Phasing the history of ancient buildings through PCA on mortars' mineralogical profiles: the example of the Sarno Baths (Pompeii)

A total of 83 wall joint mortar samples collected from the Sarno Baths complex in Pompeii (Naples, Italy) were analysed by optical microscopy (OM) and X-ray powder diffraction-quantitative phase analysis (XRPD-QPA) in order to scan the ancient construction phases and modern restorations to which the building was subject. The major issue to overcome in the research depended on the fact that the most part of the analysed mortars was taken from undated structures, while only 35 were collected from dated ones. In order to observe correlations in sample distribution which could reflect ancient building phases and modern restorations, we then processed XRPD-QPA data of the mortars through principal component analysis (PCA). A rational subdivision of the full dataset into a smaller one before performing PCA was a useful step for a proper enucleation of coherent groups. The presence in most of the resulting groups of dated samples also allowed us to place in a precise timeframe the undated ones. This study demonstrates that our approach, integrating the traditional archaeological analysis with archaeometrical methods and statistics, could be adopted as a tool with which to frame the constructive episodes in other ancient buildings in Pompeii as well as at other archaeological sites.