CHEMICAL PROPERTIES OF CLAYS. 15 



turn to the ferrous condition as the point of vitrifica- 

 tion is approached. This change is accompanied by 

 a liberation of oxygen, which is responsible for the 

 active swelling and blistering of the clay, which takes 

 place as the point of viscosity is approached. 



If treated to an oxidizing fire, the presence of fer- 

 rous salts in clay may not be considered, provided the 

 heat is raised high enough to oxidize them, but the 

 rapidity wHh which the temperature is raised is im- 

 portant, for when the heat is increased rapidly the 

 outer portion of the clay tends to shrink and become 

 dense before the air has had time to enter and oxidize 

 the iron in the center of the clay body,- the latter re- 

 maining in ferrous state. This is the cause of black 

 cores sometimes seen in bricks whose exterior is red* 



Unburned clay may be yellow, blue, brown, red or 

 gray in color, depending on the relative amount of 

 ferrous and ferric salts present, for iron is the one ele- 

 ment above all others which by itslf colors clays. 



The same variety of shades and colors may be pro- 

 duced in burning. Ferrous oxide alone produces a 

 green color when burned while ferric oxide alone may 

 give red or purple, and mixtures of the two may pro- 

 duce yellow, cherry red 1 , violet, blue and black.* 



Segar found that combinations of ferric oxidie with 

 silica had a red or yellow color wlrle similar com- 

 pounds of the ferrous salts showed blue or green. 



The color -to 'Which any given clay burns may also 

 depend on the intensity of the firing. Thus with mod- 

 erate burning the iron may color a clay yellow or yel- 

 loAvish red, \vith harder firing this will pass into deep 

 red, and on still more intense heating to blue or black, 

 this latter color is to be seen on breaking open the arch 

 brick in many kilns, but the surface of these same 

 brick may also get black, due to ashes and cinders 

 from the fire sticking to them. 



The amount of ferric oxide permissible or desirable 

 depends on the use to which tlhe clay is to be put. 



*Keramik, p. 236. 

 }$0tizbiatt, 1874. p. 10. 



