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CHARACTERIZATION OF SOME ORANGE AND YELLOW ORGANIC AND FLUORESCENT PIGMENTS BY RAMAN SPECTROSCOPY

Keywords: Raman spectroscopy , Raman spectra , organic pigment , fluorescent pigment

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Abstract:

Raman spectroscopy has become one of the most importantvibrational analytical techniques for the investigationof paint samples and notably for organic pigments. For theanalysis of modern materials, especially for synthetic inorganicand organic pigments the prospect of using Ramanspectroscopy is an alternative to e.g. Py-GC-MS. It providesenough information allowing for a broad characterization ofcolourants either in raw samples, on cross-sections or insitu.One of the advantages of Raman spectroscopy overFTIR analysis is that it allows the detection of weak IR frequenciessuch as stretching vibrations of chemical groups,e.g. the nitro group in azo pigments, which represents thebiggest family of organic pigments found in paintings.The purpose of this paper is, firstly, to complement existingdatabases with data on both yellow and orange pigments,mainly found in artist paints, and secondly, to introduce theanalysis of selected fluorescent pigments. The latter are ofaesthetic interest as they are widely employed in the industry(paints, plastics, textiles) and of conservation interestdue to their ageing behaviour. A short list of organic pigmentshas been drawn according to their use in paintings(artwork and industrial paints), to which some inorganicpigments also found in artworks, were added. This hasbeen complemented with some pigments which are mainlyfound in automotive paints. Analyses were performed withdispersive micro-Raman spectroscopy. The prospect ofinduced fluorescence background led to the preferentialuse of a diode laser at 785 nm.Results show that Raman spectra allow for characterizationof various chemical groups in organic pigments. In the caseof fluorescent pigments, differentiation is more difficultsince the formulations of such pigments/dyes are quitecomplex. Further research on this topic must be undertakenboth to further the knowledge of photostability of thesematerials and to improve characterization using vibrationalanalytical techniques. Some of the conclusions drawn from this study were used for identification of two fluorescent paint samples taken from a late 20th century Ben (Benjamin Vautier, 1935-) artwork.

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