Crystallisation processes are well understood from the thermodynamic point of view. But all desalination processes, such as reverse osmosis, are kinetically limited, and thus the kinetics of the crystallisation is of particular interest. The kinetics of scale formation, especially together with impurities like antiscalants, is poorly understood. Here a physical model of the scaling process is described which focuses on the kinetics and thus can explain a wide variety of phenomena (for example, induction time) which play a major role in module scaling.