64 CLAYS AND CLAY INDUSTRY. 



Amount of lime in clays. 



Kind of clay. Min. Max. Aver. 



Brick clays, 0.024 15-38 1.513 



Pottery clays, 0.01 1 9.90 1.098 



Fire clays, 0.03 15.27 0.655 



Kaolins, tr. 2.58 0.47 



Magnesia. 



Magnesia (MgO) rarely occurs in clay in larger quantities 

 than 1 per cent., and, so far as known, none of the New Jersey 

 clays are exceptions to this "rule. When present, its source may 

 be any one of several classes of compounds, i. e., silicates, car- 

 bonates and sulphates. 



In the majority of clays the silicates, no doubt, form the 

 most important source, and minerals of this type carrying mag- 

 nesia are the black mica or biotite, hornblende, chlorite and 

 pyroxene. These are scaly or bladed minerals, of more or less 

 complex composition, and containing from 15 per cent, to 25 per 

 cent, of magnesia. The biotite mica decomposes or rusts very 

 easily, and its chemical combination being thus destroyed, the 

 magnesia is set free in the form of a soluble compound, which 

 may be retained in the pores of the clay. Hornblende is not an 

 uncommon constituent of some clays, especially in those which 

 are highly stained by iron, and have been derived from dark- 

 colored igneous rocks. Like biotite, it alters rather rapidly on 

 exposure to the weather. Dolomite, the mixed carbonate of lime 

 and magnesia, is no doubt present in some clays, and would 

 then serve as a source of magnesia. Magnesium sulphate, or 

 Epsom salts, probably occurs sparingly in clays, and might form 

 a white coating either on the surface of clay spread out to 

 weather, or else on the ware in drying. It is most likely to occur 

 in those clays which contained pyrite, the sulphide of iron 

 (FeS 2 ), for the decomposition of the latter would yield sulphuric 

 acid, which, by attacking any magnesium carbonate in the clay, 

 might form magnesium sulphate. This substance has a char- 

 acteristic bitter taste. 



