Multi-analytical and non-invasive characterization of the polychromy of wall paintings at the Domus of Octavius Quartio in Pompeii

The materials and techniques of wall paintings decorating the Domus of Octavius Quartio, an outstanding patrician villa in ancient Pompeii, were investigated by means of a multi-analytical and non-invasive approach. Chemical and mineralogical characterization of pigments was obtained by the combined use of external reflection Fourier Transform infrared spectroscopy (ER-FTIR), sequentially shifted excitation Raman spectroscopy (SSR-RS) and portable X-ray fluorescence (XRF) spectroscopy. ER-FTIR and SSR-RS provided complementary molecular information for the characterization of both organic and inorganic pigments and helped to identify the pigments in the white, red, yellow, black, blue and pink paints. XRF spectroscopy provided elemental characterization of the inorganic pigments and assisted in the determination of pigment mixtures mainly constituting the green paints. Results pointed to the use of a characteristic Roman palette, consisting of calcite, cinnabar, red and yellow ochre, green earths, Egyptian blue and carbon black. A fresco application of the pigments was suggested for the execution of the wall paintings. The combined approach by spectroscopic techniques also recognized that wax had been applied as a consolidant or protective coating, and weathering products in the form of gypsum and calcium oxalate also occur.