The outer vial is then sealed. Vapor from the solvent of the outer vial then diffuses into the solution in the inner vial, causing the compound to grow crystals. The vertical surfaces of the inner vial should not touch the outer vial to keep the outer solution from rising by capillary action and filling the inner vial. Thermal Gradient Thermal gradient methods usually produce very high quality crystals. Such methods include slow cooling of sealed, saturated solutions, refluxing of saturated solutions, sublimation, and zonal heating.
Zonal heating is used primarily for crystallizing solid solutions or mixtures.
Small crystals may sometimes be grown larger by zonally refluxing a supersaturated solution. Sublimation may be carried out in a variety of tubes or vessels. Sealed vessels offer an advantage for sublimation in that the chamber may be evacuated or a partial pressure of some inert gas may be introduced before sealing the sample in the apparatus. Sublimation methods consistently produce very high quality crystals. Larger crystals may be grown either by decreasing the thermal gradient or by cyclic heating and cooling of the sample.