133 



use of the term " plasticity." Zschokke, who has examined the 

 subject very fully, considers that the percentage of extensibility 

 multiplied by the tensile strength of a freshly moulded clay cylinder 

 of standard size (60 mm. high by 30 mm. diameter) is a coefficient of 

 the plasticity. Modifications of this method have given excellent results 

 in the hands of several experimenters in different countries and with 

 a very large variety of clays. Grout considers that plasticity is 

 proportional to the product of (a) the load required to sink a Vacat 

 needle to a definite depth in a mass of clay ; and (b) the deformation of 

 the clay under stress, which he measures by the increase in area of 

 a clay cylinder produced by a load which just causes cracks to appear. 

 Both Zschokke and Grout" really consider plasticity to be measured 

 by the product of the deformability and force resisting deformation, 

 though they differ in the manner in which they measure these forces. 

 More recently, Ashley' has adopted the same general idea as to the 

 forces involved, but has assumed that the force-resisting deformation 

 is exerted by the colloids in the clay. He, therefore, regards the 

 plasticity of clay to be measured by the ratio :— 



Relative colloids X the shrinkage of the clay 



Jackson-Purdy surface factor. 



The term " relative colloids " is explained in the section on 

 Adsorption. 



As the ratio of the surface factor to shrinkage is approximately 

 constant, Ashley concludes that the plasticity of the clay is directly 

 proportional to the colloids present. The objection to this conclusion 

 is that it appears unUkely, from other considerations, that the whole 

 of the plasticity is due to the colloidal matter. 



Rohland^ also assuming that the colloidal matter in the clay is 

 the chief factor of the plasticity, has suggested that the ratio obtained 

 by dividing the coagulable colloids by the non-coagulable material 

 is a measure of the plasticity. He ascertains it is by measuring the 

 amount of water required to make the clay into the consistency of a 

 good modelling paste, and argues that this is a measure of the colloids 

 because as soon as sufficient water is present to dissolve the coagulable 

 colloids, a saturation point is reached and no more water can be 

 absorbed without the clay losing its stiffness. 



Stormer has stated that plasticity may be judged by the following 

 characteristics : — 



(1) The proportion of water (absorption) which must be 

 added to the clay to make a good modelling paste. This is not 

 always reliable. 



(2) The '■ feel " of the paste when rubbed between the finger 

 and thumb {binding power). 



(3) The behaviour of the paste when rolled up into a 

 " sausage " (toughness). 



(4) The adhesiveness of the clay [adhesion). 



(5) Twisting a piece of clay into a spiral and noting its 

 behaviour {torsion). j^ _^ 



