238 CLAYS AND CLAY INDUSTRY. 



quantities of iron soluble in acid. Some eastern manufacturers 

 are obliged to add magnetite ores to their clays, which are low 

 in combined iron, and No. 2 fire clays, which contain more iron 

 that the finer grades, seem to give the best results. As to the 

 effect of the physical condition of the clay, finer grinding seems 

 to> give more uniform flashing effects, and the reason that stiff- 

 mud brick flash better than dry-press ones is claimed by some to 

 be due to vitrification taking place more easily in the former. 



The following analysis gives the composition of a No. 2 fire 

 clay from Ohio used for flashed brick : 



Analysis of an Ohio No. 2 fire clay. 



Silica (SiOj), 67.14 



Alumina ( A1 2 3 ) , 19.74 



Ferric oxide (Fe20 3 ), 2.46 



Lime (CaO), 0.53 



Magnesia (MgO), 0.71 



Potash (KsO), 2.80 



Soda (NaaO), . . . ". 0.43 



Water (H 2 0), 7-OI 



Total, 100.82 



In one case the green clay showed a total of 2.15 per cent, of 

 ferric oxide, of which 0.88 per cent, was soluble in acid. The 

 flashed surface of a brick made from this clay gave, on analysis, 

 a total of 2.31 per cent, of ferric oxide, of which 0.14 per cent, 

 was soluble in nitrohydrochloric acid, thus indicating that dur- 

 ing the burning most of the iron oxide had combined with silica, 

 forming a ferrous silicate. 



2. The temperature reached must be sufficient to cause a combi- 

 nation of the iron and silica, and, therefore, it varies with dif- 

 ferent clays, the combination being aided by the presence of fluxes. 



If the kiln atmosphere is oxidizing during nearly the entire 

 burning, with only a small period of reduction at the end, the 

 temperature reached must be comparatively high in order to 

 insure union of the iron and silica, by fusion. If, however, a re- 

 ducing fire is maintained during most of the burning, then the 

 temperature need not be as high, because the clay will vitrify 

 sooner. (See Fusibility, pp. 97-106.) 



