CLAYS OF WEW YOKK 561 



Orange 2102° F. 



White 23Y2° F. 



Dazzling white 2Y32° F. 



Mechanical analysis of clays 



The mechanical analysis of a clay determines the percentage of 

 particles of different sizes which it contains. 



The method employed for this determination is partly a dry and 

 partly a wet one. Clays which are used for the finer grades of 

 ware have to be sufficiently fine to pass through a 150 mesh sieve. 

 The relative quantity of coarser particles which a clay contains 

 can be found out by sieving. 



If the clay grains are smaller than .^^-g- of an inch, the com- 

 mon method of sorting the^m is by means of a rising current of 

 water. This operation is known as " slumming " and consists in 

 brief of placing a known weight of clay in a vertical tube through 

 which a current of water passes. The velocity of the current can be 

 regulated. Careful experiment has determined the size of par- 

 ticles that are carried off by a given velocity of the current. The 

 water as it passes off at the top of the tube is conducted into jars, 

 where the suspended particles are allowed to settle, and can after- 

 wards be collected and weighed. 



The diameters of the grains commonly separated are: 



Clay substance Up to . 01 mm 



Silt 01 — . 025 mm 



Finest sand 025 — . 04 mm 



Fine sand 04: — .2 mm 



An excess of the finest particles tends to increase the shrinkage of 

 the clay, while the coarser particles have the reverse effect. For 

 thorough comparative work on the physical properties of clay, it is 

 well to make a mechanical analysis. This has been done with a 

 number of the I^ew York samples. 



Chemical effects of heating. On heating a clay to redness, it 

 undergoes certain changes, which exert an influence on the physical 



