A comparative appraisal of Raman band ratioing and chemometric analysis for classification of ancient papyri

Abstract

Papyri from past civilizations represent key information sources to understand historical societies and their culture. In general, the most critical information usually derives from the inks rather than the supports. In this work, we combine several analytical tools, including vibrational spectroscopies (FT-IR and Raman) and SEM/EDX spectroscopy, to characterise the inks of various papyri in the Palau-Ribes collection (Barcelona, Spain). The Raman spectra corroborate the presence of soot-based inks as the primary type. We use a five-peak spectral model for the deconvolution of the first-order region of the Raman spectra, which allows us to derive information about the carbon type of the soot-based inks. Further, we propose various Raman band ratios for a thorough evaluation of the graphitization process of carbon and possible classification of the ancient inks. At the same time, we use multivariate analysis, mainly assisted by cluster analysis, principal component analysis, and the Fisher discriminant functions, to enable an excellent alternative way to verify the classification and differentiation of the inks safely as a function of the historical time.