68 CLAYS AND CLAY INDUSTRY. 



Other minerals, such as hornblende or garnet, might serve as 

 sources of the alkalies, but are unimportant, as they are rarely 

 present in clays in large quantities. 



Orthoclase, the potash feldspar, contains 17 per cent, of 

 potash (KoO), while the lime-soda feldspars contain from 4-12 

 per cent of soda (Na 2 0), according to the species. The lime- 

 soda feldspars fuse at a lower temperature than the potash ones, 

 but are also' less common. 1 



Muscovite mica contains nearly 12 per cent, of potash and may 

 contain a little soda. Muscovite flakes, if heated alone, seem to 

 fuse at cone 12, but, when mixed in a clay, they appear to act 

 as a flux at different temperatures, according to< the size of the 

 grains. If very finely ground, the mica seems to vitrify at as 

 low a temperature as cone 4, 2 but, if the scales are larger, they 

 will retain their individuality up to cone 8, or even 10. The 

 latter is true oi the micaceous talc-like clays found in the Miocene 

 formation around Woodstown, which are composed chiefly of 

 white mica. We, therefore, see that the minerals supplying 

 alkalies are all silicates of complex composition. Each has its 

 fixed melting point, and the temperature at which the alkalies 

 flux with the clay will depend on the containing mineral, and 

 also on the size of the grains. If the alkali-bearing mineral 

 grains decompose, the potash or soda are set free and form 

 soluble compounds. 3 



Alkalies are considered to be the most powerful fluxing 

 material that the clay contains, and, if present in the form of 

 silicates, are a desirable constituent, except in clays of a re- 

 fractory character. On account of their fluxing properties, they 

 serve, in burning, to bind the particles together in a dense, hard 

 body, and permit a white ware, made of porous-burning clays, 

 to be burned at a lower temperature. In the manufacture of 

 porcelain, white earthenware, encaustic tiles and other wares 

 made from white-burning clays, and possessing an impervious 

 or nearly impervious body, feldspar is an important flux. 



1 See Seger Ges. Schr., p. 413. 



2 Trans. American Ceramic Society, Vol. IV, p. 255. 



3 See Origin of Clay, Chap. I. 



