Research paper Insights into the EPR characteristics of heated carbonate-rich illitic clay Corina Ionescu a, ⁎, Volker Hoeck a,b , Cristina Gruian c , Viorica Simon c a Department of Geology, Babeş-Bolyai University, 1 Kogălniceanu Str., 400084 Cluj-Napoca, Romania b Division Geography and Geology, Paris Lodron University, 34 Hellbrunner Str., A-5020 Salzburg, Austria c Faculty of Physics and Institute of Interdisciplinary Research on Bio-Nano-Sciences, Babeş-Bolyai University, Cluj-Napoca, 400084, Romania abstract article info Article history: Received 18 October 2013 Received in revised form 23 May 2014 Accepted 26 May 2014 Available online 13 June 2014 Keywords: Carbonate-rich illitic clay Thermal treatment EPR Clay-based ceramics Archaeometry The response of carbonate-rich illitic clay heated up to 1200 °C was investigated by means of electron paramag- netic resonance (EPR) in order to define the factors influencing the shape of the resonance signals and to establish whether this method can be used for evaluation of firing temperature for clay-based ceramic objects. The results show that the dominating hyperfine sextet, at g ≅ 2, due to Mn 2+ , is replaced over 700 °C by a large signal, mainly due to Fe 3+ . Oxidation of Mn 2+ (EPR active) to Mn 3+ (EPR silent) or Mn 4+ , and Fe 2+ (EPR silent) to Fe 3+ (EPR active) respectively, combined with changes in their environment produce the resonance signals. The destruction of the carriers such as Fe-oxihydroxides, clinochlore, calcite, dolomite, altered biotite, illite and muscovite, as well as the formation of new minerals and glass are the main mineralogical processes influencing the width of the res- onance signals. The results of this study can be used in conjunction with mineralogical and microstructural data for the investigation of technological conditions such as firing temperature and atmosphere related to archaeo- logical ceramic objects. Data gathered from other methods may also help to constrain the EPR signal shape. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Clays and ceramic objects play a very important role in the develop- ment of human society. Since the first ceramic statuettes from Central Europe dated to the Late Palaeolithic (~26,000 B.P.; Vandiver et al., 1989) and the invention of pottery in South China around 20,000 B.P. (Wu et al., 2012), clays have been regarded as raw materials. They are widely distributed and in many cases available close at hand. More re- cently, clays have also been involved in the study, restoration and pres- ervation of cultural heritage, especially archaeological ceramic objects. Mudstone – here referred as ‘clay’– involved in obtaining so-called ‘traditional’ or ‘clay-based’ ceramics, consists in most cases of clay min- erals (illite, kaolinite and smectites) associated with quartz, feldspars, micas, carbonates, sulfate, Fe- and Mn-oxihydroxides, heavy minerals, as well as rock fragments and organic material. Their compositional, structural and textural changes recorded at various temperatures are used for estimating the technological conditions of firing (e.g., Cultrone et al., 2001; Maggetti, 1982; Riccardi et al., 1999). The most used investigation methods are polarized light optical microscopy (OM), electron microprobe analysis (EMPA), scanning electron micros- copy (SEM), X-ray powder diffraction (XRPD) and thermoanalysis. However, inferring the conditions, in particular the temperature at which clay-based objects were produced, even with a large margin of error, is a complex issue. This is due to the nonstoichiometric mineral reactions, the occurrence of phases which are not in equilibrium, and last but not least, the wide range of temperatures recorded within a sin- gle kiln load and even within the same pot (Gosselain, 1992; Maggetti et al., 2011). In search of a more precise evaluation of the firing temperature for ancient ceramics, several spectroscopic methods are used: Fourier transform infrared, Raman, and electron paramagnetic resonance (EPR). The latter, also called electron spin resonance (ESR), ever since it was invented by Zavoisky (1945), has been widely applied in various fields, such as crystal chemistry (e.g., coordination, distortion, oxidation state etc.), behaviour of transition metals compounds, as well as effects of natural or artificial radiation, and dating. Of particular interest in archaeometry are the EPR studies on clay minerals (e.g., Allard et al., 2012; Balan et al., 2000; Elsass and Olivier, 1978; Manhães et al., 2002; Mestdagh et al., 1980; Morichon et al., 2008) and fired clays and ceramics (Bensimon et al., 1999; Cano et al., 2013; Dobosz and Krzyminiewski, 2007; Felicissimo et al., 2010; Gualtieri and Del Monaco, 1996; Ionescu et al., 2010; Matsuoka and Ikeya, 1995; Mota et al., 2009; Presciutti et al., 2005). Clay minerals and clays contain paramagnetic ions, paramagnetic defect centres and organic free radicals (Lück et al., 1993), which produce resonance sig- nals, therefore clays are suitable for EPR investigation. “A key point in the characterisation of pottery is to assess firing conditions” (Nodari et al., 2004) and this can be achieved by various methods, including OM, XRD, SEM and EMPA. Some of these methods need a relatively large amount of material, others reveal thermal changes only in isolated Applied Clay Science 97–98 (2014) 138–145 ⁎ Corresponding author. E-mail addresses: corina.ionescu@ubbcluj.ro (C. Ionescu), volker.hoeck@sbg.ac.at (V. Hoeck), cristinagruian@yahoo.com (C. Gruian), viosimon@phys.ubbcluj.ro (V. Simon). http://dx.doi.org/10.1016/j.clay.2014.05.023 0169-1317/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay