CHEMICAL PROPERTIES OF CLAY. 57 



Coloring action of iron in unburned clay. — Many clays show a 

 yellow or brown coloration due to the presence of limonite, and a 

 red coloration due to hematite; magnetite is rarely present in 

 sufficient quantity to color the clay ; siderite or pyrite may color 

 it gray, and it is probable that the green color of many clays is 

 caused by the presence of silicate of iron. In New Jersey this is 

 specially true of glauconitic clays. The intensity of color is not 

 always an indication of the amount of iron present, since the 

 same quantity of iron oxide may, for example, color a sandy 

 clay more intensely than a fine grained one, provided both are 

 nearly free from carbonaceous matter; the latter, if present in 

 sufficient quantity, may even mask the iron coloration completely. 

 The coloring action will moreover be effective only when the iron 

 is evenly distributed through a clay in an extremely fine form. 

 It is probable that the limonite coloring clays, is present in an 

 amorphous or noncrystalline form, and forms a coating on the 

 surface of the grains. 



Coloring action of iron oxide on burned clay. — All of the iron 

 ores will in burning change to< the form of oxide, provided the 

 clay is not vitrified, and so affect the color of the burned material ; 

 if vitrification occurs, the iron oxide enters into the formation oi 

 silicates of complex composition. The color and depth of shade 

 produced by the iron will, however, depend on 1st, the amount of 

 iron in the clay; 2d, the temperature of burning; 3d, condition of 

 the iron oxide, and 4th, the condition of the kiln atmosphere. 



1. Clay perfectly free from iron oxide burns white. If a 

 small quantity, say 1 per cent., is present, a slightly yellowish 

 tinge is imparted to- the burned material, but an increase in the 

 iron contents to 2 or 3 per cent, produces a buff product, while 4 

 or 5 per cent, of iron oxide makes the clay burn red. 



2. If a clay is heated at successively higher temperatures, it is 

 found that, other things being equal, the color usually deepens 

 as the temperature rises. Thus if a clay containing 4 per cent, 

 of iron oxide is burned at a low temperature, it will be pale red, 

 and harder firing will be necessary to develop a good brick red, 

 which will pass into a deep red and then reddish purple. 



3. Among the oxides of iron, two' kinds are recognized, known 

 respectively as the ferrous oxide (FeO) and ferric oxide 



