prepared with coloured and white historical pigments

The main goal of the paper is the optical and colorimetric characterization of the pigment mixtures realized with pure pigments of the principal artist's hues and white pigments through spectrophotometric analysis. This research work represents a first step in a methodology study aimed to the verify potentialities and limits of spectrophotometric techniques in the identification of pigments in mixtures. The analysis regarded the trend of Spectral Reflectance Factor (SRF%) in visible spectrum and of the derivative of SRF, d(SRF%). In particular, the analysis of d(SRF%) of the mixture in the function of achromatic pigment concentration has the aim of highlight shift of the extrema points in relation to the relative quantities of the mixture components. To this aim, painting samples were prepared in laboratory mixing the coloured pigments with achromatic ones in variable weight concentration from 10% to 90% for each of the components. This is necessary to evaluate the minimum quantities of white that could change the coloured pigments SRF% spectra. Also the analysis of colour coordinates is performed to put in evidence the L* a* b* trend in CIELAB 1976 space.