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Synthetic organic pigments: detection and identification in paint samples


Untitled, by Clyfford Still, 1951-1952, San Francisco Museum of Modern Art


The explosion of organic synthesis at the middle of XX century permitted the introduction on the market of a wide range of new synthetic molecules including organic pigments, thus enormously widening the range of organic colouring materials available to artists and manufacturers, with respect to the limited number of natural dyes and lakes. Synthetic organic pigments are difficult to distinguish and identify by analytical techniques, particularly in complex matrices such as paint samples.
The DCCI-UniPI and CNR-Pg Unit, in collaboration with the Getty Conservation Institute (LA, California), used Py-GC/MS, HPLC-DAD and HPLC-ESI-Q-ToF and spectroscopic investigations for the identification of organic synthetic pigments in paint materials. 76 synthetic pigments, 45 of them never previously reported as being analysed by this technique, were characterised by Py-GC/MS and classified in the following classes: metal complex, β-naphthol pigment lakes, BONA pigment lakes, disazopyrazolone, triarylcarbonium, dioxazine, anthraquinone, indanthrone, isoindoline and thioindigo. This permitted to expand the data on the naphthol AS, benzimidazolone, phthalocyanine and perylene classes, with respect to the literature. An approach based on HPLC allowed us to resolve complex mixtures of pigments also when natural dyes are mixed with synthetic pigments.
The investigated case studies include artworks by Julian Schabel, Keith haring, 20th century American artists, Clyfford Still (1904-1980) and Jackson Pollock (1912-1956). The CNR-PG Unit synthesised several eosinY lakes and used them to prepare reference paint layers in linseed oil. The paint layers were fully characterized by spectrophotometric techniques and subjected to artificial ageing. DCCI focused on the quantification of eosin in the different paint layers, before and after ageing, and on the characterization of eosin photo-degradation products.

Published papers
Ghelardi E., A multi-analytical approach for the characterization of contemporary paint materials, PhD Thesis, 2014, University of Florence.
Ghelardi E., Degano I., Colombini M.P., Mazurek J., Schilling M., Learner T. Py-GC/MS applied to the analysis of synthetic organic pigments: characterization and identification in paint samples, Analytical and Bioanalytical Chemistry, 2014, in press.
Ghelardi E., Degano I., Modugno F., Colombini M.P. An integrated approach to the study of Ri de Pomme, a painting by Julian Schnabel, 2014, in preparation.


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