By this scheme of setting twenty-four clays could be tested in one series of four burns, there being in each draw two slabs of the same group in each of the four saggars. This scheme of burning was made possible by the fact that the openings in the flash wall leading into the firing chamber, and openings in the opposite side of the firing chamber leading into the draft flue caused, with the down draft, an equal lateral distribution of heat. In no instance was there a failure to have the center test cone bent, although in some cases in the same draw it was bent more than in others.
Testing of the Trial Pieces—The cones were detached from the slabs, marked with lead pencil, weighed one at a time on a jolly balance and then placed in clear hydrant water. After twenty-four hours of satura tion, the wet and immersed weights of each cone were made and from the data so obtained, their porosity and apparently specific gravity cal culated.
Difficulties Encountered—First, when the cones were detached from the slabs many broke into two or more pieces; second, a few of the cones were bloated at the base, clue to a lack of oxidation; third, the cones were invariably vitrified more at the top than at the base, thus causing ir regularity of results in those that were broken; fourth, in those cones which had softened sufficiently to cause them to bend over, the pore sys tem was not normal, owing to the strain set up on the upper side and compression on the under side of the bent cone; fifth, we were not suc cessful in making a jolly balance spring that was heavy enough to pre vent the weight of a cone stretching it beyond its elastic limit, give suffi ciently delicate reading.
Data Obtained—Although the test as a whole was unsatisfactory, it is believed that the data obtained has a value. The porosity data were ploted on a diagram as shown in Figure 29. In this the linear distance between points on the abscissa, indicating difference in melting periods of the standard cones, is equal to the linear distance assigned to repre sent a difference of two per cent in porosity. The solid line is drawn through points the average data obtained on the two dup licate cones, and the points indicated but not on the heavy black line represents in each case the data obtained from each of the two cones. In case the data for one of the duplicate cones were missing, as in K 2 for instance, the heavy black line traces the points representing the de termined data.
The dotted line was drawn through all possible combinations of three points that were found to lie in line with each other. In some cases
there was only one light line and in others more than one, as shown by the data given in Table XL. The lines drawn through three points lying in a straight line are taken as representing the slope of the curve describing the change in porosity with regularly increasing intensity of heat treatment. Where there is a possibility of more than one slope, as indicated by the light lines, each is recorded and their average cal culated. The data in Table XXX is, therefore, the slope or tangent of the angle that the light lines drawn through three points makes with the abscissa.
To obtain this data a protector was so placed on the line that the angle could be read. The natural tangent of the angle was then ob tained from a logarithmic table of natural functions. Since the tangent of the angle which a line makes with a given base line, is the slope or inclination of that line, this tangency can be taken as representing the rate at which the porosity decreases with increasing heat treatment.
In the following Table will be found the values; first, of each of the tangents of the angles made by the dotted line and abscissa; second, the average of the tangents; and third, the rattler loss as determined on bricks obtained from factories using the several clays.
Summary—Owing to the unavoidable inaccuracies of the work and the erroneous assumption that a porosity graph would trace a straight line, the data given in the above Table has but little value. Its principal value lies in the developed fact that as a rule the slower the clay fuses the tougher appears to be the mass.
Failing to solve the problem at hand in the above test, another and more thorough investigation was at once started, using not only a large number, but also a larger variety of clays. The manner in which the test pieces for this latter study were prepared was as follows : Wedging—Approximately one pound of dry clay was placed on a dampened plaster-covered table and sufficient water from the University mains added to develop the plasticity required to permit batting the clay into loaves. This was accomplished by adding water in small quantities, and thoroughly working it into the clay each time, until the mass had the desired plasticity. It was then thoroughly wedged by kneading and batting until, on cutting the mass open, it appeared to be compact, i. e. without air blebs.