Report of the State Geologist. 



often increases with the amount of organic matter or combined water present, 

 and diminishes with the amount of sand which the shale or clay contains. 

 Lime in excess exerts the same influence as sand. 



Between the points at which air shrinkage ceases and fire shrinkage 

 begins, the clay shrinks little or not at all, therefore in burning clay wares 

 the heat can be raised rapidly between these two points, but above and below 

 them it should be raised slowly as long as any water is passing off, to prevent 

 cracking. 



Fusibility. 



No clays, on being subjected to a rising temperature, pass suddenly 

 from a solid to a fluid or viscous condition; on the contrary, they change 

 slowly from the condition of solidity to that of viscosity. This change may 

 occur within a range of 75° F., as in very marly shales, while in others it may 

 require a rise of 400° F. to convert the material from solidity to viscosity. 



As the heat is raised to a temperature varying from 1,500 to 2,000° F. 

 in different shales and clays, the particles of the clay soften somewhat, and 

 become tightly stuck together. In fact, the individual grains may no longer 

 remain distinct, and the clay can barely be scratched with a knife. This is 

 the point of incipient fusion and, as Wheeler* has suggested, this is a good 

 term to use in defining this stage. 



As the temperature is raised from 100° to 200° F. higher (also depending 

 on the clay), the clay becomes completely vitrified; the body resembles one 

 solid mass and is impervious or nearly so. The clay has also acquired its 

 maximum toughness and maximum shrinkage. With an additional varying 

 rise of temperature, viscosity occurs. The point of vitrification is generally 

 midway between incipient fusion and viscosity, and these two latter points 

 may be from 100° to 400° F. apart; the nearer 400*-' the better. It should 

 not be less than 200°, otherwise there is the danger of loss in burning, for 

 with so little margin between vitrification and viscosity it is hard to run the 

 burning to the former point without passing it and reaching the latter. 



Manufacture of Paving Brick. 



As the most extensive use of shale is for the manufacture of paving 

 brick, it may not be out of place to describe briefly the methods employed. 



Owing to its hard nature, shale generally has to be mined by blasting, 

 although in some cases, as at Galesburg, 111., a steam-shovel has been found 



* Vitrified Paving Brick, Indianapolis, 1H'J5 



