CLAYS OF NEW YOEK 571 



Brick clays often have more or less organic matter which may 

 sometimes reduce the iron. Thus the clay from EathenoAV, when 

 ignited in a closed crucible, showed 2.20/"^ of ferrous oxid. In 

 the presence of air, however, it was converted to ferric oxid, for 

 after being ignited for one hour, exposed to the air, the same clay 

 showed only .76^ of ferrous oxid. 



The black color of bricks is due to the reduction of iron in the 

 last stages of burning. 



One interesting result of Soger's experiments is that oxidation 

 of the iron can take place within a clay which has been burned 

 to vitrification. This was explained by an experiment in which 

 he took a prism 2 cm thick, and burned it to vitrification. After 

 burning, the surface of the prism was cherry red, but in passing 

 from the middle to the surface the colors encountered were cherry 

 red, gray red, gray green, black violet, gray green, gray red, and 

 cherry red. An analysis of these different colored portions showed 

 the following: 



Ferric oxid 



Ferrous oxid 



Cherry red 



Gray green 



Black violet 



5.35 



3.43 



2.14 



.12 



1.85 



3.01 



The above is explained by supposing that the flame of the fire 

 caused a reducing action of the iron, which did not extend the 

 entire distance to the core; later, on cooling the outer portion of 

 the brick was reoxidized. 



