Effect of firing temperature and atmosphere on ceramics made of NW Peloponnese clay sediments: Part II. Chemistry of pyrometamorphic minerals and comparison with ancient ceramics

The influence of firing conditions on the chemical composition of the major pyrometamorphic phases of Ca-, Fe-rich Plio-Pleistocene sediments of NW Peloponnese is studied. These sediments have been used in the production of ceramic artefacts for a large historical period and are still in use by the local heavy clay industry. Firing conditions were oxidising (CO/CO2, %, ranging from 0.2 to 4.4) or reducing atmosphere (CO/CO2, %, ranging from 11.2 to 23.0), for a temperature range 850–1050 °C. At 850 °C, for both atmospheres, fassaite and gehlenite are the predominant new minerals with anorthite and wollastonite also formed in a small quantity. Up to 950 °C, the chemistry of minerals in oxidising and reducing conditions does not exhibit considerable differences. Fe3+ is present even in reducing atmosphere entrapped in the structure of fassaite, gehlenite, anorthite. At 1050 °C, in reducing atmosphere, more hedenbergite component (CaFe2+Si2O6) incorporates in fassaite, whereas the ferrigehlenite component (Ca2Fe23+SiO7) is absent from gehlenite structure. Fe3+ is detected only in fassaite and anorthite structure. The work concludes with a summary of the results from Part I and Part II, in the form of a reverse engineering table for the determination of the firing conditions in a ceramic body by using analytical information. Three examples are given that compare ancient ceramic sherds with ones reproduced in laboratory conditions and demonstrate the effectiveness of this approach.