Identification of synthetic organic pigments: the role of a comprehensive digital Raman spectral library

Identification of pigments in modern and contemporary arts is indispensable to determine correct conservation strategies, to study degradation processes and to answer authenticity-related questions. Since the early 20th century, the introduction of synthetic organic pigments (SOPs) has enormously increased the number of available pigments. Micro-Raman spectroscopy has proven to be the first line technique for the identification of these pigments, which often only show small variations on the same basic chemical structures. To ensure a correct identification, however, an as complete as possible library of reference spectra is needed. Although reference spectra of SOPs have been published before, they have always been limited to a certain number of pigments. Some publications discuss only one or a few chemical classes, and others are limited to a certain number of pigments belonging to different classes; none, however, have attempted to create an extensive library of commercially available pigments. Moreover, most of the reference spectra published so far are only available as small imprints or as peak lists and, as such, hardly usable for spectral matching algorithms. Often, flow charts have been developed on the basis of the pigments studied, to help in identifying unknown SOPs. In this paper, a digital spectral library that comprehends almost 300 spectra of different SOPs is presented. The library was tested by means of non-invasive analysis of four contemporary paintings from the collections of the Stedelijk Museum voor Actuele Kunst (Ghent, Belgium). Published flow charts have been evaluated in relation to digital search algorithms by using the extensive library of SOPs. To enable fellow research and conservation institutes to make use of these data, the nearly 300 reference spectra of synthetic organic pigments are available in digital format on http://modern.kikirpa.be. Copyright (C) 2012 John Wiley & Sons, Ltd.