RELATION OF RATE OF ABSORPTION TO POROSITY.
Aside from exposing the irregularities in our method of analysis, this data gives evidence of the lack of relation of total porosity and rate of absorption in the green or unburned bricks. Since all the bricks were subjected to the same oil immersion treatment, it must follow that clays differ in the rate at which they can be saturated, and that this rate is not wholly a function of porosity.
The writer is not aware of tests ever having been made to investigate this property, which we may call "absorption ratio," but its significance in connection with the drying behavior of clays is obvious.
Johnson' has said, "Obviously, too, the quantity of liquid in a given volume of soil affects not only the rapidity, but also the duration of evaporation. The following table, by Schubler, illustrates the peculiar ities of different soils in these respects. The first column gives the per centages of water absorbed by the completely dry soil. In these experi ments the soils were thoroughly wet with water, the excess allowed to drip off, and the increase of weight determined. In the second column are given the percentages of water that evaporated during the space of four hours from the saturated soil spread over a given surface." "It is obvious that these two columns express nearly the same thing in different ways. The amount of water retained increases from quartz sand to magnesia. The rapidity of drying in the air diminishes in the same direction." Johnson affirms' that "these differences—(in the imbibing power of clays) —are dependent mainly on the mechanical texture or porosity of the material." That Johnson's statement, when applied to unburned bricks, is incorrect, is shown by the data in table IV. That there are other factors affecting the difference in rate of absorption and evapora tion in different clays is quite evident.
Value of the Porosity Determination on Raw Clay Lump.—It has been contended at various times in ceramic literature that a porosity termination on a raw lump of clay would give evidence, concerning such properties, as slaking, weathering, amount of water required to de velop plasticity, etc., and thus indirectly the shrinkage. Such claims have never been based on data, nor are they substantiated by the data secured by this Survey. As will be shown in later paragraphs, neither data nor sound reason would warrant such statements.
Value of the Porosity Determination on Green Brick.—Before the value of knowing the porosity of a green brick can be discussed it is neces sary to show the correlation of that property with those which it affects. If porosity, fineness of grain, and drying behavior are in any degree re lated functions, curves plotted from data should show such relations. Such a relation is shown in Fig. 4 where there seems to be an inverse ratio between fineness of grain in surface or loose grained clays, and the porosity of the green or unburned brick. The data from which this curve was plotted was obtained from the work of Beyer and Williams'. The surface factor was calculated from their data by the method given on page 113. The porosity data and calculated surface factor are as follows: This reciprocal relation between fineness of grain and porosity could be taken as evidence in proof of the close relation of fineness of grain and porosity of the green brick to drying, shrinkage and other properties that are peculiar to wares manufactured from fine but loose-grained clays or mixtures. The writer hesitates, however, to affirm the truth of such a relation from evidence obtained on a few samples of a single type of clay. Observation of the working behavior of boulder clays in build ing brick manufacture does not lead one to believe in such an exact rela tion.
In the manufacture of bricks by the ordinary dry process—where there is present only from 6 to 12 per cent of mechanical water, and the grains of the clay are not surrounded by slippery media that permit the particles to slide easily and freely upon one another—the clay cannot be formed into as compact a mass as when there is sufficient water present to permit of manufacture by the stiff mud process. If dry pressed bricks be formed in a hammer machine, or press, where the brick is subjected to repeated blows by a heavy hammer, the clay particles, even though nearly dry, would be forced over one another until the mass assumes a much closer or denser structure than is possible by the ordinary dry press process. The difference in structure, and its consequent effect on the burning properties of dry press bricks manufactured by these two methods in the St. Louis district is more evident than the difference between the structure of the dry and stiff mud or stiff mud and soft mud bricks.