Development of the Polish ceramic industry over the last ten years has resulted in a sharp increase in demand for basic raw materials, especially for ceramic clays – both white- and light-firing as well as red-firing varieties. Polish sources of white-firing clays are rather scarce. Their production is based on Santonian sandy-clayey sediments of the North-Sudetic Trough (Lower Silesia, SW Poland). Light-firing varieties occur mainly in Lower Jurassic sediments on the northern margin of the Holy Cross Mountains (central Poland). Red-firing well-sintering clays of Triassic age are known in the northern margin of the Holy Cross Mountains as well as in the Cracow-Silesian Monocline.
The white-firing Santonian clays contain mainly highly-ordered kaolinite (~50%) and quartz (~30%) with minor amounts of illite. Their grain-size median ranges between 1.9 and 2.5 μm. They show moderate plasticity (bending strength after drying at 1.7–1.9 MPa) and weak sinterability (water absorption after firing ~12%), but greater lightness after firing (L parameter ~86%). The light-firing Jurassic clays are kaolinitic-illitic in character, with a variable content of quartz. Kaolinite is represented by the low-ordered variety. The grain-size median is also variable, ranging between 1.1 and 3.4 μm. They show good plasticity (bending strength after drying 2.6–2.8 MPa), good or very good sinterability (water absorption after firing <6%) but moderate lightness (L ~79%).
Triassic red clays are highly polymineralic, with illite, kaolinite, smectite, chlorite and mixed-layered minerals occurring in variable amounts. Their specific features are: large hematite content (5–12%), commonly small CaO content (<0.5 wt.%) and an extremely small organic matter content (⩽0.1 wt.%). Their grain-size median usually varies between 2 and 3 mm. Using such clays, sintered ceramics (e.g. stoneware tiles) are obtained after firing in the temperature range 1150–1200ºC.