DIFFERENTIATION BETWEEN CLAYS ON BASIS OF DIFFERENCE IN RATE AND MANNER OF DECREASE IN POROSITY AND SPECIFIC GRAVITY.
Importance of Slow Vitrification—It is the consensus of opinion among those who have given serious thought to the vitrifying proper ties of ceramic mixtures, whether natural, as ordinary clay, or artificial, as pottery bodies, that those mixtures which vitrify most slowly and at a uniform rate, all other things being usual, will produce the strongest and toughest ware. Chemical analysis and synthetical mixtures have failed to reveal the happy combination of minerals or chemical ele ments that will produce this slow, uniform rate of vitrification. A few general rules can be stated as to combinations of ingredients required to produce tough bodies, but none of them can be applied with absolute assurance that they will operate in a given case. With our present in formation empirical trials have to be resorted to find the proper com bination in each case.
It is commercially impractical to alter the composition of clays used for paving brick manufacture except in so far as different strata permit of the use or rejection of materials that effect the character of the ware. This the paving brick manufacture has learned by experience, so• that the composite "dry pan" sample, before described, is supposed to repre sent "mix" that is commericiallv possible in a given case. On the supposition that, according as its rate of vitrification is slower, one clay is more suited for vitrified paving brick than another and that there is no means of obtaining information that bears on this problem other than determining this very pyro-physical property in paving brick clays, clays were molded into cones having the same shape and dimen sions of Seger pyrometric cones manufactured by Prof. Edward Orton, Jr.
Manufacture of Test Cones—The clays in this experiment were dry ground in a mortar to pass a 40 mesh screen, wetted with water from the University mains, wedged thoroughly and molded into cones with a spatula in a regular cone die as used by Orton. On the upper face of each cone was scratched its sample and serial number. After removal from the die the cones were placed in a cool place protected from drafts to dry.
Setting of Test Pieces After Drying—One cone each of four different clays was set in a row in the center of a fire clay slab. On either side of the row of test cones was placed a row of three standard Seger cones arranged in opposite order from one another. There were eight groups of such slabs for each set of four test cones, thus allowing eight heat treatments of different intensities' on each clay.
The eight groups with the standard cones were as follows: First group 010-09-08. Second group 07-06-05. Third group 04-03-02-01.
Fourth group 01-1-2. ' Fifth group 2-3-4. Sixth group 4-5-6. Seventh group 6-7-8. Eighth group 8-9-10.
Special saggars were prepared, being 31/2 inches deep and 8 by 8 inches in area, and having only three sides. These saggars were placed in four bungs in the side down-draft kiln designed by the writer for the ceramic department of the University of Illinois, and shown in Fig. 28. Four of these special saggars were placed in each bung, making 16 sag gars in all.
separate burns were made, one of the first four groups, one of the last four groups, and a duplicate or check burn on each.
The kiln was fired with coke, in a manner that maintained oxidizing conditions throughout the entire burn. In all four burns the fire clay slabs were burned to a clean buff color showing no evidence of having been subjected at any time to reducing influences. Inasmuch as the buff color of a fire clay is very sensitive to reducing action, and if once re duced the buff tint is irrevocably bleached, confidence is felt that in these burns we were successful in maintaining oxidizing conditions.
When a temperature had been reached sufficient to cause cone 09 to bend, the wicket was opened and the top saggers from each of the four bungs were drawn and placed in the ash pit of the kiln where they cooled slowly. After placing a cover over the exposed cones left in the kiln, the wicket was resealed and the heat raised until cone 06 was bend ing, and so on until the center standard cones of the last set of four sag gers were bending.