FeIII in pottery: Identifying firing temperature and ambiguity

Abstract

Firing parameters for archaeological pottery were estimated from the coincidence between iron concentration (FeIII) in the pottery and in a clay specimen, both chemically and mineralogically identical, fired under controlled conditions. Most pottery specimens analysed here were produced in a reducing environment, and one of them has been used as a pan. For reference clays, heating causes FeIII signal intensification, followed by reduction at high temperatures in reducing atmosphere. This behaviour differs from the model to identify firing parameters (Mangueira et al., 2013, 2016, 2018) and results in ambiguity to identify the firing temperature for the archaeological pottery. Pure hematite (Fe2O3) shows the same behaviour in reducing environment and indicates that the iron oxide in the clays is responsible for this kind of variation. The ambiguity of the firing temperature of some ceramics could not be clarified neither by producing the ceramics in the laboratory nor by X-ray diffraction and infrared spectroscopy techniques. However, electron paramagnetic resonance provided the firing atmosphere and the possible firing temperatures.