Both methods require finding two solvents or solvent mixtures in which the compound is soluble in one system but insoluble in the other. The two solvent systems should be immiscible or nearly immiscible for liquid diffusion and should be miscible for vapor diffusion. Crystal growth may be slowed somewhat by cooling the apparatus. Liquid diffusion usually requires that the less dense solvent system be carefully layered on top of the more dense system. The sample can be dissolved in either solvent system.
Crystals grow at the interface between the solutions.
When compounds precipitate immediately upon being formed, it is possible to slow down the reaction and thus grow larger crystals by putting the reactants in different liquid layers which are separated by a third solvent layer that is not miscible with either of the layers or with the sample. Note that the top layer should be added very slowly to assure a minimum of mixing of the layers. Vapor diffusion is carried out by dissolving a small amount of the sample in a small vial, then placing this inner vial inside a larger vial that contains a small volume of a solvent system in which the sample is insoluable. 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.