Mineralogical and thermal analyses of the Hellenistic ceramics from Laodicea Temple, Iran

In this study, thermal behavior of the Hellenistic ceramics from Laodicea Temple (Seleucid period), Iran, was investigated through spectroscopic, thermogravimetric, mineralogical and chemical analyses. The results indicate the same calcareous raw material was used for the ceramic production. On the basis of X-ray diffraction (XRD), four firing temperature ranges can be established: T < 750 °C, 850-900 °C, 950-1000 °C and 1000-1050 °C. These ceramics were differentiated based on different mineralogical phases resulting from the different firing temperatures. For low temperature fired ceramics, the main crystalline phases were calcite and quartz. Gehlenite is the predominant new mineral at 850 °C. CaO reacts with gehlenite and forms anorthite at around 950 °C. Diopside grows with increasing firing temperature up to 1050 °C in the Ca and Mg-rich clayey compositions and an oxidizing atmosphere, corroborated by trace hematite. The contemporaneous presence of secondary calcite and newly formed crystalline phases (diopside, gehlenite) at high temperatures indicates fast heating rate and/or short soaking time as well as coarse-grained primary calcite. Fourier Transform-Infrared (FT-IR) spectrometry indicates a considerable change in phase structures and a progressive decrease and broadening in calcite bands as a function of firing temperature.