All the clays for both the "slip" and "wedge" process were made to pass through a 10-mesh sieve.
The clay for slip process was cast as in the Fox method.
The clay for the wedge process was thoroughly wedged by hand while at its state of maximum plasticity, and then worked into a sheet thick on the plaster slab by pounding it with a flat board. Briquettes were cut and forced into the mold under constant pressure as in the case of the slip clay.
The results are shown in Table XIII : • The following conclusions were reached as a result of these tests: First—In every case except that of the Terre Haute not grooved, the wedging process gave higher results.
Second—The variation is considerably lower in the slip than in the wedge process.
Third—The increased strength due to wedging was not sufficient to warrant the accompanying increase in percentage of variation.
Fourth—Grooving the briquettes did not materially better the results in the slip process and actually made the results worse in the wedge process. It must be remembered in this connection, however, that the results of briquettes that did not break at the necks were rejected. All grooved briquettes broke at the neck.
Fifth—Grooving increased the variation in coarse non-plastic clays, such as K-14 and K-9, but did not seem to effect the finer grained clays.
Effect of fine view of the fact that grooving aids ma terially in reducing the variation in all, except the less plastic, coarse grained clays, it was thought that perhaps the comparison would be more just if all were finely ground.
The dry-pan 'samples of the two plastic clays, K-10 and K-11, and the two coarse and less plastic clays, K-14 and K-9, were ground of the variations are due to them.
wet and also dry until they passed through sieves of 10, 20, 40 and 80 mesh, as follows : A quantity of clay sufficient to make six briquettes was taken from the stock by quartering, making ample allowance for waste. This sample was first crushed to pass a 10-mesh sieve. It was then sieved through the desired mesh and the residue placed in a small Bonnot mill with 100 Iceland pebbles. Both the wet and the dry samples were taken from the mill every five minutes, and the particles fine enough to pass through the desired mesh were sieved out. The residue left on the sieve was then placed in the mill and ground for another five minutes. This grinding was continued until all the clay passed through the desired mesh. In this manner there was prepared, by both wet and dry grinding, stocks that would just pass the 10, 20, 40 and 80 mesh sieves.
The clays that were ground were kept at casting consistency, i. e., quite thick slush, so that when completely ground they were cast into slabs as quickly as convenient. The clays prepared by the dry method were allowed to stand in water until they assumed the thick slip state and then cast on plaster of Paris slabs after standing from 10 to 24 hours.
Briquettes were cut and pressed by the Fox method. In table XIV will be found the results of this experiment.
The work was done by a man not accustomed to it who could not at first be made to realize the importance of taking the greatest pains to in sure constant conditions and accurate results. This may account for the higher variations.
From these results the following conclusions may be drawn : First—The variations with the grooved briquettes are on the whole lower than those with the ungrooved.
Second—The average strength of the grooved is practically equal to that of the ungrooved.
Second—The average strength of the grooved is practically equal to that of the ungrooved.
Third—Finer grinding either wet or dry does not materially better the constancy of the results. The fact is, in this experiment, the varia tions in the finer ground samples were higher in many cases than in the coarsely ground samples.
Fourth—The average strength of the clay was not materially altered by finer grinding.
Fifth—The results by wet grinding differed but little, if any. from those by dry grinding.
In the, light of the foregoing tests it was decided to dry-grind the clays in a jaw crusher to pass a 20-mesh sieve. In this the whole sam ple, including the fine and coarse particles, was passed through the jaw crusher. Six briquettes were made by the slip method, as designed by Fox, and grooved to insure breakage at the neck. In this manner the following data was obtained : The results here reported are exceptionally good. The variation in the strength of dry clay, as made by other methods; usually runs from 25 to 50 per cent in nearly every reported instance. In fact, it is seldom, if ever, that a report on tensile strength will show a lower variation than 25 per cent. The placing of 15 per cent as the maxi mum variation to be allowed would be very severe standard ordinarily, but the general character of the work as here reported justifies the limit.