566 NEW YORK STATE MUSEUM 



color of claj and specially influence the coloring action of iron. 

 Calcareous clays in burning develop a yellow, instead of a red color, 

 and at the temperature of vitrification this passes into a yellowish 

 green. Seger has shown that the color of a hard burned clay der 

 pends on the relation of iron oxid to alumina., and in calcareous 

 clays on the ratio of iron oxid tO' lime. 



Diimmler in his table of analyses^ shows the ratio of iron and 

 manganese oxid to the' sum of nonvolatile constituents, and the 

 ratio of lime and magnesia oxid to iron and manganese oxids. From 

 this it follows that in all clays in which the combined iron and 

 manganese oxids are more than t- j- of the amount of the total non- 

 volatiles, a distinct red color is produced, if at the same time the 

 sum of the two is more than twO' and a half times greater than the 

 combined magTiesia oxids. 



Of course the grade of firing has an influence on the color, and 

 in addition the composition of the kiln gases might exert a marked 

 influence. Thus, for instance, clays high in iron burned slate blue 

 in a reducing fire, while yellow burning noncalcareous clay takes 

 on a distinct red color, if subjected to alternating reducing and 

 oxidizing action. (Diimmler. Die ziegel fabrikatiorij, p. 42) 



The shades which ferric oxid takes in burning vary partly with 

 the nature of its formation. According to Seger that which is made 

 from ferric nitrate bums brown red, that from iron sulfate by 

 ignition is reddish orange. 



Heating deepens the color of the ferric oxid with increase of the 

 temperature; and this holds true for all ferruginous clays, so that 

 in general the color of clay products containing iron will be darker 

 the higher the temperature to which they are burned. 



A small percentage of iron in a clay produces a buff color when 

 burned to, say 2000° F., but might give a red if burned to 2500° F. 



If a clay contains enough iron to color it red when burned to 

 incipient fusion, it may become deep red or brownish at the tem- 

 perature of vitrification, and black at the temperature of viscosity. 



1 Die ziegel fabrikation. 



