The rate at which drying is accomplished is governed by the rate at which the two processes proceed. Process 1 depends
strongly on external conditions such as temperature, air humidity and flow, area of exposed surface and pressure, whereas process 2 depends on the physical nature, temperature and moisture content of the solid. Surface evaporation in process 2 is controlled by the diffusion of vapor from the surface of the solid to the surrounding atmosphere through a thin film of air in contact with the surface. Excessive surface evaporation, after the initial surface moisture removal, causes a
high moisture gradient from the interior to the surface, sometimes causing over-drying and excessive shrinkage. These
phenomena lead to high tension within the material, resulting in cracking and warping. Excessive surface evaporation
can be retarded by employing high relative humidities of the drying air while maintaining a relatively high rate of internal
moisture movement by heat transfer.