An overview of Principal Components Analysis approaches in Raman studies of cultural heritage materials

The present overview answers the need of assessing the current state of the art concerning the application of principal components analysis (PCA) to Raman spectroscopy investigations of cultural heritage and related materials. An increment of the employment of this multivariate statistic technique to Raman results in the mentioned field began between 15 and 10 years ago, after a very slow start at the turn of the millennium. A delay of about a decade was observed with respect to PCA applied to elemental quantitative data of archaeometric analyses, likely a consequence of the required spectral pre-treatment and to results of complex interpretation. Therefore, it is by now the time to summarize this evolution in a comprehensive, yet very specific way. In this overview, painting constituents were considered, both colouring materials and binders, in addition to natural and synthetic glasses, and biogenic and mineral gemmological materials. A marked unbalance between the studies pertaining to the different sections has been noticed, revealing a concentration of the work mainly on painting materials, including the study of ageing and alteration. The different aims of PCA application to Raman spectra, the various approaches and the achievable results, with the possible arising problems, were underlined, too. Special attention was given to the pre-treatment of the spectra, which was observed to be essential to overcome the influence of several issues concerning bands intensity, spectral noise, background, fluorescence and so on.