3.5A: General Procedure You will often be asked to follow a crystallization procedure that may be written in one of two varieties: Solid A was crystallized using Y solvent, or Solid A was crystallized using X mL of Y solvent. When no volume of solvent is specified, the process should be conducted using the minimum volume of boiling solvent to dissolve the solid. If a volume is specified, it probably means that previous trials have shown that a greater than minimal solvent volume produce higher quality crystals. 3.5B: Heat Source and Bump Prevention Crystallizations are generally conducted using a hotplate or steam bath, and the crystallization solvent may affect which heat source is appropriate. Boiling stones, boiling sticks, or stir bars must be used for "bump" protection as crystallization involves heating solutions to a boil. Boiling stones are often used for solvents, and may be used with solutions if a hot filtration step is planned. 3.5C: Charcoal Activated charcoal is sometimes used to remove small amounts of colored impurities from solution. Activated charcoal has a high affinity for conjugated compounds, whose flat structures wedge themselves well between the graphene sheets. The quantity used should be limited, as charcoal adsorbs all compounds to some extent and could lead to a lower recovery of the desired compound. Charcoal should of course not be used if the product itself is colored. 3.5D: Cooling Slowly After a solution is dissolved in the minimum amount of hot solvent and filtered (if applicable), the solution should be cooled as slowly as possible (keeping in mind time limitations in a lab). 3.5E: Initiating Crystallization At times, crystals will not form even when a solution is supersaturated, as there is a kinetic barrier to crystal formation. At times crystallization may need to be initiated, for example if the solution becomes faintly cloudy as it cools, or if the solution fails to produce crystals even when it is noticeably cooler than originally. The methods described in this section preferably should be used on solutions that are still warm to avoid a too rapid crystallization.