The premise behind this approach is that contaminants are more soluble in molten metal than in solid metal. A rotating circular heater slowly glides over an impure metal rod or bar in zone refining. The impurities flow into the adjacent molten zone as the circular heater moves, and the pure metal crystallises out. This is performed numerous times, resulting in contaminants being segregated at one end of the bar. After then, the impure end is cut off. Boron, silicon, gallium, and other elements are refined in this fashion.