When the grains are made to lie closer together, either by strong force, or by a lighter force supplemented by a floating medium, better opportunity is offered for the grains to fuse with one another. This is shown nicely by the fact that hammered brick can be burned in the colder parts of a kiln to a degree of hardness that is equal to and often exceeds the hardness of a brick made from the same clay by the dry press method and burned in the hotter portions of the kiln.
In the illustrations just cited, the difference in the burning properties of bricks made from the same clay by different processes, has been used as a means of noting that porosity of green brick is not wholly a func tion of size of grain. It is clear that it is also very largely a function of process of manufacture.
While there may be some relation between the size of grain of loose or soft clays and the porosity of the brick manufactured from them, it is still doubtful if a similar relation can be observed in the hard rock like fossil clays, such as shales, where the mineral particles are so cement ed as to very stubbornly resist separation by the crushing force of dry pan mullers as well as the disintegrating influence of the water used in pugging.
In Fig. 5 are shown curves plotted from data obtained with shales in the same manner as the data for surface clays in Fig. 4. The porosity is taken from Table I, and the surface factor from Table VIII.
The clay from which these shale brick were made had been crushed to pass through a dry pan and then screened. In the laboratory they were known as dry pan samples. These "dry pan samples" were then in the same state of mechanical subdivision as the clay used by the manu facturer.
In making the bricks from which the data in Table I were obtained, considerable time was expended in pugging or wedging the clays by hand, first in a large bulk, and later in quantities just sufficient for one brick. The operator batted a quantity of clay that would make approxi mately 60 bricks on a plaster top table until it was as compact as he could make it. Then by use of a trowel in some instances and a wire in others, he cut off from the large mass a quantity sufficient to fill the die of the press. This smaller piece was again thoroughly wedged by hand until all air blebs had been worked out and the whole took on the shape of a compact loaf. This loaf was then placed in the
die, using care to see that it cleared the sides so as to prevent a shearing off of any portion of the loaf on the edge of the die when the plunger descended. The loaves were pressed into bricks on a slow screw tile press, so that the clay did not receive much compression, but yet sufficient to cause it to flow in shreds up around one side or another of the plunger. From this flowage of the clay past the plunger, together with the unusual amount of wedging by hand, it was considered that the clay had been sub jected to a treatment that was approximately comparable to the pugging it would have received in the factory, so that the data as given in Table I show approximately the physical structure except for lamination that would be developed on a regular manufacturing basis.
It is commonly known by paving brick manufacturers that some shales require inordinate pugging before they develop sufficient plasticity to permit the production of a perfect bar in the die of the brick machine. In fact it is not uncommon to see a battery of two pug-mills through which the clay must pass before it enters the brick machine proper. In the brick machine the clay receives further pugging before it issues as a bar from the die. It is also not uncommon to hear the manufacturers claim that they cannot pug clay sufficiently unless they use hot water. Not all manufacturers have to resort to this extra care in the pugging process, for some shales develop plasticity with sufficient readiness to allow them to emerge from the first pug-mill in a workable condition. This same difference in the working property of the various shales was perhaps more noticeable in the laboratory than in the factories.
This difference in the working properties of shales is considered to. be due to the fact that the grains of clay are cemented by substances that differ in their solubility in water. It is now well known that soils and clays contain soluble salts that are adsorbed by, or, to use a more homely expression, smeared over the particles, and are not easily extracted by water. It has been learned by experiment that clays can take on or adsorb soluble salts from solutions and so retain these salts in their sub microscopic pores that they cannot again under ordinary conditions be dissolved from the clay.