Frits are prepared mainly by melting raw materials in a tank melting furnace, at high temperatures of 1350 up to 1550 ºC. The material is charged in the rear wall by a screw charger forming a pile, melted and then quenched in water, thus turned into a solid, insoluble, fragmented material. In comparison to the glass industry are these tanks rather small 8-15m² producing app. 20 t/d in average. The combustion air has a low recuperative air preheat of 350 - 500 °C.