Silicon as it is used in semiconductor manufacturing, is made up of quartz. Oxygen which reacts very fast with silicon even at room temperature, and which is present in quartz associated with silicon as silicon dioxide SiO2, must be removed. This is done just above the melting point of silicon (1414 °C) in furnaces using carbon. At 1460 °C oxygen cleaves of the silicon and reacts with carbon C to carbon monoxide CO:
Iron prevents the reaction of silicon and carbon to form silicon carbide. At these temperatures the carbon monoxide is in gaseous state and can be separated from the molten silicon easily. However, the raw silicon is still heavily polluted. There are up to 5 % impurities, such as for example iron, aluminum, phosphorus, and boron. These substances must be removed in additional processes.