Thermal Behaviour of a Carbonatic Clay: A Multi-Analytical Approach

A Miocene carbonatic clay quarried in Transylvania (Romania) has been used for more than 100 years for the production of traditional ceramic ware, bricks, and tiles. To investigate the mineralogical and microstructural changes of this clay when heated between 700 °C and 1200 °C, a combination of polarized light optical microscopy, X-ray powder diffraction, scanning electron microscopy coupled with energy dispersive X-ray spectrometry, and Fourier transform infrared spectroscopy was applied. Primary mineral phases such as illite, muscovite, feldspar, carbonate, Fe oxyhydroxides, and quartz undergo a gradual thermal alteration and form, besides a glassy phase, a wide range of minerals such as gehlenite, clinopyroxene, feldspar, maghemite, hematite, mullite, and α-cristobalite. These firing phases can be regarded as ‘ceramic markers’. A comparison between the data obtained by several methods is discussed. The combination of the optical appearance and the microstructure on one side, and the specific associations of primary phases and ceramic markers on the other side, can be used as a ‘ceramic thermometer’ in estimating the firing temperature for ancient ceramics.