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Caking is a powder's tendency to form lumps or masses rather than flow smoothly.

Factors involved in the degree of caking include the substance's particle surface chemistry, particle size and particle-size distribution, particle shape, and environmental factors such as temperature and humidity. Caking also depends on any pressure on the powdered substance, and how long the substance has been stored.

Physically, the caking process involves electrostatic attraction or the formation of weak chemical bonds between the particle surfaces. This gives the bulk substance the strength to crack and collapse abruptly, to form vertical surfaces, and so on.

Usually caking is undesirable, but is useful when pressing powdered substances into pills or briquettes. Granular materials can also be subject to caking, particularly those that are hygroscopic such as salt, sugar, and many chemical fertilizers, because of the effects of moisture on grain surfaces.

Caking properties must be considered when designing and constructing bulk material handling equipment. Powdered substances that need to be stored, and flow smoothly at some time in the future, are often pelletized or made into pills. Anti caking agents are also commonly added to control caking.