56 CLAYS AND CLAY INDUSTRY. 



Iron Oxide. 



Sources of iron oxide in clays. — Iron oxide is one of the com- 

 monest ingredients of clay, and a number of different mineral 

 species may serve as sources of it, the most important of which 

 are grouped below: 



Hydrous oxide. Limonite. 



Oxides. Hematite, magnetite. 



Silicates. Biotite, glauconite (greensand), hornblende, garnet. 



Sulphides. Pyrite. 



Carbonates. Siderite. 



In some, such as the oxides, the iron is combined only with 

 oxygen, and is better prepared to enter into 1 chemical combination 

 with other elements in the clay when fusion begins. In the case 

 of the sulphides and carbonates, on the contrary, the volatile 

 elements, viz., the sulphuric acid gas of the pyrite and the car- 

 bonic acid gas of the siderite, have to be driven off before the iron 

 contained in them is ready to enter into< similar union. In the 

 silicates, the iron is chemically combined with silica and several 

 bases, forming mixtures of rather complex composition and all 

 of them of low fusibility, particularly the glauconite. Several 

 of these silicates are easily decomposed by the action of the 

 weather, and the iron oxide which they contain combines with 

 water to form limonite. 



The range of ferric oxide as determined from a number of clay 

 analyses is as follows r 1 



Amount of Ferric Oxide in Clays. 



Kind of clay. Min. Max. Aver. 



Brick clays, 0.126 32.12 5.311 



Fire clays, 0.01 7.24 1.506 



Kaolins, 6.87 1.29 



Effects of iron compounds. — Iron is the great coloring agent 

 of both burned and unburned clays. It may also serve as a flux 

 and even affect the absorption and shrinkage of the material. 



1 Bull. N. Y. State Museum, No. 35, p. 520. 



