Calcium silicate units are produced by mixing sand, lime and water. This mixture of natural ingredients is moulded into shape by mechanical or hydraulic presses. After moulding, the "green" material is hardened in an autoclave. In these autoclaves steam is introduced at pressures of 8 to 16 bars to raise the temperature up to approximately 200 °C. After some hours of autoclaving the units have developed their final properties, especially the strength, and are ready for packing and dispatch.