A purification technique in which a molten or high-temperature zone is moved along a length of material to be purified to bring about impurity segregation; the impurities become concentrated in the slowly moving hot zone, leaving behind the cooler solidified material that has a higher purity than the original material. The technique depends on differences in composition of liquids and solids in equilibrium and may be repeated to attain high degrees of purity. It usually is applied to crystalline materials, such as germanium or silicon.