The most commonly used procedure for the purification of  a solid material by  recrystallisation from a solution involves the following steps:
(a) The impure material is dissolved in  a suitable solvent, by  shaking or vigorous stirring, at or near the boiling point, to form a near-saturated solution.
(b) The hot solution is filtered to remove any  insoluble particles.  To prevent crystallisation during this filtration, a heated filter funnel can be used or the solution can be diluted with more of the solvent.
(c) The solution  is then allowed to cool so that the dissolved substance crystallises out.
(d) The crystals are separated from the mother liquor, either by  centrifuging or by  filtering, under suction, through a sintered glass, a Hirsch or a Buchner, funnel. Usually, centrifugation is preferred because of the greater ease and efficiency of separating crystals and mother liquor, and also because of the saving of time  and  effort, particularly when  very  small crystals are formed or when  there  is entrainment of solvent.
(e) The crystals are washed free from mother liquor with a little fresh cold solvent,  then dried.

If the solution contains extraneous coloured material  likely to contaminate  the crystals, this can often be removed by adding some activated charcoal (decolorising carbon)  to the hot, but not boiling, solution which is then shaken frequently for several minutes before being filtered. (The large active surface of  the carbon makes  it a good adsorbent for this purpose.) In general, the cooling and crystallisation steps should be rapid so as  to give small crystals which occlude less of  the mother liquor. This  is usually satisfactory with  inorganic material, so that commonly the filtrate is cooled in an ice-water bath while being vigorously  stirred.  In many  cases, however, organic molecules crystallise much more slowly, so that the filtrate must be set aside to cool to room temperature or left in the refrigerator. It is often desirable to subject material that is very impure to preliminary purification, such as steam distillation, Soxhlet extraction, or sublimation,  before recrystallising  it.  A greater degree of purity is also to be expected if  the crystallisation process is repeated several times, especially if different solvents are used.  The advantage of several crystallisations from different solvents lies in the fact that the material sought, and its impurities, are unlikely to have similar solubilities  as  solvents  and temperatures are  varied.

For the final separation of  solid material, sintered-glass discs are preferable to filter paper. Sintered glass  is unaffected by strongly acid solutions or by oxidising agents.  Also, with filter paper, cellulose fibres are likely to become included in the sample. The sintered-glass discs or funnels can be readily cleaned by washing in freshly prepared chromic acid cleaning mixture.  This mixture is made by  adding  l00mL of concentrated sulfuric acid slowly with stirring to a solution of 5g of  sodium dichromate (CARE: cancer suspect) in  5mL of water. (The mixture warms to about 70°C).

For materials with  very  low melting points  it  is  sometimes convenient to use  dilute  solutions  in  acetone, methanol, pentane, diethyl ether or CHCl₃-CCl₄.  The solutions are cooled to -78°C in a dry-ice/acetone bath, to give a slurry which is filtered off  through a precooled Buchner funnel. Experimental details, as applied to the purification of nitromethane, are given by Parrett and Sun.

Where  substances vary  little  in  solubility with temperature,  isothermal  crystallisation may  sometimes be employed. This usually takes the form of a partial evaporation of a saturated solution at room temperature by leaving  it under reduced pressure  in a desiccator.

However, in rare cases, crystallisation  is not a satisfactory method of purification, especially  if  the impurity forms crystals that are isomorphous with the material being purified.  In fact, the impurity content may even be greater in such recrystallised material. For this reason, it still remains necessary to test for impurities and to remove or adequately lower their concentrations  by  suitable  chemical manipulation prior to recrystallisation.