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. Shrinkage is the loss in volume of a clay as it is dried or fired. Drying shrinkage is dependent on the water content, the character of the clay minerals, and their particle size.
Most plastic clays shrink appreciably with drying, which tends to produce cracking and warping.
Sandy clay or clay having low plasticity has low shrinkage properties but tends to produce a weak porous body. Firing shrinkage depends on the density of the clay, volatile materials present, the types of crystalline phase changes that take place during firing, and the dehydration characteristics of the clay minerals. The temperature range of vitrification or glass formation during firing is a very important property of clay and shale used in structural clay products. Vitrification is a process of gradual fusion in which some of the more easily melted constituents produce increasing amounts of liquid as the temperature is increased. This liquid produces glass as the material is cooled, and the glass is the bonding material in the final fired product.