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 aggregate and binder, aggregate size distribution, aggregate/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. Rock types 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.