For the formation of silicon, the carbon requirements have to be precise. Excess carbon results in SiC formation. On the other hand insufficient carbon results in excessive SiO formation which is lost in the off-gas. As the raw materials are consumed, a cave is formed around the electrode, and the top of the charge bed becomes a hard crust. This crust must be physically stoked (rabled) from the top in order for additional raw materials to be fed. The energy required for the process is provided by the electric arc between the electrodes.