<P> Once the first small crystal, the nucleus, forms it acts as a convergence point (if unstable due to supersaturation) for molecules of solute touching--or adjacent to--the crystal so that it increases its own dimension in successive layers . The pattern of growth resembles the rings of an onion, as shown in the picture, where each colour indicates the same mass of solute; this mass creates increasingly thin layers due to the increasing surface area of the growing crystal . The supersaturated solute mass the original nucleus may capture in a time unit is called the growth rate expressed in kg / (m * h), and is a constant specific to the process . Growth rate is influenced by several physical factors, such as surface tension of solution, pressure, temperature, relative crystal velocity in the solution, Reynolds number, and so forth . </P> <P> The main values to control are therefore: </P> <Ul> <Li> Supersaturation value, as an index of the quantity of solute available for the growth of the crystal; </Li> <Li> Total crystal surface in unit fluid mass, as an index of the capability of the solute to fix onto the crystal; </Li> <Li> Retention time, as an index of the probability of a molecule of solute to come into contact with an existing crystal; </Li> <Li> Flow pattern, again as an index of the probability of a molecule of solute to come into contact with an existing crystal (higher in laminar flow, lower in turbulent flow, but the reverse applies to the probability of contact). </Li> </Ul> <Li> Supersaturation value, as an index of the quantity of solute available for the growth of the crystal; </Li>

The property of a system that makes a process occur consists of two driving forces