Ries — Physical Tests of Devonian Shales. 



Ferrous iron compounds are more fusible than ferric ones, and conse- 

 quently with reducing fire the clay will fuse at a lower temperature ; at the 

 same time, it will not burn to as bright a red. If treated to an oxidizing 

 fire, the presence of ferrous salts need not be considered. lion salts are 

 affected by varying conditions in burning. If the temperature is raised too 

 rapidly the outer portion of the piece of clay being burned may shrink and 

 become dense before the air has had time to permeate the clay and oxidize 

 the iron in the centre of the body. The centre of a brick or other piece of 

 clay ware may thus be dark and porous while the surface is red and dense. 

 A further result of this will be a differential shrinkage between centre and 

 exterior and possible cracking. 



Unburned shales may be yellow, blue, brown, red, gray or green in color, 

 depending generally on the relative amounts of ferrous and ferric salts present. 

 The same variety of shades and colors is produced in burning. Ferrous iron 

 alone may impart a green color to burned clay, and ferric oxide red or, with 

 hard firing, purple. The higher the temperature to which a clay is subjected, 

 the deeper usually is the color produced by the same amount of iron. 



Non-Fluxing Impurities. 



These include silica, titanium, organic matter and water. 



Silica. This may be present either as quartz combined with alumina 

 and water in the form of kaolinite, or combined with other elements, as 

 in feldspar and mica. Silica renders a' clay more refractory, lessens its 

 shrinkage in burning and decreases the plasticity. On this account plastic 

 clay or shale is added to very siliceous ones. 



Titcmivm is a seemingly rare element in clays, due to the fact that it is 

 seldom looked for in chemical analysis. It is never present in great quanti- 

 ties, rarely over one per cent., and only exerts a fluxing influence at high 

 temperatures and when six or seven per cent, of it are present. 



Organic matter. Very common in black and in some grey shales, and 

 may mask the color which any iron present might produce. It may be pres- 

 ent as finely disseminated particles or in the form of stems or other plant 

 remains. Organic matter burns off at a bright red heat. Its chief influence 

 lies in the increased plasticity of a clay or shale, provided an excess of sand 

 is not present. 



Water is present in shales in two forms, viz., chemically combined water 

 and mechanically combined water or moisture. The moisture in unweathered 

 shales is generally low, but in mellowed portions of their outcrops it may be 



