Blue Shades in Plasters: Archaeometric Role in Dating of the Casamassima (Southern Italy) Historical Facades

Casamassima old town, locally known as "blue town", is widespread stratified blue paint covering the facades of the historical buildings, not reported in the archival data and historical sources. The archaeometric results presented in this study aimed to investigate raw materials, in particular pigments, used to cover the facades to contribute more precisely to the historical reconstruction of this local custom. A set of nine samples was collected from two representative historical buildings and observed by reflected light optical microscopy and scanning electron microscope (SEM-EDS) and their mineralogical characterisation was carried out by means of X-ray powder diffractometry (XRPD). The comparison of the mineralogical and elementary results and the stratigraphic, morphological, and microstructural observations made it possible to highlight a significant stratification of these plasters and to recognize the artificial ultramarine blue applied with lime, as the main mineral pigment, together with red ochre and blanc fixe. The identification of pigments was crucial to date the blue plasters.