Ultramarine is made from simple, relatively cheap materials, typically china clay, feldspar, anhydrous sodium carbonate, sulfur, and a reducing agent (oil, pitch, coal etc.).
3.5.3.1
Clay Activation
The transformation of kaolinite to metakaolinite is brought about by heating the clay to about 700 °C causing hydroxyl ions to be removed as water. The rate of dehydroxylation as a function of heating has been studied [3.164]. The reaction can either be a batch process with the clay in crucibles in a directly fired kiln, or a continuous process in a tunnel kiln, rotary kiln, or other furnace.
3.5.3.2