Caking mechanisms depend on the nature of the material. Caking is a consequence of chemical reactions of grain surfaces.  Often these reactions involve adsorption of water vapor or other gases.  Crystalline solids often cake by formation of liquid bridge between microcrystals and subsequent fusion of a solid bridge. Amorphous materials can cake by glass transitions and changes in viscosity.  Polymorphic phase transitions can also induce caking. The caking process can involve electrostatic attractions or the formation of weak chemical bonds between particles.