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Illitic clay from the locality of Liepa, Latvia, was investigated using dynamic thermomechanical analysis during the heating and cooling stages of firing. Differential thermal analysis, thermogravimetry, thermodilatometry, X-ray diffraction and porosimetry were also performed to shed light on the processes influencing the elastic properties of clay. The increase in the Young's modulus (YM) at low temperatures was linked to the release of physically bound water. Above 850°C, the bulk density and YM both increased as a consequence of sintering. The YM was more sensitive to the progress of sintering compared to dimension changes. The YM values continued to increase during cooling until the glass-transition temperature was reached. At this temperature, the first microcracks caused by the differences in thermal expansion coefficients of the present phases were expected to appear. The YM showed a sharp V-shaped minimum at the β → α transition of quartz, which was a result of alternation of the mechanical radial stresses around the quartz grains. When the transition of quartz was completed, the YM continued to decrease because microcracks were still being created at the boundaries between the different phases. The decrease of the YM during cooling from the glass-transition temperature down to room temperature was ~50% for all of the firing temperatures and isothermal periods applied.

Dynamical thermomechanical analysis of 5 illite-based clays from deposits in Slovakia, Estonia, Latvia, and Hungary is presented. The clays consist of illite (37–80 mass%), quartz (12–48 mass%), K-feldspar (4–13 mass%), kaolinite (0–18 mass%), and calcite (0–3 mass%). Young’s modulus is measured during the heating and cooling stages of firing (25 °C → 1100 °C → 25 °C). The liberation of the physically bound water increases Young’s modulus by ∼70% for all studied clays. By increasing the temperature, dehydroxylation and the α → β transition of quartz take place without a significant effect on Young’s modulus. Sintering, which starts at 800 °C, leads to an intensive increase in Young’s modulus up to the highest temperature (1100 °C). The increase remains also in the early stage of cooling (1100 °C → 800 °C). This increase of Young’s modulus is also the result of solidification of the glassy phase, which is finished at ∼750 °C. A sharp minimum of Young’s modulus is observed at around the β → α transition of quartz. Then, Young’s modulus still decreases its value down to the room temperature. The physical processes observed during heating and cooling do not differ in nature for the studied clays. Values of Young’s modulus vary at around 8 GPa, up to 800 °C. During sintering, Young’s modulus reaches values from 30 GPa to 70 GPa for the studied clays. The microstructure and composition given by the origin of the clay play a cardinal role for the Young’s modulus of the final ceramic body.

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