16 GENERAL DISCUSSION OF CLAYS. 



The clays which are used for making white ware 

 should not contain over one per cent ferric oxide. 

 And those with even three-quarters of one per cent, 

 are apt to burn grayish at a high temperature, such 

 as 2700 deg. Fahr. It is true that the reddish color- 

 ation of a small percentage of iron would be neutral- 

 ized if any excess of carbonate of lime were present, 

 but in this case even we should not get a pure white 

 tint, but a yellowish one. 



Brick clays should contain sufficient iron oiide 

 to give a good red color to the ware when burned. 



The bleaching of the iron coloration by the presence 

 of lime wUl be mentioned later, an excess of alumina 

 also tends to exert a decolorizing action upon the iron 

 contained in the clay. 



LIME IN CLAYS. 



Lime is a most wide-spread constituent of clays, and 

 occurs either in a finely divided state or else in the 

 form of pebbles. An excess of lime in the clay in the 

 former condition causes it' to pass into marl, and in 

 certain regions such clays are extremely abundant. 



Lme may occur in clays either as a constituent of 

 silicate minerals such as feldspar ; in the form of car- 

 bonate as exampled by calcite or dolomite; or thirdly 

 it may be present as a sulphate, which is the mineral 

 gypsum. 



The first two classes of compo"unds include minerals 

 which are primary constituents of the clay, but the 

 third type, gypsum, is usually of secondary origin, be- 

 ing the result of chemical processes, wh^ch took place 

 in the clay mass. 



The condition of lime is important, for in one case, 

 it may be desirable, and in another it may do injury. 



The presence of lime as a constituent of some silicate 

 mineral is not infrequent, especially if the clay has 

 been derived wholly or in part from crystalline rocks, 

 such as gneisses and granites. The common feldspar, 

 orthoclase, contains no lime, but the other sipecies of 

 feldspar do, and in addition there are other lime bear- 



