The amount of water used in the pugging of shales is not sufficient to dissolve or loosen all of the cementing salts in a clay even by continued pugging, so that at best, only a portion of the clay particles are separated from one another, but the manufacturers must continue the pugging until a sufficiently large number of grains are separated to form a slippery medium, by virtue of which the unslaked or undisintegrated bundles of particles can slip past one another freely enough to permit a flowage of the mass under pressure. The difficulty encountered by manufacturers in breaking down the cementing bond in shales is increased many-fold when an attempt is made to disintegrate a clay into its ultimate grains, as is done in mechanical analysis. The data for texture or size of grain used in plotting the curves in Figs. 4 and 5 were obtained by mechanical analysis and are supposed to represent the subdivision of the clays into their ultimate particles. While it is comparatively easy to obtain separa tion of the particles in loose-grained clays in the laboratory and in the factory, it is obvious that it is not possible to obtain a similar separation of the particles of the hard rock-like clays in the factory, and very diffi cult to obtain much more than an approximation of ultimate subdivision in the laboratory. It is owing to this indefinite degree of solution of the
natural bond in pugging that we have, in the case of shale bricks, a dis cordant relation between the porosity of the brick and the fineness of grain, shown in Figs. 2 and 5, as contrasted with the semingly concord ant relation in the case of the loess bricks, as shown in Fig. 4.
Although a porosity determination on a green brick may not be of value as direct evidence of the so-called "working properties" of a clay, it can be shown that it is of indirect value, in that the data can be used as the basis of many interesting and valuable calculations. For practical demonstration of the commercial possibilities, or exposition of the work ing properties of a clay, the writer has failed to find wherein porosity data on unburned bricks separately considered are of use.