24 GENERAL DISCUSSION OF CLAYS. 



clays low in alumina and high in organic matter are 

 not only 'highly plastic but also absorb a large quanti- 

 ty of water. 



In the manufacture of clay products the moisture 

 is partly expelled by exposing the ware to the sun or 

 putting it in heated tunnels or rooms, while the last 

 traces of moisture a~e driven off in the early stages of 

 burning. 



Moisture may play another important and injurious 

 role in clay working by its tendency to dissolve the sol- 

 uble salts in the clay and bring them to the surface in 

 drying, where they are left in the form of a white 

 coating. It may also permit the acids which are con- 

 tained in the fire gases; of the kiln, to act on the min- 

 eral ingredients of the clay, and thus form soluble 

 compounds, especially clorides and sulphates. 



Combined water is present in every clay. In pure 

 kaolin there is nearly 14 per cent, of it, in other clays 

 the percentage depends on the amount of clay base and 

 the presence of other hydra ted minerals, such as 

 limonite. 



Combined water is driven off at a low red heat, and 

 when this occurs the clay suffers an additional shrink- 

 age. It is a curious fact that although the combined 

 water does not determine the degree of plasitlcity of 

 the clay, nevertheless when once driven off the clay 

 can no longer be rendered plastic. The greater the 

 amount of combined water, the greater the shrinkage, 

 and in the burning the Alabama clays it varied from 

 2-J to 12 per cent. 



PHYSICAL PROPERTIES OF CLAYS. 



These are fully as important as chemical ones, if 

 not more so, plasticity for instance being a character 

 of enormous value. 



The physical characters which are of the most im- 

 portance from the practical standpoint, are plasticity, 

 fusibility, shrinkage, tensile strength, slaking, absorp- 

 tion and density. 



