466 grim. CERAMICS AND CLAY COMPOSITION [Ch. 25 



applies to the clay minerals, electrolyte content, organic content, and 

 non-clay mineral content. 



RELATION OF THE CERAMIC PROPERTIES OF CLAYS TO THEIR 



COMPOSITION 



The object of the following discussion is to provide some under- 

 standing of the wide variation in the properties of clays and the causes 

 of such variations. An understanding of the properties of clays is es- 

 sential to any satisfactory evaluation of economic potentialities of a 

 clay deposit. No attempt is made to consider the theory of such 

 properties or the methods for their specific evaluation, since there is 

 an abundance of information in the ceramic literature concerning these 

 matters. 



Plasticity 



When mixed with certain moderate amounts of water, clays generally 

 can be deformed under pressure without rupturing, and the deformed 

 shape is retained after the pressure is removed. A clay that becomes 

 plastic readily with a small amount of water and that deforms easily 

 over a wide range of pressures is in general most usable. 



Plasticity values for various common clay minerals are given in 

 Tables 2 and 3. The range of plasticity values is a result of variations 

 due to particle size, character of exchangeable base, and the like. In 

 the case of halloysite, the high and low hydration forms have sub- 

 stantially no plasticity, whereas the intermediate stage has some 

 plasticity. 



Clays composed of illite and kaolinite usually have good working 

 properties, and the plasticity tends to increase as the particle size 

 decreases. Montmorillonite clays are frequently exceedingly plastic 

 and require large amounts of water to develop the plastic state. Small 

 amounts (±5 percent) of montmorillonite in a clay material will gen- 

 erally provide a material with considerably higher plasticity than it 

 would otherwise have. Similarly, small additions of montmorillonite 

 clay greatly enhance the plasticity of a relatively non-plastic material. 



A clay material that contains a considerable amount of non-clay 

 material frequently has better working properties than a clay material 

 composed solely of clay minerals. The application of this generality 

 depends somewhat on the kind of clay, the type of ceramic ware to be 

 provided, and the method of manufacture. It is most applicable to 

 montmorillonite clays, and particularly to structural clay products. 

 Clays containing as much as 50 percent non-clay minerals often have 



