Influence of firing conditions on ceramic products: Experimental study on clay rich in organic matter L. Maritan a, ⁎ , L. Nodari b , C. Mazzoli a,c , A. Milano b , U. Russo b a Dipartimento di Mineralogia e Petrologia, Università di Padova, C.so Garibaldi 37, I-35137 Padova, Italy b Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, I-35131 Padova, Italy c Istituto di Geoscienze e Georisorse, CNR, Padova, C.so Garibaldi 37, I-35137 Padova, Italy Received 24 May 2004; received in revised form 15 April 2005; accepted 12 August 2005 Available online 6 October 2005 Abstract 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. © 2005 Elsevier B.V. All rights reserved. Keywords: Organic matter; Heating rate; Firing atmosphere; Mössbauer spectroscopy (TMS); Diffractometric analysis (XRD); Fourier transformed infrared spectroscopy (FTIR) 1. Introduction Plasticity is an essential parameter, which determines the attitude of the clayey material moulded and modelled in pottery production, a criterion for clay classification (Marsigli and Dondi, 1997) determined by measuring the Atterberg limits. The presence of organic matter may significantly increase clay plasticity (Husein Malkawi et al., 1999). Organic matter occurs as humus in soils excavated for pottery production (Nannipieri, 1993; Rivero et al., 1998) or may be deliberately added by the potter (Johnson et al., 1988) in order to reach the required rheological features of the raw material. If adequate conditions occur during firing, organic matter may completely burn, contributing toward increasing secondary porosity. But what is the role of organic matter in the firing process and how does it affect the sequence of reactions in the resulting ceramic products? Applied Clay Science 31 (2006) 1 – 15 www.elsevier.com/locate/clay ⁎ Corresponding author. Tel.: +39 049 8272032; fax: +39 049 8272010. E-mail address: lara.maritan@unipd.it (L. Maritan). 0169-1317/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.08.007