100 = per cent of porosity.
j (W—D) Sp. gr. + D In this formula W=saturated weight; D=dry weight; and Sp. gr. the composite specific gravity of the clay particles, as calculated from dry, saturated, and suspended weights of the briquette.
This formula, however, can be simplified by substituting for the value of D in the denominator its value in terms of the Sp. gr. and suspended weight (S) as given in the formula for specific gravity where D=D (Sp. gr.)—S (Sp. gr.).
W—D The Buckley formula then simplifies to the expression 100 = W—S porosity. This formula holds true no matter what liquid is used in the saturation of the brick, so long as the same liquid is employed in ob taining the suspended weight.
The method is accurate but very slow and tedious, unless it is carried out with small pieces on the jolly balance.
If a jolly balance is used in this determination, the weight of the bri quette or piece must not exceed that which would stretch the spring be yond its elastic limit. If any other than a light weight spring is used the difference between the several readings will not be sufficient to permit of very accurate determination. This method was used in the determina tion of the rate of vitrification, which will be described under the gen eral heading of "Pyro-Chemical Tests," so will not be discussed in de tail at this time.
Third Method, Calculation.—It has been noted that the specific grav ity of the powdered clay by the pycnometer method is uniformly higher than that calculated from data obtained on the green bricks. It has also been noted that this difference between the specific gravities is due to the incomplete saturation of the brick. Since the formula for speci fic gravity is : Dry weight (W) divided by the combined volume of the W particles (V) or —=Sp. Gr., the true volume of the particles in the V brick can be obtained by the formula : Dry weight divided by the pycno D meter specific gravity, or —=Vol. Then the volume of the whole Sp. Gr.
brick (Vb) minus the volume of the clay particles (Vc) would give the volume of the pore spaces (VP), or Vb—Vc—VP. To obtain the frac- • tional amount of pore space in a brick, the volume of the pores (VP) VP must be divided by the volume of the brick, or —. But since
Vb Vp V° W — we have 100 1-- =per cent pore space where Vc= Vb Vb Sp. Gr.
The economy and accuracy in determining porosity by this method lies in the fact that it is not necessary to saturate the brick and obtain the saturated weight. It is obvious, therefore, that the bricks would either have to be partially saturated or covered with a thin coating of paraffin and their volume determined in a volumeter. Without a volu meter this method cannot be used.
If the specific gravity has been determined by the pycnometer method and a volumeter is not accessible, the porosity is best calculated by the Buckley formula. In this, however, complete saturation of the brick must be assured, and the true specific gravity of the clay particles used Neither the Buckley method nor the indirect method here proposed is usable on any other than a green or unburned lump of clay. For the W—D porosity of a burned lump or briquette, the formula 100 is W—S the only one that will give accurate results, as will be shown under the discussion of Pyro-Chemical and Physical Properties of Clays.
In the following table are given porosity data obtained, first, by the usual volumeter method without taking into account the hygroscopic water; second, by the indirect method described above, without taking into account the hygroscopic water, and third, by the indirect method on a basis of absolute dryness of the bricks. The percentage of increase of porosity obtained in the second and third instance, over that obtained in the first is also shown.
The data in table IV shows the inaccuracy of the usual method of de termining the porosity in dried clay wares. It has been stated' that three to six hours is sufficient to saturate with oil unburned briquettes that measure inches. Forty-eight hours was therefore considered ample time in which to saturate 'a brick that cubically was about eight times as large. From the fairly close agreement in the specific gravities as determined by the pycnometer and the volumeter, it was thought that the briquettes had been fairly well saturated. Such, however, was evi dently not the case.