Insights into the EPR characteristics of heated carbonate-rich illitic clay

• Raw and thermally treated carbonate-rich illitic clay were investigated by EPR
• The clay was heated between 400°C and 1200°C with 2 hours soaking time
• Main changes in resonance signals are due to oxidation of Fe2 + and Mn2 +
• The environment of the resonance sources varies with temperature
• EPR might be used for inferring technological conditions for ancient ceramics

The response of carbonate-rich illitic clay heated up to 1200 °C was investigated by means of electron paramagnetic 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 Mn2 +, is replaced over 700 °C by a large signal, mainly due to Fe3 +. Oxidation of Mn2 + (EPR active) to Mn3 + (EPR silent) or Mn4 +, and Fe2 + (EPR silent) to Fe3 + (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 resonance 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 archaeological ceramic objects. Data gathered from other methods may also help to constrain the EPR signal shape.