The mosaic of the Frigidarium of “Villa Bonanno” in Palermo: mineralogical and petrographic analyses for in situ conservation and restoration interventions

Abstract

The topic of this study was the mineralogical and petrographic characterization of bedding mortars (made of different layers) and tesserae of Roman age (3rd Century A.D.), taken from the mosaic of the Frigidarium of “Villa Bonanno”, brought to light by archaeological excavations conducted in the historical centre of Palermo. The collected samples have been analysed by thin-section optical microscopy (PLM), and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS). The study was aimed to define the “recipe” (composition of temper and binder, temper size distribution, temper/binder ratio), in order to assess the provenance of raw materials (supply site/area) and to acquire useful information in order to formulate "restoration mortars" that should be most comparable with the original ones, for replacements and/or integration. Lithotypes constituting the coloured tesserae were also characterized by thin-section optical microscopy. The mineralogical and petrographic investigations allowed establishing two different recipes used for the formulation of the studied mortars in terms of both compositional and textural features. The aggregate is composed by diverse proportions of detritic calcareous granules (both bioclasts and limestone fragments deriving from the local outcropping biocalcarenites and limestones), siliceous sand (monocrystalline quartz, chert and quartzarenite fragments), volcanic ash (pozzolana) and sometimes cocciopesto. The resulting hydraulic binder was the product of the ‘pozzolanic reaction’ between volcanic ash and the aerial lime (specifically made by the calcination of locally available magnesian limestone or dolostone). The coloured tesserae can be all classified as compact limestones of Mesozoic or Cenozoic age, likely of local provenance. Detailed micropaleontological analysis (nannofossils) is ongoing for a more precise stratigraphic assignment. It was found a satisfactory technological correspondence with the more or less contemporary manufactures within Palermo and western Sicily (Montana et al., 2016). Concerning the sandy aggregate, it was shown a substantial compositional and textural equivalence with the plasters used in the baroque palaces of Palermo demonstrating a remarkable continuity in the criteria of selection of raw materials, mainly dictated by the qualitative characteristics of locally available geomaterials.