METHODS OF _MEASURING PLASTICITY.
General—There have been many methods devised for measuring plasticity. The methods suggested by and seem to be the most rational of any, for in them the resistance to deformation and amount of flow before rupture, two characteristic properties of plastic bodies, are measured. These methods are based on the same principle as the well-known but crude method of testing plasticity by squeezing a ball of plastic clay between the tips of the forefinger and . thumb, and making a mental note of the amount of pressure required to affect a given degree of deformation.
The test developed by this Survey involves the tensile strength of the plastic mass rather than its resistance to compression, 'as in the Zschokke and Grout methods. It is believed that a tensile test gives a more accurate rating of the tenacity with which the several grains cling to one another, for in this, friction between the non-plastic grains and interference to flowage by the larger ones crowding into one another is entirely eliminated.
Shape of the test piece—The special features of the shape and size of the briquette employed in this test are, first, narrow neck, , wide ends (—") and straight sides to fit the jaws, as explained later. The smallest portion is cubical in shape, being x x The clips—In previous experiments it was learned that the Standard Fairbanks clips, using the standard shape and size of briquette, would permit the stretching of the briquettes until they would slip out of the jaws. Special clips were therefore made to fit the briquette. These clips were designed after Orton', differing from his only in dimensions. angle of nip between the jaws, and manner of adjustment.
Manufacture of briquettes—Clay was mixed to a thick slip, cast and cut into briquettes by the Fox method, as described under tensile strength. When the cast slab had, in the opinion of the operator, as sumed its maximum plasticity, the briquettes were cut and forced into steel molds under a constant pressure of fifty pounds. This weight was
applied slowly but the briquette was not allowed to remain under pres sure after it had received its full load.
Adjustment and calibration of the machines—Before the Fairbanks machine could be used, the balance beam had to be poised to allow for the difference between the standard and our special clips.
For measuring the stretch which the briquettes suffered, the small adjusting wheel was calibrated so that the peripheral distance through which the wheel was turned would represent the distance the under clip had been lowered. The amount of stretch which the briquettes suf fered at any time during the test was measured by the fractional num ber of turns of the adjusting wheel required to lower the under jaw sufficiently to keep the beam in a predetermined position.
Method of procedure—The plastic briquette was carefully placed in the clips and the jaws adjusted to it, care being taken to see that the jaws on either side were at the same. angle. The lower clips, suspended by counterpoise, were kept in a vertical line by hand guidance. Very small shot was allowed to run into the pail slowly until a rupture oc cured at the neck of the briquette. As soon as a rupture occurred, the beam dropped with a suddenness that shut off the flow of shot. At the moment of rupture the amount of initial stretch was noted by the fractional number of revolutions through which the adjusting wheel had been turned. Before removing the "load" the adjusting wheel was slowly turned until the briquettes was completely torn apart. The sec ond peripheral distance through which the wheel had been turned was noted as "final stretch." The weight of the shot required to cause rupture was obtained on a balance that is accurate to one centigram. While the weight thus ob tained is not the force that was required to cause rupture, it does bear a constant ratio to that force. The shot was not weighed on the Fair banks machine because it was not sufficiently sensitive.