The properties desired vary with the structural clay product made. For example, a clay used in making conduit tile must be very plastic and have high green and dry strengths and uniform shrinkage, but for drain tile or common brick these properties do not have to be controlled so closely. Most clays are plastic when naturally wet or mixed with water. Plasticity can be defined as the property of a material to undergo permanent deformation in any direction without rupture under stress beyond that of elastic yielding. The type of clay mineral, particle size, particle shape, organic matter, soluble salts, adsorbed ions, and the amount and type of nonclay minerals influence the plastic properties of clay.
Green strength is the strength of the clay material in the wet or plastic state.
Dry strength is the strength of the clay after it is dried. Plasticity and green strength are closely related and are influenced mainly by the same variables. Dry strength is dependent on the proportion of fine particles present, the shape of the individual particles, the degree of hydration of the clay fraction, the method of forming the ware, and the rate and thoroughness of the drying. The presence of a small amount of smectite, which occurs in very fine hydrated particles, generally increases dry strength. Both drying and firing shrinkages are critical properties of clay used for structural clay products.