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

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 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 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.