82 CLAYS AND CLAY INDUSTRY. 



Several theories have been advanced to explain the cause of 

 plasticity. For a long time it was supposed to be directly con- 

 nected with kaolinite (the hydra ted silicate of alumina) and clays 

 high in kaolinite were said to be very plastic, but it was found 

 before long that the plasticity does not stand in close relation 

 to the amount of kaolinite. High-grade koalins containing 13. 

 per cent, to 14 per cent, of combined water, and therefore a large 

 percentage of kaolinite, are usually lean, while clays with but 

 4 per cent, or 5 per cent, of combined water may be very plastic. 

 That there is some connection between plasticity and the pres- 

 ence of minerals containing combined water would appear from 

 the fact that if a clay is heated sufficiently high to lose its com- 

 bined water, it also- seems to lose its. plasticity. Another theory 

 considers it due to fineness of grain. If, however, we take quartz 

 and grind it very fine in a ball mill it is not plastic, although it 

 is true that if an excess of water is mixed with it, it becomes soft 

 and pasty. If this wet mass is pressed it will pack to a hard mass,, 

 entirely lacking the mobility of form possessed by a plastic clay, 

 so that fineness of grain is not the sole cause of plasticity. Still 

 another theory considers that it is due to* a plate structure, the 

 idea being that the clay particles are plate-like in form, and that 

 the plasticity is caused by these plates sliding over each other. 

 One serious objection to this theory is that many clays when 

 examined under the microscope do' not appear to be made up 

 entirely of plates. 



In studying clays from different formations, and different 

 parts of the same layer even, some interesting facts bearing on: 

 this subject were obtained. 



A residual clay (kaolin from Webster, near Dillsboro, N.. 

 Car.) was compared with a sedimentary clay of the same com- 

 position (Florida ball clay). The residual clay was found to 

 possess a decidedly less degree of plasticity than the sedimentary 

 clay, the particles of which had been transported long distances 

 and had been more or less rubbed and ground together previous 

 to their deposition. Inasmuch as the kaolinite particles, as 

 shown by the microscope, are more or less bunched, it is to be 

 expected that this rubbing and grinding action during trans- 

 portation would break up the bunches to some extent, and it is 



