More sophisticated variants of sublimation apparatus include those that apply a temperature gradient so as to allow for controlled recrystallization of different fractions along the cold surface. Thermodynamic processes follow a statistical distribution, and suitably designed apparatus exploit this principle with a gradient that will yield different purities in particular temperature zones along the collection surface. Such techniques are especially helpful when the requirement is to refine or separate multiple products or impurities from the same mix of raw materials. It is necessary in particular when some of the required products have similar sublimation points or pressure curves.