Testing of clay raw material. Profiles of locations 1, 2 and 3 of ceramic raw material sampling (1–3); thin section microphotographs of ceramic raw materials (a-f); a–sample Sur35, depth approx. 0.6 m, clay rich in fine grains, iron oxide and hydroxides and fine mica flakes, PPL; b–sample Sur36, depth approx. 0.8 m, smaller content of silt fraction, orange color of clay matrix, PPL; c–sample37, depth approx. 0.6 m, high content of silt fraction and fine opaque minerals, PPL; d–sample Sur38, depth approx. 1.1 mm, smaller content of silt fraction, very fine grained clay, PPL; e–sample Sur39, depth approx. 0,6 m, medium sorted, fragments of sedimentary rocks occur, PPL; f–sample Sur40, depth approx. 0.8 m, clay with the smallest content of silt fraction, fragments of sedimentary rocks occur.