CHEMICAL PROPERTIES OF CLAYS. 13 



bring tlie particles of the clay together into the dense 

 hard body, and also permit of the ware being burned 

 at a lower temperature. If present in kaolins to the 

 extent of several per' cent, it is no detriment, provided 

 no iron is present; an excess of feldspar, however, 

 when added to a white burning clay will tend to pro 

 duce a creamy tint. 



In the manufacture of pprcelain, white earthen 

 ware, encaustic tiles and other products made from 

 kaolins or white burning clays, and having a white 

 body, which is impervious, or nearly so, the alkalies 

 for the fluxing of this body are added in the form of 

 feldspar. • 



Much feldspar is mined in this country for potters? 

 use, but all of 't is the ortholase or potash feldspar. 



iKON COMPOUNDS IN CLAYS. 



Iron is not simply a fluxing impurity, but it is also 

 the great coloring agent of clays in either their burned 

 or unburned condition, and furthermore when in the 

 form of the hydrated oxide or limonite it may serve 

 to increase the absorbtive power of clay. * 



The compounds in which iron may exist in the clays 

 are as follows: Oxides: — ^limonite, hematite, magne- 

 tite, ilmenite. Silicates: — mica, hornblende, garnet, 

 etc. Sulphides : — pyrite and marcasite. Sulphate : — 

 melanterite. Carbonate : — siderite. 



The iron oxides, limonite and hematite, are present 

 in" all clays, and may be introduced by percolating 

 waters or be set free by the deccomposition of any of 

 the iron-bearing silicates which the clay may contain. 

 Not infrequently they are distributed through the 

 clay in a very finely divided condition, or may form a 

 thin film around the other mineral grains. Limonite 

 tends to color .the clay (unburned) brown or yellow, 

 while hematite imparts a red color to it, and carbon- 

 ate of iron may give gray tints. 



The more sandy the clay the less the amount of the 



' A. B. Smith, Alabama Geological Survey, Agricultural Eeport, p. 45. 



