CHEMICAL PROPERTIES OF CLAY. 73 



out the compounds in solution, leaving them behind when it 

 vaporizes. It may also help the fire gases to act on certain 

 elements of the clay, a point explained under burning. 



Chemically combined water. — Chemically combined water, as 

 its name indicates, is that which exists in the clay in chemical 

 combination with other elements, and which, in most cases, can 

 be driven out only at a temperature ranging from 400 C. 

 (752 F.) to 6oo° C (1112 F.). 1 This combined water may 

 be driven from several minerals, such as kaolinite which con- 

 tains nearly 14 per cent., white mica or muscovite with 4 per 

 cent, to 5^ per cent., and limonite with 14.5 per cent. Unless 

 a clay contains considerable limonite or hydrous silica, the per- 

 centage of combined water is commonly about one-third the 

 percentage of alumina found in the clay. In pure or nearly pure 

 kaolin, there is nearly 14 per cent., and other clays contain vary- 

 ing amounts, ranging from this down to 3 per cent, or 4 per 

 cent., the latter being the quantity found in some very sandy 

 clays. The loss of its combined water is accompanied by a slight, 

 but variable shrinkage in the clay, which reaches its maximum 

 some time after all the volatile matters have been driven off. 



Organic Matter. 



Lender this head are included all fragments of vegetable origin, 

 large and small, found in clay, which have been washed into the 

 water where the clay was being deposited, and settled down with 

 it. In many cases the material has settled down in layers, and 

 these, on account of their coarseness and foreign character, 

 destroy the cohesiveness between the clay particles, causing the 

 clay to split along these planes when taken from the bank. 



Indeed, the gray or black color given by organic matter to 

 many clays is so strong as to obscure the presence of other color- 

 ing agents, such as iron oxide, so* that two clays both colored 

 black, may burn nearly white and red, respectively. The black 



1 See Bourry, Treatise on Ceramic Industries, p. 103, also W. M. Kennedy 

 Transactions American Ceramic Society, Vol. IV, p. 146, and, further, experi- 

 ments under Fire Shrinkage (Chap. IV). 



