CLAYS OF NEW YORK 519 



If the actiooi of the fire is oxidizing, the presence; of ferrous salts 

 need not be considered, provided the heat is raised high enough to 

 oxidize them. 



The rapidity with which the temperature is raised is important, 

 for if the heat is raised too quickly the outer portion of the clay 

 may shrink and become dense before the air has had time tO' per- 

 meate the clay and oxidize the iron in the center of the body. This 

 is the ca.use of the black cores sometimes seen in bricks whose sur- 

 face is red. 



The same variety of colors seen in the raw clay may be similarly 

 produced in the burned clay, the result being conditioned on the 

 relative amounts of ferrous and ferric compounds. Ferrous oxid 

 alone produces a green color when burned, while ferric oxid alone 

 may give a purple or red, and mixtures of the two may produce 

 yellow, cherry red, violet, blue and black.-^ The more intense the 

 heat, the deeper the color produced by the iron. At very high 

 temperatures it is diihcult or "impossible to obtain an oxidizing 

 action in the kiln or furnace. 



Seger^ found that combinations of ferric oxid with silica pro- 

 duced a yellow or red color in the burned clay, while similar com- 

 pounds of the ferrous salt showed blue and green. 



The black coloration produced by iron oxid in hard firing is 

 often to be seen on breaking open the arch bricks of a kiln. The 

 surface of such bricks may frequently get black, this being due in 

 part to the slagging action of the ashes from the fire which stick to 

 them. * j 



The coloration of clays by iron in burning will be farther dis- 

 cussed under that head. 



The amount of ferric oxid permissible or desirable in a clay de- 

 pends on the use to which it is to be put. Kaolins or plastic clays 

 to be used in the manufacture of white bodies should contain less 

 than 1^ if possible. A greater amount might be present, provided 



1 Keramik. p. 256. 

 2 NotizUatt. 1874. p. 16. 



