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The historical pigments, contrary to modern ones, are not constituted by particles having all the same size and this influences the colour of the paint layers. The hiding power and colouring power of a pigment depends, in fact, on its particle size. The aim of this study was to evaluate the influence of particle size on optical characterization of paintings in terms of reflection of light and related colour specification. Starting from the qualitative observation, we have attempted to quantify the pigments colour variations induced by grinding and then attributable to granularity. Powdered pigments of principal colours have been selected in specific particle size range by mechanical sieving. The measurements were performed both on pigment pellets and on paintings realized with binder casein. All samples were characterized by an optical and colorimetric point of view through spectrophotometric analysis and for the surface morphological observation through scanning electron microscopy.

An on-line database was compiled for the Raman and fluorescence spectra, obtained using three different excitation wavelengths (531.5, 632.8 and 780 nm), of 99 pigments analysed dry and already applied with four painting techniques (egg tempera, casein tempera, oil and fresco). The database makes it possible to determine easily the most suitable excitation wavelength for the identification of pigments already applied with these painting techniques.

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