58 CLAYS AND CLAY INDUSTRY. 



(Fe 2 3 ). In the former we see one part of iron united with one 

 of oxygen, while in the latter one part of iron is combined with 

 one and one-half parts of oxygen. The ferric oxide, therefore, 

 contains more oxygen per unit of iron than the ferrous salt, and 

 represents a higher stage of oxidation. In the limonite and 

 hematite the iron is in the ferric form, representing a higher stage 

 of oxidation. In magnetite both ferrous and ferric iron are 

 present, but in siderite the ferrous iron alone occurs. In the 

 ultimate chemical analysis the iron is usually determined as ferric 

 oxide, no effort being made to find out the quantity present as 

 carbonate or sulphide. 



Iron passes rather readily from the ferric to> the ferrous form 

 and vice versa. Thus, if there is a deficit of oxygen in the inside 

 of the kiln, the iron does not get enough oxygen and the ferrous 

 compound results, but the latter changes at once to the ferric 

 condition, if sufficient air carrying oxygen is admitted. Similarly 

 if ferric oxide is present in a clay containing considerable carbon- 

 aceous matter, the latter will, if it cannot get enough oxygen from 

 the kiln atmosphere, take it from the ferric oxide and so reduce 

 the latter to the ferrous condition. The same change may be 

 produced by smoky fires. The necessity for recognizing these 

 two forms of iron oxide is because they affect the color of the 

 clay differently. Ferrous oxide alone is said to produce a green 

 color when burned, while ferric oxide alone may give purple or 

 red, and mixtures of the two may produce yellow, cherry red, 

 violet, blue and black. 1 Seger 2 found that combinations of ferric 

 oxide with silica produced a yellow or red color in the burned 

 clay. We may thus get a variation in the color produced in 

 burning clay depending on the character of oxidation of the iron, 

 or by mixtures of the two oxides. 3 



It is found sometimes that bricks after burning show a black 

 core, due to the iron in the centre of the brick being prevented 

 from oxidizing, but this should not be confused with the black 

 coloration seen on the ends of many arch brick, which is caused by 

 the slagging action of the impurities in the fuel. 4 



1 Keramic, p. 256. 



- Notizblatt, 1874, p. 16. 



3 See "Flashing of brick," Chap. X, and "Burning pottery," Chap. XV. 



4 See "Burning bricks," Chap. X. 



