38 GENERAL DISCUSSION OF CLAYS. 



conditions during burning, with the results obtained, 

 may lead to a discovery of those conditions that will 

 produce the best product. 



CHEMICAL EFFECTS OF HEATING. 



While the fusion of a clay may be looked upon in 

 part as a chemical action, there are ot'her changes 

 which take place in the clay before the temperature 

 of fusion is reached. These changes are : 



The driving off of the chemically combined water. 



The burning of the organic matter. . 



The change of limonite to hematite by the loss of 

 its combined water. 



The oxidization of pyrite to sulphate which by 

 further heating loses its sulphur and is also converted 

 into hematite. 



The driving off of carbonic acid from any carbonates 

 of lime or magnesia which may be present. 



The general effect of these changes is first to make 

 the clay more porous, but subsequently to increase its 

 s'hrinkage, and in addition the color of the clay is 

 changed. 



A chemical interaction between the components of 

 the clay only begins with incipient fusion. 



SLAKING. 



Clays, when thrown into water, break up more or 

 less completely, or in other words, they slake. The 

 process is simply one of mechanical disintegration, 

 which, however, has important practical bearings. 

 Some 'homogeneous clays on being immersed split into 

 a number of angular fragments, while others flake off 

 into scaly particles, while still others crumble down to 

 a powder. This slaking action proceeds slowly or 



