The purpose of the refractory lining is to insulate the steel shell from the high temperatures inside the kiln, and to protect it from the corrosive properties of the process material.  It may consist of refractory bricks or cast refractory concrete, or may be absent in zones of the kiln that are below approximately 250 °C.  The refractory selected depends upon the temperature inside the kiln and the chemical nature of the material being processed.  In some processes, such as cement, the refractory life is prolonged by maintaining a coating of the processed material on the refractory surface.  The thickness of the lining is generally in the range 80 to 300 mm.  A typical refractory will be capable of maintaining a temperature drop of 1000 °C or more between its hot and cold faces.  The shell temperature needs to be maintained below around 350 °C to protect the steel from damage, and continuous infrared scanners are used to give early warning of "hot-spots" indicative of refractory failure.