METHODS OF DETERMINATION The true specific gravity of the clays included in this report was ob tained by three methods: By Seger volumeter, using unburned bricks; by pycnometer; by chemical balance, using unburned bricks.
Determination by Seger's volumeter was used in the determination of the volume shrinkage, porosity and specific gravity .on the several clays, as noted in Table I .
The accuracy of the tests in the volumeter is shown in Table I by the percentage of variation volume shrinkage, specific gravity and porosity. These variations are very small considering the conditions under which the determinations were made.
The specific gravity of any substance is the ratio of the weight of that substance to the weight of an equal volume of some substance taken as a standard. In the metric system, distilled water at 4°C. is taken as the standard. At this temperature a cubic centimeter of distilled water weighs one gram. Therefore, when using this system, volume and weight of water may be interchanged, i. e., 1 c.c.=1 gram.
With this understanding the formula by which the specific gravity of a clay can be obtained could be expressed as where W=dry weight in grams and V=voluine in cubic centimeters. If the porosity of the brick has been determined the formula for the specific gravity could be written: Where W=dry weight as before, V=volume of the brick in cubic centimeters and P=percentage porosity.
It will be noted by comparing the specific gravities in Tables I and II, that those obtained by the volumeter are lower than those obtained by the pycnometer. This can be accounted for perhaps by the operator's in ability completely to saturate .a brick, that is, to fill all the pore spaces with oil without resorting to the use of a suction or vacuum pump to re move all the air from the pores so that oil could enter. If the air is not entirely exhausted it will pass through the oil very slowly, requiring a period extending over several weeks in which to esape. In .ordinary labor atory practice sufficient time can not be given to permit the ddthplete es cape of the included air. In the porosity and specific gravity tests here reported, no attempt was made to fill the pores completely. The bricks were simply soaked in coal-oil for 48 hours, with one face exposed at the level of the surface of the oil. This incompleteness of saturation under these conditions is shown by the difference in the specific gravity as de termined by the volumeter and pycnometer.
Determination by Pycnometer.—A pycnometer, or specific gravity bottle, as it is often called, is a small flask of known capacity, usually 25 to 100 c.c. When filled up to a given mark with air-free water at normal room temperature, its weight is noted. The flask is then partly emptied, a known weight of clay added, and the whole carefully boiled to exclude all the entrapped air, then cooled, filled up to the mark and weighed. By the formula, weight of dry sample (a) plus weight of
bottle filled with cold air-free water (b) minus weight of bottle filled with sample and water (c), or a+b—c, will give the weight of water having the same volume as the sample or true total volume of the clay particles. Knowing the dry weight and true volume of the grains, their composite specific gravity is readily calculated by the formula (dry weight - volume).
This may be illustrated by the following calculation: Weight of bottle filled with water = 143.22.
Dry weight of sample = 3.41.
Weight of bottle + sample + water required to fill to mark = 145.35. 143.22 + 3.41 — 145.35 = 1.28 total volume of the particles.
3.41 - 1.28 = 2.68 specific gravity of the sample.
In the following table will be found specific gravity of the clays by the pycnometer method.
Determination with Chemical Balance.—The dry, saturated and im merscd weights of briquettes were determined by using a chemical bal ance. For this it was found that briquettes of the size could be used. Obviously the larger the briquette the more nearly true will be the determined specific gravity. Sizes larger than that given, however, cannot be used to advantage on the ordinary chemical balance. This method was used for but a small number of samples.
The briquettes were dried to constant weight in an air bath at 120°C. cooled in a dessicator and their dry weight obtained as rapidly as possible. After weighing, the briquettes were immersed in clarified coal-oil with one face above the level of the oil. After standing thus for 20 to 24 hours, they were placed under a bell jar and the air kept exhausted for fifteen minutes, it having been found in previous work that this treat ment was sufficient to attain nearly complete saturation. The briquette was then suspended by a silk thread from the beam of a chemical bal ante and its saturated weight noted. A breaker partially filled with oil was then so placed that the briquette could swing clear and be com pletely immersed. In this manner the immersed weight of the briquette was obtained.
By the formula then of dry weight (D) divided by (dry weight (D) minus suspended weight (S) ) or D - (D—S), the specific gravity of the material in the briquette was readily obtained.
The comparative accuracy attained in the determination of the speci fic gravity of clay by these three methods may be seen in the table fol lowing.
It is evident from this table that the essential fault in the first and third method of determining specific gravities lies in the fact that the brick was not completely saturated, and therefore, gave low specific gravities. The closeness in agreement, however, suggests that by the use of extra precaution in the saturation of the bricks the specific gravities of the clays could be made quite accurately by either of these two methods.