title: Identification of copper phthalocyanine blue polymorphs in unaged and aged paint systems by means of micro-Raman spectroscopy and Random Forest.
abstract: Copper phthalocyanine (CuPc) blues (PB15) are largely used in art and industry as pigments. In these fields mainly three different polymorphic modifications of PB15 are employed: alpha, beta and epsilon. Differentiating among these CuPc forms can give important information for developing conservation strategy and can help in relative dating, since each form was introduced in the market in different time periods. This study focuses on the classification of Raman spectra measured using 532 nm excitation wavelength on: (i) dry pigment powders, (ii) unaged mock-ups of self-made paints, (iii) unaged commercial paints, and (iv) paints subjected to accelerated UV ageing. The ratios among integrated Raman bands are taken in consideration as features to perform Random Forest (RF). Features selection based on Gini Contrast score was carried out on the measured dataset to determine the Raman bands ratios with higher predictive power. These were used as polymorphic markers, in order to establish an easy and accessible method for the identification. Three different ratios and the presence of a characteristic vibrational band allowed the identification of the crystal modification in pigments powder as well as in unaged and aged paint films.
authors.affiliation: Institute of Science and Technology in Art, Academy of Fine Arts Vienna, Schillerplatz 3, A-1010 Vienna, Austria; Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/161, A-1060 Vienna, Austria. Electronic address: m.anghelone@akbild.ac.at.
authors.forename: Marta
authors.initials: M
authors.lastname: Anghelone
authors.affiliation: Institute of Science and Technology in Art, Academy of Fine Arts Vienna, Schillerplatz 3, A-1010 Vienna, Austria.
authors.forename: Dubravka
authors.initials: D
authors.lastname: Jembrih-Simbürger
authors.affiliation: Institute of Science and Technology in Art, Academy of Fine Arts Vienna, Schillerplatz 3, A-1010 Vienna, Austria; Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/161, A-1060 Vienna, Austria.
authors.forename: Manfred
authors.initials: M
authors.lastname: Schreiner
keywords: Modern artists’ paints; Phthalocyanine polymorphs; Random Forest; UV ageing; μ-Raman spectroscopy