In seeming contradiction to this statement regarding the fineness of grain as a sense for plasticity, is the fact that finer grinding of given clay increases its plasticity; but quoting Wheeler': "While it is true that fine clays are usually very plastic and coarse clays much less so, there are very many exceptions." And again, says that while the majority of clays improve on fine grinding, some are unchanged.
Wheeler' reports the physical structure of a few clays as follows : Moberly shale (400 diam.): Mainly clusters of thick plates with minor portions split off; moderately plastic; suggests fine grinding to develop plasticity.
Aldrich shale (325 diam.): One-third dolomite crystals; bulk in coarse thick crystals or plates; rest in fine state of division; moderately plastic.
Unweathered Leasburg flint fire clay (950 diam.) : Almost all fine particles; no plates or scales; devoid of plasticity. Weathered Leasburg flint fire clay (950 diam.) : Numerous coarse plates present and occasionally, apparently a. few thin plates. Came from same bank the same day a few feet from the unweathered sample.
Hartwell loess clay (400 diam.): Large angular fragments which were undoubtedly sand, and apparently some clusters of plate crystals, with only a minor portion of small plates; very plastic.
There is sufficient evidence in the above citations to show that any theory so far discussed other than that of molecular attraction, is in sufficient to account for the presence or absence of plasticity.
Plate Structure Theory of Plasticity—Grout' has recorded the face that in the case of the Thornton Brick Company's plastic clay the amount by weight of the particles below 0.005 mm. in diameter rose from 7.7 per cent to 17.8 per cent by weight in one wetting and drying. Fox, in our laboratories, found that the plates, although not disintegrat ed by twenty-four hours of shaking in water, would break down by mechanical crushing or by disintegration in acids and caustic alkali, and that when so broken down the mass became considerably more plastic. Wheeler' not only advises fine grinding in the case of the Moberly shales, but relates a most remarkable instance of a clay in which the grains on weathering formed themselves into clusters re sembling plates. It seems highly probable, therefore, that these plates or coarse grains are bunches or bundles of minute grains cemented to gether by salts that are to a greater or less extent soluble in water, and that, depending upon the solubility of the cementing salt in a particular case, or the peculiar compactness of the grains • in another, it requires a greater or less amount of time to cause a breaking down of these bundles. It can be readily conceived that the adsorptive power of the
particles when combined with their axes in a certain general direc tion, for instance, has greater power in holding certain of these cement ing salts than the solvent action exerted by the water can overcome. The solvent power of water, in other words, is not sufficient to overcom' the adsorptive power of the kaolinitic grains.
These coarse grains add to the plasticity of the clay as a whole in a ratio to the surface exposed. Every exposed kaolin particle is as effec tive in enhancing plasticity as the very small independent particles. The extent to which the larger grains would affect plasticity would, there fore, be in proportion to the exposed surface of the particles of which the bundle or cluster is composed.
Further, it is fair to challenge the plate theorist to demonstrate that these small grains when cemented together in a bundle or cluster have not a tendency to line up one with another so that their longest axes will lie in the same relative plane, just as they are in the natural kaolin crystals, i. e., in plate forms. The plate theorist must admit that when these bundles are thus formed they are well-nigh indistinguishable from mica crystals, and that the very large majority of so-called plates or scales of kaolin in a clay are most likely to be mica. It is certainly strange that on one page of a report there will be a statement to the effect that "the clays of this state are quite micacious," and another page will report the scales that appear on the stage of the micro scope as "kaolin scales or plates." If the idea that has been put forward in the foregoing is correct, then we must agree with Dr. Laddl when he says : "The question of fineness of grain and shape of the particle becomes, then, largely but modifying factors, affecting degree, and being, within large limits at least, modifiers, rather than determinants of plasticity." It is quite evident that the peculiar physical make-up of a kaolin grain, so far as the eye by the aid of the microscope can discern, is not fundamentally responsible for their individuality, as expressed in their power to develop plasticity. If the structure of the grains which en ables a mass of them to develop plasticity is not detectable by the micro scope, direct observation and measurement are obviously inadequate in finding the true cause.