When non-plastic kaolins are wetted with water, they are compressed into shapes only with difficulty, and when dried they either fall to pieces, as would so much fine sand, or have so weak a bond that they are easily crumbled. The finer the particles, as with the case of sand, the more readily can they be shaped into pieces that will retain their form, but no matter how finely sub-divided the grains may be, the mass is still very friable. In this fine condition the kaolin no doubt possesses every chem ical and physical property possessed by the plastic kaolin (ball clay) save that of plasticity. It has water chemically combined, molecular attraction, and adsorbing properties. It becomes plastic only when it has adsorbed salts, the solution of which exhibits a high surface tension, or as Whitney would express it, which have a. relatively high potentiality.
Clays having adsorbed salts and consequently, plasticity are no longer friable when molded but, on the contrary, they are exceedingly hard.
It is because of this adsorption property which in kaoline grains seems to be manifested to a higher degree than in any other mineral substance, with perhaps the exception of zeolites, that many find reason to believe that plasticity is due to a pectoidal structure of the kaolin grains. Since, however, they cannot show that those substances which are known to have a pectoidal or colloidal structure can be made to show or develop plasticity, and since colloids cannot be extracted from plastic clays, rendering them non-plastic, nor added to non-plastic kaolins rendering them plastic, we must conclude that this theory is hardly tenable.
To what this great adsorptive power of clays is due has not as yet been determined. We, however, must accept the existence of this property as a proved fact. We must also concede that when water is added to a clay, that portion which envelops the very minute solid particles having a relatively large surface area in proportion to their volume, and hold ing salts by absorption, will be highly concentrated, that the potential of this film will be very much more than those portions of the water farthest away from the solid particles; and finally, as shown by the flocculation of the clay particles, the potential of this saturated film of water is higher than the potential of the kaolin particles.
The writer bases his assumptions as to the cause of plasticity on known facts : Adsorption of salts by the kaolin 'grains and the con sequent high potential of the water film which surrounds the grains when a clay mass is in a plastic condition. On these assumptions, the cause of the latent plasticity when clay is dry and the developed plasticity when it is wet, are obvious.
Fineness of grain, molecular attraction, adsorptive property, are conditions that permit of the adsorption of salts. In other words, they are necessary conditions.