Ries — Physical Tests oe Devonian Shales. 



077 



Most shales contain a large percentage of fusible impurities and it is this 

 which enables them to burn to a hard, dense, tough body so essential to 

 vitrified wares, and especially to paving brick. Their action under heat, how- 

 ever, varies with their physical and chemical properties, which may be briefly 

 stated. 



Chemical Properties of Shales. 



Pure clay is fusible only at extremely high temperatures, but the presence 

 of a slight amount of impurities may lower its melting point considerably. 

 The impurities found in clays and shales are silica, alumina, iron, lime, mag- 

 nesia, alkalies, water, organic matter, titanic acid, phosphoric acid, sulphur 

 and sometimes manganese. All these impurities may be placed in one of 

 two classes, depending on whether they are active or fluxing impurities, or 

 inert or non-fluxing ones. 



Fluxing Impurities. 



These, in the order of their effectiveness, are alkalies, lime, magnesia and 

 iron. The manganese, sulphur and phosphorus would come under this head, 

 but are seldom present in sufficient quantities to be worth considering. 



' Alkalies. The alkalies in clay may be ammonia, potash, soda and lithia. 

 The lithia is extremely rare and need not be considered. The ammonia is 

 frequently present in shales, but as it volatilizes at low temperatures its only 

 effect is to give a slight, characteristic odor to the material when moist. 



Soda and potash are powerful fluxes, which are found in nearly all shales. 

 They vary in quantity from a trace up to nine or ten per cent. This variation 

 depends on the quantity of feldspar grains present, for this mineral is the 

 usual source of the alkalies in shales and clays. Aside from the feldspars, 

 mica may also furnish potash. The mica shows itself as small glistening 

 scales scattered through the shale. 



The alkalies are sometimes present in soluble form as sulphates, which 

 may cause considerable trouble. In the drying of the clay the water coming 

 to the surface to evaporate will leave these soluble alkaline salts on the sur- 

 face as a white coating. In salt-glazing sewer-pipe the sodium vapors from 

 the salt thrown into the fire unite with the silica and alumina of the clay, 

 thus forming the glaze. If, now, a coating of these soluble sulphates is pres- 

 ent, it prevents this chemical union and formation of the glaze. Furthermore, 

 if the heat is not raised sufficiently to decompose any sulphate salts 



