Thermal analysis as a method of characterizing ancient ceramic technologies

Abstract

Ceramic materials represent manufacturing techniques which were improved consistently during the course of time. The components of ceramic materials are the “fingerprint” of the stable and/or metastable solid phases formed during the firing; the production processes of antique ceramics and pottery can be derived from their assemblage. There are many recognizable phases and their association depends, more than on their chemistry, upon the mineralogy of the raw materials, their grain-size distribution, maximum heating temperature, heating ratio, duration of firing and kiln redox atmosphere. All these factors help in understanding the “course” of reactions. Heating also affects the contact between the fine-sized clayey matrix and mineral clast fragments, appearing in reaction rims, sometimes showing newly-formed phases. The temperature at which ancient ceramics and pottery were fired varies over a wide range (600–1300°C) depending on the type of clay used and the kiln available, although firing temperatures not above 300–400°C have also been suggested. Clay minerals, as the main material for production of ceramics and pottery, show some characteristic reactions (dehydroxylation, decomposition, transformation) in the course of firing (heating effects) and several thermoanalytical criteria can be used for reconstruction of former production conditions. In the present work DTA, TGA and XRD results from byzantine and medieval ceramics are examined and information derived on ceramic technologies concerning raw materials and production conditions is validated by SEM observations concerning the extent of vitrification, as well as by the microstructural data provided by porosimetric measurements.