Influence of firing conditions on ceramic products: Experimental study on clay rich in organic matter

The effect on ceramic products of firing conditions in the presence of abundant organic matter is assessed here experimentally by reproducing two ancient firing techniques: pit firing, characterised by a high heating rate and short residence time in a reducing atmosphere; kiln firing, with a low heating rate and long residence time in an oxidising atmosphere. As expected, pit firing conditions produced uniformly reduced ceramics, and gehlenite, diopside and spinel occurred at suitable temperatures (above 900 °C). Interestingly, in kiln firing conditions, sandwich structures formed in the ceramics, in which mineral assemblages and Fe oxidation state turned out to be different in the black core with respect to the margin. Spinel, cordierite and metallic Fe formed in the core, whereas hematite and spinel, in addition to gehlenite and diopside, crystallised in the margin at suitable temperatures (above 800 °C). Therefore, despite the oxidising atmosphere of the firing, reducing conditions occurred in the core, suggesting sluggish oxygen diffusion within the pottery. In addition, decomposition of reactant mineral phases and crystallisation of new mineral products in pit firing conditions occurred at temperatures about 50 °C higher than in kiln firing conditions, indicating that residence time strongly controlled reaction kinetics, as higher temperatures (i.e., a higher degree of reaction overstepping) are required when residence time is lower. On the contrary, illite broke down at a lower temperature than expected in both sets of firing conditions. This is interpreted as the effect of reduced water fugacity caused by burning of the organic matter.