Characterization of component pigments in a mixture

This work is part of Raman spectra database constitution of pigments and materials of Cultural Heritage interest for polychrome artworks characterization. The colours of paintings are often obtained mixing pigments of palette artist’s. The attribution of a work of art to an author and the individuation of fakes are related to the identification of used pigments and also to the detection of mixture constituents. The main research objective is the extension of PH3DRA laboratory database with information useful to detect not only each single pigment but also quantitative data regarding its relative ratio in the examined mixture. The availability of spectra as revealed represents the primary limit in this context. In addition to usual procedures of background and baseline subtraction, the step of intensity correction relative to the specific experimental setup used for signals acquisition (optical components, monochromator, CCD,…) is indeed necessary. After the selection of pigments really mixed by painters in the past from our wide laboratory chromoteque, different specimens were prepared using different and known proportions. Measurements were carried out using both a microraman system equipped with 542 nm and 785 nm laser sources and a portable tool with 542 nm, 633 nm and 785 nm excitation wavelenghts. Preliminary results on mixtures show two different behaviours. The spectra of the first group present well separated peaks allowing the immediate identification of constituent pigments, multiple convolutions make instead impossible this step for the second type of signals. The data elaboration requires in this last case, an appropriate deconvolution procedure in order to isolate the peaks of each pigment.