14 GENERAL DISCUSSION OF CLAYS. 



limoiiite required to produce any given intensity of 

 color. 



Mica is found in most clays, and hornblende and 

 garnet are probably wanting in few, while the pyrite 

 is often present in many clays, especially in stoneware 

 and fire clays, its yellow, glittering, metallic particles 

 being easily recognizable. When large, the lumps of 

 pyrite can be extracted by hand-picking, but if very 

 small, they can only be separated by washing. Un- 

 der weathering influences the pyrite changes to sul- 

 phate of iron. In all of the iron-bearing minerals rthe 

 iron is present in either the ferrous or the ferric stage 

 of oxidization, and the fusibility of the clay is in- 

 fluenced somewhat by this fact, for ferrous com- 

 pounds are more easily fusible than ferric ones. In 

 the burning of the clay the ferrous salt will be con- 

 verted into the ferric state, provided the action of the 

 fire is oxidizing. But if it is reducing the clay will 

 fuse at a lower temperature. 



The action of weathering agent in nature is often 

 sufficient to oxidize the iron in clays so that more 

 ferric than ferrous iron will' be found in most of them. 

 This change is often noticeable in many clay banks 

 where the upper, and at times more porous layers, are 

 colored red or yellow, while the lower layers cire blue 

 or bluish gray. 



It should be noticed, however, that a gray color may 

 be produced by the presencce of organic matter, and 

 the same material present in a dense clay, to which 

 the air can not get access, may serve to retard the oxi- 

 dation of the iron. Whenever iron exists in clay in 

 combination Avith s ; lica it is present probably as a 

 complex silicate, for pure ferric silicate u very rare 

 in nature. 



Ferric hydrate increases the absorbing power of 

 clay for both gases and liquids, but it as well as the 

 carbonate change to the oxide in burning. 



The general tendency in burning is to convert the 

 iron compounds into ferric oxides, provided a certain 

 temperature, depending on the fusibility of the clay, 

 is not exceeded, for in every clay the iron seems to re- 



