POROSITY.
The percentage of porosity expresses the relation between the volume of pore space and the combined volume of the particles of which the clay is composed. It is the ratio, in terms of volumes, of void spaces to solid particles. If determined on an unburned brick it would measure the degree of consolidation of the mass.
The porosity of an unburned clay mass may be determined directly by two methods: first, by use of the Seger volumeter; second, by the use of a chemical balance, and indirectly, or by calculation on basis of the pycnometer specific gravity determination.
To obtain percentage of porosity by either of the direct methods, the briquette or lump must be dried to constant weight, and the dry weight obtained, then saturated in kerosene and the saturated weight obtained. The difference between the saturated and dry weights is obviously the weight of petroleum that is required to fill the pores. This weight divided by 0.8, the density of the oil, gives the equivalence of oil in terms of water. Thus far, therefore, the actual amount of pore space in the brick in terms of water by weight is known. If the metric system has been used throughout, this amount of water by weight is equivalent to its amount by volume, since one cubic centimeter of water at room temperature weighs practically one gram.
Complete saturation of a lump of unburned clay even with kerosene, for which clay seems to have a peculiar physical attraction, cannot be obtained without resorting to the use of a vacuum pump. Standing in oil for 48 hours is not sufficient to cause complete saturation, as has been shown on proceeding pages by the specific gravity so obtained, as well as by the discrepancy between the directly measured and the calculated porosity as given in table IV, page 144.
In obtaining the dry and saturated weights, the two direct methods are alike. The data for actual volume of the pores thus obtained are, however, of no value in themselves, and cannot become of value until calculated to parts of 100 unit volumes of the whole brick. For this,
it is more practical to determine the volume of the whole mass, i. e., pores plus solid particles. It is in the determination of the volume of the mass that the two direct methods above mentioned are differen tiated.
First method, Volumeter.—After complete saturation the brick is placed in the volumeter and its volume determined in cubic centimeters. W—S By the formula 100 ( ) where W=weight of oil taken up by the V brick, S=the specific gravity of the oil, and V=the volume of the brick, there is expressed the part of 100 unit volumes of the brick as a whole which consists of open pore. In other words, it is the percentage por osity.
By referring to Table I it will be noted that the percentage of varia tion in the porosity determination was relatively small. Since the data given in Table I represents determinations made on 60 bricks of each clay 47.5 as the maximum, 0.6 per cent as the minimum, and 11.5 as the average percentage variation, is considered as being excellent.
These percentages of variation in results are not surprising in view of the fact that an error of 1cc. in determination of volume, or an error of 1 gram in obtaining either the dry or saturated weight, makes a dif ference of 0.3 in the porosity.
It is obvious, therefore, that when the dry weight of the bricks are obtained they must be absolutely dry, i. e., oven dried at 120°C. so as to expel all of the hygroscopic water. This was not done in obtaining the data given in Table I.
Second method, Chemical Balance.—When the porosity of the brick is determined on a chemical balance the volume of the briquette is found by the apparent loss of weight of the briquette when suspended in the oil. The briquette appears to lose weight when thus suspended, and this loss of weight is equivalent to the weight of a quantity of oil equal to that of the briquette. This method was used by Dr. E. R. Buckley in the test on the Wisconsin clays, and the porosity calculated by him using the formula: W—D) Sp. gr.