Ceramic technology: how to characterise black Fe-based glass-ceramic coatings

Abstract

The study of the ancient black ceramic coating that decorates the surface of Classical and Hellenistic pottery from Attica and other main production centres in the Mediterranean has been the subject of more than three centuries of research. The physicochemical characterisation and analysis of the black glaze/gloss (BG) with the use of a broad spectrum of analytical techniques has gradually revealed its nature as an iron-based glass-ceramic material. At its best, it is coloured by polycrystalline nanoparticles of magnetite or mixed magnetite/hercynite spinels, dispersed in the amorphous K-aluminosilicate phase doped by Fe2+/Fe3+. We discuss the technique to produce the BGc (black glass-ceramic, hereafter), known as the “iron reduction technique”, with emphasis on the 3-stage firing under oxidising-reducing-oxidising (ORO) conditions as well as the latest analytical results through XPS, high-resolution TEM, SR-micro XRD and XANES. The discussion is based on data and research experience accumulated over the years from the application of conventional techniques (such as SEM-EDS and hhXRF) on ancient samples, laboratory reproductions and contemporary full-scale reproduction of similar artefacts. We show that the parameters affecting the quality of the BGc (sheen, colour, micromorphology) depend on the nature of the clay-slip, the application process, the ORO firing scheme and weathering mechanisms due to burial in humid terrestrial or marine environment. The proposed analytical protocol for the routine characterisation of the micromorphology and the analysis of the BGc and the ceramic body involves the use of non-destructive or minutely destructive techniques with special emphasis on the pottery sample classification and documentation.