Ries — Physical Tests of Devonian Shales. 



t>81 



quantity of water, and ''lean 1 ' ones usually needing but little. Of course 

 tliere are exceptions to this rule. 



The production of maximum plasticity has a practical bearing in the 

 tempering of the clay, for if too little water is added, the clay will frequently 

 crack in molding. 



Tensile Strength, or Cohesion. 



The plasticity of a clay has been found t<> be due to the interlocking of 

 its particles,* consequently a mass of clay, when air dried, offers a resistance 

 (which may be great) to any force tending to pull it apart. This tensile 

 strength or cohesion stands in close relation to the plasticity, and consequently 

 serves as a measure of it. The tensile strength is expressed in pounds per 

 square inch and is determined by forming the wet clay into briquettes of the 

 same shape and size as those used in testing cement, allowing them to air dry 

 and then pulling them apart in a cement testing machine. 



Clays which appear moderately plastic when worked in the hand, will 

 show a tensile strength of 100 to 150 pounds per square inch when air dried. 

 Lean clays may run about fifty pounds per square inch, often lower. Very 

 plastic (days may show 250 to 300 pounds tensile strength per square inch. 

 Very fine and very coarse grained shales or clays show a low strength. 



Behavior under Increasing Temperature. 



Shrinkage. The amount of shrinkage that clay materials undergo in 

 drying, depends somewhat on the amount of water absorbed or the porosity 

 of the clay. Coarse grained clays may absorb much water and yet shrink 

 very little. Having larger pores, they will also permit the water to escape 

 more rapidly and consequently can be dried quicker, w hile fine grained ones, 

 ow ing to the smaller size of their pores, must dry slowly. 



The air shrinkage of a clay begins as soon as it is molded and set out in 

 the open air or put in tunnels to dry, and continues until all the moisture is 

 driven off. It may be as. low as two per cent, in lean, coarse grained clays, or 

 reach tw r elve or thirteen per cent, in others. The highly plastic clays do not 

 always shrink the most. 



The fire shrinkage begins w hen the combined water commences to pass 

 off, or at a temperature of about 1,200° F. It may also vary within the same 

 limits as the air shrinkage. It is, however, affected by several factors. It 



* Olcliewsky. Tupfer untl Ziegler Zeitung, 1882, No. 29. 



