The object of the present investigations is a clay from a locality in Kunda, Estonia. It was successfully exploited for brick manufacturing in the past, and its re-exploiting is attractive for its use as the main part of green ceramic bodies. The goal of this paper was to focus on the internal processes which occur in the sample during heating and cooling and their influence on elastic and inelastic mechanical properties. The clay contains 52.3 % of illite, 28 % of quartz, 7.5 % of kaolinite, 5 % of chlorite (as a plastic component), and 5 % of K-feldspar (as a flux). The clay contains residual physically bound water (~2 mass %) that is released from the clay during its heating up to 200–250 °C. This process was accompanied by an increase in Young’s modulus by ~50 %. Dehydroxylation, which runs between 450 and 700 °C, caused a slight expansion and a very small decrease in Young’s modulus. The solid-state sintering, which was not accompanied by contraction, starts at ~600 °C, as follows from an increase in Young’s modulus. At ~900 °C, a glassy phase was created which was confirmed by an increase in the logarithmic decrement. The liquid-phase sintering began and continued even at a cooling stage of the firing down to 900 °C, which can be considered as the temperature of the glass transformation. During the cooling, only the β → α transformation of quartz was observed. The logarithmic decrement had a positive correlation with Young’s modulus. The logarithmic decrement decreases with the release of the physically bound water, then passes through maximum in dehydroxylation region, and steeply rises up when the glassy phase is created. During cooling, the logarithmic decrement steeply decreases up to reaching the temperature of the glass transition and then low logarithmic decrement stays up to room temperature. Only β → α transition of quartz is expressed by a small maximum of the logarithmic decrement.