CLAYS OF KEW YORK 5 i3 



A fourtli plan is to press the mixed clay into a form of given 

 thickness, and then bring a metal cylinder to bear on the upper 

 surface. This cylinder can be weighed, and the weight be noted 

 which has to be added in order to force the cylinder through the 

 clay in a given time; or the determination may be made by measur- 

 ing the amount of water mixed with the clay in order to produce 

 the proper consistency to permit the passage of the cylinder through 

 the clay in a certain time under given pressure. 



Olchewsky^ states that the amount required for lean clays is 

 as low as lY^, while for very plastic clays it is not uncommonly 50/^. 



Bischof has suggested forcing the wet clay from a circular open- 

 ing in the lower end of a vertical cylinder, and observing the length 

 of the clay which would issue before the mass broke. This is not, 

 however, an accurate method. 



The difference in plasticity between residual and sedimentary 

 clays is dwelt on by Seger, who says: 



In clays which show a mixture of plastic and nonplastic parti- 

 cles the degree of plasticity depends on the relations existing be^ 

 tween the two. We also know that the true clay substance [meaning 

 kaolinite], even when of constant composition, may show con- 

 siderable variation in plasticity. One clay substance may be lean 

 and permit of very little admixture of nonplastic particles, while 

 another may be very fat and permit considerable material being 

 mixed in, and still be not only plastic but dry to a hard mass. The 

 former case is generally to be found in residual clays, the latter 

 in sedimentary. 



Seger considered it unsettled whether this is due to a finer 

 state of division, or the introduction of plastic particles not derived 

 from the feldspar. 



The degree of hardness which clays assume on drying also stands 

 in direct relation to the plasticity. Slightly plastic kaolins when 

 dry give only a loosely knit mass. This point as well was com- 

 mented on by Seger in comparing two clays of nearly the same com- 



' Post. Chem. ttch. analyse. 1890. v. 2, pt 1, p. i6. 



