OPLS multivariate regression of FTIR-ATR spectra of acrylic paints for age estimation in contemporary artworks

In recent years the interest and demand for artworks has been increasing as they are an interesting commercial investment due to their growing value in the market. This explains the increasing number of counterfeits dealing with artworks that has led to the development of new methodologies for their characterization. The material characterization of these types of works can provide relevant information for both authentication and conservation/restoration. Thus, in this study multivariate chemometric methods were applied to FTIR-ATR spectroscopic data for artwork dating purposes. To that end, ageing prediction models were developed for Liquitex^® and Hyplar^® brands. Paint samples containing the green synthetic organic pigment (PG7), were exposed to artificial ageing and analysed with FTIR-ATR and Py-GC/MS for characterization and monitorization of the main components (binding medium, pigment and additives). Although the OPLS ageing models were mainly characterized by the modifications suffered by both the binder and the surfactants, a universal model could not be developed due to differences in the modification trends of the different brands. The applicability of the OPLS modelling for artwork dating purposes was tested in artworks provided by internationally recognized contemporary Basque artists. For Liquitex^® a significant correlation (p < 0.05) between natural and accelerated ageing could be established, in which approximately 50 h of accelerated ageing under the applied conditions were equivalent to one natural year. This correlation might have possible applications in the dating of artworks for up to at least 22 years. Thus, the study demonstrates that FTIR-ATR combined with chemometrics is a potential method for artwork dating and a valuable source of information about the chemical processes involved in paint ageing, which can be of great help in the conservation and restoration steps.