72 CLAYS AND CLAY INDUSTRY. 



action, and fills all the spaces between the clay grains. When 

 these are all small, the clay may absorb and retain a large quan- 

 tity, because each interspace acts like a capillary tube. If the 

 space exceeds a certain size, they will no longer hold the mois- 

 ture by capillary action, and the water, if poured on the clay, 

 would fast drain away. The fine-grained clays and sands, for 

 these reasons, show high powers of absorption and retention, 

 while coarse sandy clays or sands represent a condition of 

 minimum absorption. This same phenomenon shows itself in 

 the amount of water required for tempering a clay. Thus, a 

 very coarse sandy clay mixture from near Herbertsville required 

 only 15.9 per cent, of water, while a very fat one from Wood- 

 bury took 45 per cent, of water. It is not the highly aluminous 

 ones, however, that always absorb the most water. The total 

 quantity found in different clays varies exceedingly. In some 

 air-dried clays it may be as low as 0.5 per cent., while in those 

 freshly taken from the bank it may- reach 30 to 40 per cent., 

 without the clay being very soft. 



Water held mechanically in a clay will pass off partly by 

 evaporation in air, but can all be driven off by heating the clay 

 to ioo° C. (212 F.). The evaporation of the mechanical water 

 is accompanied by a shrinkage of the mass, which ceases, how- 

 ever, when the particles have all come in contact, and before 

 all the moisture is driven off, because some remains in the pores 

 of the clay. This last portion is driven off during the early 

 stages of burning, and this part of the burning process is referred 

 to as water-smoking or steaming. The shrinkage that takes 

 place when the mechanical water is driven off varies, and ranges 

 from 1 per cent, or less in very sandy clays up to 10 per cent, or 

 12 per cent, in very plastic ones. 



Since most clays having a high absorption shrink a large 

 amount in drying, there is often danger of their cracking, 

 especially if rapidly dried, owing' to the rapid escape of the 

 water vapor. Mechanical water may hurt the clay in other 

 ways. Thus, if the material contains any mineral compounds 

 which are soluble in water, the latter, when added to the clay, 

 will dissolve a portion of them at least. During the drying of 

 the brick, the water rises to the surface to evaporate, and brings 



