Pigment characterization of drawings and painted layers under 5th-7th centuries wall mosaics from Ravenna (Italy)

The results of a multi-analytical investigation on preparatory drawings below three late antique wall mosaics in Ravenna (Italy) are reported and discussed. Examinations were made on two sinopiae in mosaic substrates detached from the apse of the basilica of St. Apollinare in Classe and on the undercoloring characterizing some finds of wall mosaics coming from the basilicas of St. Agata Maggiore and St. Croce. Due to the historic and archaeological relevance of the finds, the research was carried out mainly through noninvasive techniques, such as fiber optics reflectance spectroscopy (FORS) and energy dispersive X-ray fluorescence spectrometry (EDXRF). Analyses by Fourier transform infrared spectroscopy (FTIR) and Raman micro-spectroscopy (μRaman) were only performed on incoherent material and small loss particles in the interstices among the tesserae. In some selected painted area, visible-induced infrared luminescence (VIL) was employed to verify if Egyptian blue was used and how was distributed, if present. By comparing the results obtained with the different techniques, a proposal about the nature of the pigments was formulated: the pigments employed to make the sinopiae from St. Apollinare in Classe are iron-based pigments, while the analyses of the undercoloring show a more complex character, where the use of a broader palette of pigments was recognized (red ochre, green earth, Egyptian blue).