Surface enhanced Raman scattering for the analysis of red lake pigments in painting layers mounted in cross sections

Cross sections are very useful in the scientific investigation of paintings and polychromies as they show the sequence of the layers and allow the detection of colorants in each layer through micro-Raman spectroscopy. Surface-enhanced Raman scattering (SERS) presently represents the most promising tool to overcome the limits of a traditional micro-Raman approach in the investigation of painting materials containing fluorescent organic dyes. Among the SERS substrates, silver colloidal pastes are suitable to analyze minute samples by drop-coating them with the paste, without the need of specific pretreatments. The application of silver colloidal pastes has been explored here on two cross sections obtained from the 13th century painted wooden statue and from the 16th century mural painting. The main issue when employing such a substrate consists in achieving a suitable coating on the complex surfaces under investigation. In this work, some tests were devoted to enable the display of coated samples through the microscope coupled to the Raman spectrometer and to enhance the signals from lake pigments. The analytical procedure was successful in gaining SER spectra from the sections: the red, highly fluorescent pigment grains were identified as lake pigments obtained by kermes and by a purpurin-rich plant dyestuff. SER signals were discussed by considering SER spectra obtained from pure substances and/or reference materials and data reported in the literature. The work represents a further step toward the possibility of fully exploiting the high spatial resolution of the micro-Raman spectrometer to investigate samples from painted objects.