520 NEW YORK STATE MUSEUM 



there were three times as mucli lime toi destroy the red color, but 

 even then the resulting tint would be yellowish. Even a very small 

 amount, below 1^, may produce a grayish tint at high temperatures. 



Brick clays should contain sufficient iron to give a good red color, 

 provided that is desired in the product. 



For fire clays a small iron percentage is desirable, in fact the 

 total of fluxes should be low, and in every case the permissible 

 quantity of iron, so far as its fluxing effect is concerned, depends on 

 the relative amounts of the other fluxes contained in the clay. 



The following is the range of ferric oxid contained in a number 

 of clays. 



Kiudofclay Max. Min, Aver. 



Brick clays 32 . 12 .126 5 . 311 



Fireclays .01 Y.24 1.506 



Kaolins tr 6 . 87 1 . 29 



Lime 



Lime is a very common impurity of many clays, specially of low 

 grade ones, 



A large number of minerals may serve as its source, but in all 

 of these it is present in one of three conditions. 



1 As a silicate in certain feldspars, hornblende, garnet '. 



2 As a carbonate, limestone or calcite, dolomite 



3 As a sulfate in gypsum 



The first two classes include primary mineral constituents of 

 clays, but the third, gypsum, is most commonly of secondary origiLn, 

 having resulted from chemical action within the clay. 



In many clays, lime probably occurs as a constituent of some 

 silicate mineral, a lime soda feldspar, hornblende or garnet. This 

 would be the case if the clay was derived from an igneous or meta- 

 morphic rock. There are other silicates containing lime, but their 

 presence in clay is probably not very frequent. Lime when present 

 in a silicate acts as a flux, but is seldom liable to exert a decolorizing 



