The rheology of clay may be divided into three almost distinct fields; the study of liquid “slips,” i.e., the relatively dilute liquid suspensions used for casting, mainly in the manufacture of pottery, or of plastic bodies, mouldable but capable of retaining their moulded shape under their own weight, or of the deformation of the fired material at high temperatures. Consideration of time available made it necessary to restrict the present considerations to the second of these, the plastic state.