Plasticity modulus—It is obvious that since all three factors, initial stretch, final stretch, and load required to cause rupture, must be consid ered as being affected by the degree of plasticity of the clay, a modulus must be devised that includes all three factors. The one used in our tests was constructed as follows: The central portion of the briquette is a perfect cube On the assumption that the volume of this portion of the briquette mains constant throughout the test', and that its cross section decreases proportionally as the length increases, the decrease in cross section in 1.93 (E) centimeters due to the initial stretch would be or C 1.9+a 1.9+a where a is the initial stretch. The decrease in cross section after the final stretch (here it is figured as though there has been no rupture) would be equal to (—) or, by reduction, 1.9+a 1.9+a+b J where b is the final stretch.
Now a measure of the tension that is holding the grains together would be directly proportional to the load and inversely proportional to the decrease in cross section of the briquette due to stretching. The modulus must, therefore, represent a value that is directly as the Ioad and inversely as the product of the decreases in cross section due to the initial and final stretch. Performing- this calculation and collecting
terms the following plasticity modulus is obtained : 1 (6.859 + 10.83 b+5.7 a3+3.8 ab+a3+azb)=M 24.76b in which L — load in centigrams, a the initial stand, b the final stretch.
While the modulus is very formidable looking it was found that the test could be made and the plasticity factor calculated quite readily. In fact the entire test required less time than did the Grout test as carried out in our laboratories.
With the heavy and far from delicate Fairbanks machine and the clumsy clips, plasticity factors were obtained that varied for anyone clay not more than 50 per cent and in some cases only 13 per cent on six briquettes. This percentage of variation was considered too high to at tach any value to the obtained data, and they are, therefore, not reported.. It is believed that with a more delicate apparatus this method of measur ing plasticity would give very close results and that the data obtained would be a true measure of plasticity.