Geography of Ohio 171 



classified as clays, there is a wide gap. This fact accounts for 

 several varieties of clay, depending upon the percentage of given 

 constituents. 



Clay produced by the decay of crystalline or other rocks in situ 

 is termed ''residual." Clays which have been deposited in water, 

 or by running water, are ''sedimentary." The sedimentary 

 clays are either unconsolidated, or are in the form of more solid 

 rock; surface clays belong to the former, while shale represents 

 the second class. The clays of Ohio are all sedimentary in origin; 

 we have extensive outcrops of shale or clay. Over much of the 

 state the glacial drift is also a source of clay; locally "bowlder" 

 clay is well developed. Another form of glacial clay occurs in the 

 northern counties; this was deposited in the ice-front lakes, and is 

 called "lake" clay. Wherever rivers are making deposits, clays 

 accumulate; the flood plains of valleys are accordingly a source 

 of clay. 



Properties of clay. The character of a clay depends upon its 

 constituents. These are numerous, involving commonly, lime, 

 magnesia, silica, oxides of iron, alkalies, titanic acid, alumina, 

 organic matter, and combined water. As the per cent of a par- 

 ticular constituent varies, the character of the clay changes accord- 

 ingly. If nmch iron is present, the burned clay has a stain; 

 red brick represent a clay carrying an appreciable amount of 

 iron oxide. If lime is in excess, it acts as a flux in burning and often 

 gives the product a cream color. Silica makes the clay more 

 refractory, and lowers its plasticity, while organic matter or 

 colloids increase its plasticity. The tensil strength of clay varies 

 from a few pounds to four hundred pounds or more per square inch. 

 The fluxes, such as magnesia, alkalies and lime, cause the clay to 

 fuse at lower temperatures. Thus, it is seen that products made by 

 burning clay must vary greatly in accordance with the con- 

 stituents. 



In the early days of clay working, men could learn only through 

 experimentation. The chemistry of clays was little understood. 

 At the present time, the haphazard method of using clays will 

 not enable the manufacturer to cope with competition. He should 

 know the chemistry of his clays and treat them accordingly, if 

 he is to hold a place in business. 



When properly burned, many clays are very enduring. Chemi- 

 cal changes are introduced by burning, the manner of the burning 



