TENSILE STRENGTH.
One of the vital factors affecting the drying behavior of clays is their cohesion. Many ways have been devised to measure this cohesion, but the tensile strength test seems to be the most popular. Determinations of tensile strength as usually made and reported, have so large a per centage of variation that they are practically worthless. This has been justly attributed to the personal factors entering into the preparation of the test pieces. It is indeed surprising how variable the results can be even when the operator uses all, the care possible in wedging the clay and pressing the briquette. The personal factors have been largely eliminated in the tests here reported by following a method for making briquettes devised by H. B. Fox, of the University of Illinois.
Fox Method.—The Fox method is, in the main, as follows : The clay is mixed with just sufficient water to make a thick paste. It is allowed to stand in this condition for some time, generally twelve or more hours, and is then poured onto a slightly moistened plaster slab and allowed to harden until it has assumed about the consistency of "stiff mud" It is then cut into briquettes by a cutter similar to a biscuit cutter. The clay is forced out of this cutter into the briquette mold by a plunger under a given load; in our case about 50 pounds. While the load is still on, the cutter is removed and the briquette struck off with a wire. By this means the briquette is formed and pressed under uniform conditions without the introduction of personal factors, with the possible exception of the making up of the slip.
The briquettes are then room-dried. In this, care is exercised, for the fine-grained clays and the exceptionally weak clays can be dried so fast as to cause them to "dry check." It is not always possible to see these "dry checks," but there is no doubt that a considerable proportion The briquettes are then grooved to a slight depth by the use of a file operated in a making a uniform cross section in all cases. The object of this grooving is not to obtain a uniform cross section primarily, but to insure the breaking of the briquette at the narrowest section. Be
ing uniform, the cross section can be considered as a constant factor, thus making easier the calculation of the results. This grooving was not trusted to give us a constant cross section, however, but each briquette was measured with a vernier shrinkage scale that reads to three places.
The results of grooving the briquettes may be noted in the table given below. There it will be seen that the strength per square centimeter cross section is not materially different. In fact the only difference in strength between the grooved and the ungrooved can be said to be within the limits of errors that are unavoidable in this test. The usual contrast between the variation in results in the grooved and in the ungrooved briquettes which ordinarily exists cannot be seen in the result given be low. The results are exceptionally good in all cases, irregular results due to breaking elsewhere than at the neck not being reported.
After the briquettes are grooved they are made bone dry in a hot air bath and cooled in a dessicator so as to eliminate all moisture, and then broken in a Fairbanks Tensil Strength Machine.
Wedging Versus Slip Process.—Clay workers, especially the old potters who make large jars by "throwing" on a wheel, recognize a difference in the working properties of clay when prepared by the slip process and when prepared by the "chaser," wet pan, or the old-time stamping pro cess. In fact the difference in the clay when prepared in slip, or in one of the "plastic" methods, is so marked, that where ware is to be thrown they install special machinery on which to prepare the clay, and in one of the most up-to-date terra-cotta factories in the west, they keep four men tramping the wet clay with their bare feet, in preference to using the cheaper slip method. In the manufacture of glass pots, tramping with bare feet is the method most generally used in preparing the clay. For this reason the fairness to all clays in casting the slab from which the briquettes were cut was questioned, and the following tests were made to throw light on this point.