Thermal analysis of ancient ceramics using the microchemical and microstructural alterations of foraminifera

Thermal degradation of planktonic and benthic foraminifera within the coarse wares of Qizlar Qaleh and clay from Khangiran Formation in Kopet Dagh basin, Iran, were examined at different temperature rates. Hyaline and agglutinated tests of foraminifera were investigated through Thermogravimetry (TG), Derivative thermogravimetry (DTG), Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscope coupled with Energy Dispersive Spectrometry (SEM-EDS) to identify their thermal stability and chemical transformation from 500 °C to 1000 °C. TG/DTG and FTIR analyses show significant weight loss in the range of 750–800 °C and a progressive decrease in the intensity of carbonate bands at 1424, 2514 and 875 cm− 1 (from 700 °C, to 800 °C) which indicates the release of CO2. In addition, the chemical characteristics of the agglutinated tests indicate high thermal stability due to their high silicate component. The complete decarbonization of hyaline tests occurred at around 800 °C, whereas the agglutinated tests are still preserved at 1000 °C. Finally, microstructural and microchemical analysis of foraminifera from the ancient ceramics and Khangiran clay indicated firing temperature of the ceramics at around 700–750 °C. The X-ray diffraction (XRD) and a comparative TGA and FTIR analysis of the ceramics and the fired clay samples from Khangiran Formation also show a high intensity of calcite bands, indicating a firing temperature < 750 °C. This approach provides a new tool for estimating the firing temperatures of ancient ceramics.