Focusing on red and black engobes in Roman pottery from Cumae (southern Italy): Pompeian Red Ware and Graue Platten ceramic productions

This short paper provides additional details to a recent minero-petrographic investigation on local and exotic ceramic artefacts collected from the Forum of Cumae and particularly about the composition of the ceramic slips of locally produced Pompeian Red Ware (PRW) and the imported pottery known as Graue Platten (GP). These pottery fragments were characterised via non-destructive techniques (portable XRF and Raman spectroscopy), the results of which have been validated with micro-destructive X-ray diffraction, and conventional electron and optical microscopy techniques. As a whole, the internal red slip of PRW samples should be made using a slurry of a levigated, iron-rich clay material (i.e., ochre) suspended in water and then fired in oxidizing atmosphere in order to obtain the typical red hue. Clear differences in the technological manufacturing between the older (dated from the 1st century BCE to the early 1st century CE) and later (from the 1st to the 4th centuries CE) PRW ceramic productions have been highlighted. The dark colour of the ceramic slip in fragments of Graue Platten (dated between the 1st century BCE and the 1st century CE) seems to be conferred by the occurrence of carbon black material, detected via Raman spectroscopy and EDS spectrometry. Probably, presence of carbon black could be due to the thermal decomposition, in absence of oxygen (pyrolysis), of colloidal organic matter. This investigation sheds new light on production technologies of engobes of these ancient Roman ceramic productions, although further investigations are required to infer the provenance of clayey materials used for the manufacturing of PRW coating in archaeological workshops of the Campania region