Molding—The loaf was then subdivided into smaller portions, each just sufficient to fill a mold 3/4 inch by by inches. The slabs were made to fill the mold by pressure applied in a screw press. They were then placed in a miter-box and cut into briquettes inches by inch by inches.
Marking—The laboratory sample number and a serial number was stamped on each briquette.
Drying—The briquettes were dried in an open room at summer heat. It had been found possible to dry even the most tender of clays in this manner, so it was assumed that all clays used in this test could, with out detriment, be subjected to this treatment.
Firing—Twenty-four briquettes of each clay were prepared. The ones on which the serial numbers 1 and 2 had been stamped were placed in a saggar to be drawn at cone 010, those on which the serial numbers 3 and 4 were stamped were placed in a saggar to be drawn at cone 08 and so on—each successive pair of briquettes of each clay being placed in a saggar to he drawn at a predetermined heat treatment as follows : The briquettes in the saggars to be fired from cones 3 to 11 were packed loosely in coarse white placing-sand, as to prevent their stick ing nne to another. Only those known to be fire clays, or at least sufficiently refractory to withstand severe heat treatment were placed in the saggars to be drawn at the higher cones.
The eleven saggars were placed in a coke-fired, side down-draft kiln in a manner convenient for drawing. The "spy" cones were centrally located in the kiln in a shield that protected them at all times from direct contact with the flame. When cone 010 was bent over sufficiently, to touch the plaque, the wicket was opened enough to draw the cone 010 saggar, the wicket replaced, and the heat slowly raised as shown in the above table.
Cooling—The saggars in which the briquettes were placed were "tile setters" 2 inches deep and 8 inches by 8 inches in area. Before plac ing, another saggar was inverted over the one containing the briquettes, so that on drawing, the briquettes were at no time exposed to the rela tively cold temperature of the room, except in one case of accident.
The saggars were placed, uncovered, in the ash pit of the kiln, where they were exposed to the direct radiation from the hot grate bars above. In this manenr, the briquettes were cooled rapidly at first, thus pre venting the fused portions in the briquettes from crystallizing very much, but from dull redness down to blackness the cooling extended over a considerable period.
The method of cooling pursued in this investigation was not ideal. The briquettes should have been cooled slowly for the first 200 C. which, as above stated, was not the case. Inasmuch as there is danger of checking the vitrified briquettes by cooling down to room temper ature too rapidly, some attention should be given to the last as well as to the first stage of the cooling period, but more particularly to the first. It was not possible to cool the briquettes under these ideal con ditions, for the services of the kiln were in demand for other purposes, and circumstances did not permit of delaying the burning until such time as the kiln would not be in use.
Preparation of Briquettes for Testing—When cooled, sand grains were found to be fused to many of the briquettes, requiring that they be ground off on an emery wheel. Care was taken not to unduly heat the bricks while grinding off the sand, and yet as little water as possible was used. The bricks that were thus ground were washed in distilled water to remove all traces of dirt and adhering particles. From the unground briquettes all adhering particles were removed by a dry stiff brush. Each briquette was carefully examined for flaws induced during manufacture or cooling, and also in order to remove all adhering por tions, such as broken corners that might have been detached later in the test.
Up to this point, all briquettes were handled together, without re gard to sample or series number, except as before indicated.
In all, 60 clays were prepared for testing, as above described, using 16 to 22 briquettes for each. The briquettes were not sorted, those of each clay being treated as a unit,.so as to insure like conditions at all times for all briquettes of the same clay.