THE PHYSICAL PROPERTIES OF CLAY. 107 



TEXTURE. 



By the texture of a clay is meant its size of grain. Many 

 clays contain sand grains of sufficient size to be visible to the 

 naked eye, but the majority of clay particles are too small to 

 be seen without the aid of a microscope, and are, therefore, so 

 small that it becomes impossible to separate them with sieves. In 

 testing the texture of a clay it is perhaps of sufficient importance 

 for practical purposes to determine the per cent, of any sample 

 that will pass through a sieve of 100 or 150 meshes to the inch, 

 since in the preparation of clays for the market by the washing 

 process they are not required to pass through a screen any finer 

 than the one above mentioned. 



If it is desired to measure the size of all the grains found in 

 the clay, some more delicate method of separation becomes neces- 

 sary. In many clays the grains cohere more or less, forming 

 compound ones, and these have to be disintegrated by some pre- 

 liminary treatment such as boiling, or better still by agitating. 



The simplest and least accurate method of separating the grains 

 is by settling. This is done by stirring up the disintegrated 

 sample in a beaker of water and allowing it to stand just long 

 enough to let the coarse sand grains settle, and then pouring 

 it into a second beaker, where it again remains long enough to let 

 the finer sand particles drop. The sample is thus run from 

 beaker to beaker until only the finest clay particles are left in 

 suspension. This method is not very accurate. 



A second method consists in separating the pebbles and coarse 

 sand particles out of the disintegrated clay by means of sieves, 

 and then placing the finer portions in a tube where it is exposed 

 to an upward current of water. Since the carrying power of the 

 current will increase with its velocity, a current of water rising 

 very slowly in the tube will carry off only the finest particles, 

 while the heavier ones remain behind. If the velocity of the 

 current be kept at this speed it will finally become clear when 

 all the finest particles are carried off. The velocity is then in- 

 creased a slight amount, so that the next coarsest size is carried 

 off, the current being thus gradually increased until all the 



