Thermoanalytical (TA) and hot-stage microscopic techniques were employed to investigate the complicated behaviour of the non-isothermal
dehydration of single crystals of α-NiSO4·6H2O. Non-isothermal dehydration to the tetrahydrate proceeds in two stages: (1) surface nucleation and growth of nuclei, followed
by advancement of reaction fronts inward; (2) random nucleation and growth near the reaction front as well as in the bulk.
Corresponding TA curves were interpreted to represent diffusional removal of evolved water vapour through the surface layer
created in stage (1). The dehydration process of the tetrahydrate to the monohydrate is explained on the basis of textural
structures produced in the previous step. Crack formation in the surface layer and rapid escape of the water vapour were observed
in this step.