135 



an inert substance, such as sand, with a true colloid the shrinkage 

 is lessened, and the cohesion of the dried colloid, including its 

 adhesion to inert substances, are the causes of the increased 

 mechanical strength of many such mixtures. This is another charac- 

 teristic common to colloids and to all j)lastic clays. 



As there is no wholly reliable method of measuring plasticity 

 {see p. 124), it is not possible to state precisely what relationship 

 exists between the plasticity and the shrinkage of clays. Speaking 

 broadly, the most plastic clays shrink more than those which are 

 less plastic, but this is not invariably the case. For instance, the 

 Lias clays usually shrink less than would be expected from their 

 plasticity. 



When articles made of plastic clay are dried under suitable condi- 

 tions, they contract equally in all directions, the contraction in volume 

 being almost three times the linear shrinkage. Excessively plastic clays 

 crack, or twist, when dried and many moderately plastic clays will do 

 so if dried irregularly or too rapidly. 



When water is added to a dry clay, it is first absorbed by the pores, 

 but, when these are filled, any further supply of water appears to cause 

 a separation of the particles from each other so that the volume of clay 

 is increased, though not in proportion to the water added. The 

 amount of Abater which can be absorbed in this manner differs greatly 

 with different clays. The stage at which the clay contains the 

 maximum quantity of water without loss of shape is also the point 

 of maximum plasticity; it is said to be the "point of saturation 

 of the coagulated colloids (gels) in the clay." If some of this water 

 is removed, the volume of the mass begins to diminish and contraction 

 occurs. This contraction or shrinkage is chiefly, but not entirely, 

 due to the removal of water from the clay by evaporation at the 

 ordinary temperature (air-shrinkage), at a somewhat higher tempera- 

 ture in the dryer (dryer-shrinkage), or during the burning (kiln- 

 shrinkage). 



As all coagulated colloids (gels) which are saturated with water 

 shrink when the water is removed, some investigators consider that 

 the shrinkage of clay may be due in part to this cause. 



The more general idea (which states facts rather than explains them) 

 is that, as the water is removed, any which remains draws the clay 

 particles together into a smaller and denser mass. 



The amount of shrinkage appears to depend partly upon the rate 

 at which the clay is dried, for if this operation is performed rapidly 

 the shrinkage will be less, the clay particles not having time to move 

 over each other so freely as when the drying is slower. When drjdng 

 a strong, porous clay, the water first evaporates from the surface 

 and is replaced by capillary action from the interior, the mass 

 contracting by the same amount as the water diminishes. All the pores 

 remain filled -ndth water until the rate of evaporation exceeds the rate 

 at which the pores will transmit water. Tliis point occurs, when 

 the clay particles move so much less freely on each other that the 

 rate of evaporation exceeds that of the contraction. After the first 

 stage of surface-drying, the exterior loses water more rapidly than 

 the interior; in the second stage the pores are no longer filled with 



