CHEMICAL PROPERTIES OF CLAY. 55 



Amount of Silica in Clays. 



Per cent, of total silica. 

 Kind of clay. Min. Max. Aver. 



Brick clays, 34-35 90.877 59-27 



Pottery clays 45.06 86.98 45-83 



Fire clays, 34-40 96-79 54-304 



Kaolins. 32.44 81.18 55.44 



With the exception of kaolinite, all of the silica-bearing min- 

 erals mentioned above are of rather sandy or silty character, and, 

 therefore, their effect on the plasticity and shrinkage will be 

 similar to* that of quartz. In burning the clay, however, the 

 general tendency of all is to affect the shrinkage and also the 

 fusibility of the clay, but their behavior is in the latter respect 

 more individual. 



Sand (quartz and silicates) is an important antishrinkage 

 agent, which greatly diminishes the air shrinkage, plasticity and 

 tensile strength of clay, its effect in this respect increasing with 

 the coarseness of the material ; clays containing a high percentage 

 of very finely divided sand (silt) may absorb considerable water 

 in mixing, but show a low air shrinkage. The brickmaker 

 recognizes the value of the effects mentioned above and adds sand 

 or loam to his clay, and the potter brings about similar results 

 in his mixture by the use of ground flint. 



In considering the effects of sand in the burning of clays, it 

 must be first stated that the quartz and silicates fuse at different 

 temperatures, and each changes its form but little up to< its fusion 

 point. A very sandy clay will, therefore, have a low fire shrink- 

 age as long as none of the sand grains fuse, but when fusion 

 begins a shrinkage of the mass occurs. We should, therefore, 

 expect a low fire shrinkage to continue to a higher temperature in 

 a clay whose sand grains are refractory. 



Of the different minerals to be included under sand, the glau- 

 conite is the most easily fusible, followed by hornblende and 

 garnet, mica (if very fine grained), feldspar and quartz. The 

 glauconite would, therefore, other things being equal, act as an 

 antishrinkage agent only at low temperatures. Variation in the 

 size of the grain may affect these results, but this point is dis- 

 cussed under Fusibility (Chapter IV). 



