Production technology of early-Hellenistic lime-based mortars from a Punic-Roman residential area at Palermo (Sicily)

Abstract

The topic of this study was the mineralogical and petrographic characterization of lime-based mortars of Hellenistic-Roman age (3rd century BC), taken from a residential area located in the historical centre of Palermo, near the walls of the Punic-Roman Neapolis, recently brought to light by excavations carried out by the Soprintendenza BB.CC.AA. of Palermo at Palazzo del Gran Cancelliere (Palermo). The collected mortars have been analyzed by optical microscopy (PLM), X-ray powder diffraction analysis (XRPD) 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. The mineralogical and petrographic investigations allowed to establish four different recipes used for the formulation of the studied floor and wall mortars in terms of both compositional and textural features. The aggregate resulted to be composed by different proportions of alluvial calcareous and siliceous sand or else by “cocciopesto”. Aerial lime based mortars (for the most part magnesium-bearing) have been used for wall coatings and decorations, while the use of pozzolanic binder was attested for floor covering. Furthermore, it is interesting to report an unusual mosaic floor manufactured with tesserae obtained from overfired limestone scraps (locally produced). Thus, ‘recipes’ having different functionalities were established. The sandy aggregate was quarried from the coastal alluvial deposits of the river Oreto, while the binder was mostly made by the calcination of locally available magnesian limestone or dolostone. It was found a satisfactory similarity with the more or less contemporary manufactures within western Sicily. It was also 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.