PHYSICAL PROPERTIES OF CLAYS. 39 



quickly depending on the toughness or density of the 

 clay. Some clays slake completely in two or three 

 minutes, while others may be little effected by an 

 immersion in water of an hour or two. 



The practical importance of slaking i^ noticed first 

 in the case of clays which have to be washed for mark- 

 eting, for the quicker they fall apart when they are 

 thrown into water, the more rapid and sometimes the 

 more thoroughly will be the elimination of the impuri- 

 ties. 



In the tempering the easy slaking of a clay is also 

 of importance, permitting it to be more easily broken 

 up and the more thoroughly mixed with water. 



ABSORPTION. 



This varies with the amount of organic matter, fer- 

 ric hydrate, and the porosity of a clay, and increases 

 with all three. As has already been stated the more 

 water a clay absorbs the more it has to give off in dry- 

 ing and the more difficult it is, especially in the case 

 of fine grained clays, to avoid cracking. 



COLOR OF UNBURNED CLAYS. 



Ferric oxide and organic matter are the two great 

 coloring agents of the raw clay. Organic matter gen- 

 erally colors a clay gray, bluish gray, or black, while 

 iron according to the condition of the oxide, or the 

 presence of carbonate, may impart a red, yellow 

 brown, or sometimes a gray color. 



For any given amount of organic matter or ferric 

 oxide, the coloration will be much more intense the 

 more sandy the clay. 



In general it may be said that, organic matter ex- 



