514 XEW YORK STATE MUSEUM 



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In some clays containing- sulfate of iron the latter may be de- 

 composed by cliemical reactions taking place in the clay and siil- • 

 furic acid being set free. This acid is apt to attack the alnmina of 

 the clay substance, and if potash, soda or ammonia be present they 

 give rise to potash, soda or ammonia alum, which can frequently be 

 detected by tasting the clay. - ■ 



Insoluble alkaline compounds. All the sources of these in clay 

 are minerals, silicates of complex composition. Feldspar and mica 

 are the most abundant sources, but some may be derived from 

 garnet, hornblende and pyroxene, fragTiienta of which may be 

 present in nearly all impure, and specially ferruginous clays. 



The feldspars are complex silicates of alumina and potash, or 

 alumina, lime and soda. Orthoclase, the most common of the feld- 

 spars, contains about 17 fo of potash, while the lime-soda feldspars 

 have from 4^ to 12^ of soda, according to the species. Feldspars ai^e 

 the most important source of alkalis in clay, and, as the speciea 

 vary somewhat in their fusibility, they may exercise a varying in- 

 fluence on the fusing point of the clay. Thus the lime-soda feld- 

 spars are more fusible than the potash ones.-^ 



The micas are complex silicates of alumina, with iron, magnesia 

 and potash. Muscovite, the commonest species of the group, con- 

 tains nearly 12^ of potash and may contain a little soda. While 

 feldspars fuse completely at about 2300° F., mica alone is very 

 refractory, being unaffected by a temperature of 2550° F. While 

 it prohably serves as a flux, it is not known positively at just what 

 temperature it begins to act as such. 



Alkalis, specially in the form of silicates, are frequently a de- 

 sirable constituent of clay, on account of their fluxing properties, 

 as in burning they serve to bind the particles together in a dense, 

 hard body and permit the ware being burned at a lower tempera- 

 ture. 



In the manufacture of porcelain, white earthenware, encaustic 

 tiles and other wares made from kaolins, and having a body which 



1 Seger. Ges. Schrift. p. 413. 



