By heating the solution above the boiling point of water the solute concentration becomes greater. A supersaturated solution is formed when this solution, after heating to a high temperature, is allowed to cool undisturbed. Upon cooling the sugar recrystallizes into several small crystals or forms one large amorphous mass. For crystallization to occur, nuclei must form and solute must be added from the solution to these nuclei. The size of the resulting crystals depends on the number of nuclei, rate and temperature of crystallization, agitation and impurities in the solution.
Crystallization is a complex process with many interrelated factors.
The nature of the crystallizing substance is important for crystallization, although not as obvious in candy making as sucrose is almost always the substance under discussion. The rate of crystallization is the speed at which nuclei grow into crystals. At a higher temperature the rate of crystallization is slow and becomes more rapid at a lower temperature. Agitation distributes the crystal forming nuclei and hastens crystallization. Impurities in the solution usually delay crystallization and in some cases such as caramels may prevent crystal formation.