CHEMICAL PROPERTIES OF CLAY. 59 



4. Since the stage of oxidation of the iron is dependent on the 

 quantity of air it receives during burning, the condition of the 

 kiln atmosphere is of great importance. If there is a deficiency 

 of oxygen in the kiln so that the iron oxide, if present, is re- 

 duced to the ferrous condition, the fire is said to be reducing. 

 If on the contrary there is an excess of oxygen, so that ferric 

 oxides are formed, the fire is said to be oxidising. These 

 various conditions are often used by the manufacturer to produce 

 certain shades or color effects in his ware. Thus, for example, 

 the manufacturer of flashed brick produces the beautiful shading 

 on the surface of his product by having a reducing atmosphere 

 in his kiln followed by an oxidizing one. The potter aims to 

 reduce the yellow tint in his white ware by cooling the kiln as 

 quickly as possible to prevent the iron from oxidizing. 



Fluxing action of iron oxide. — Iron oxide is a fluxing im- 

 purity, lowering the fusing point of a clay, and this effect will 

 be more pronounced if the iron is in a ferrous condition or if 

 silica is present. A low iron content is, therefore, desirable in 

 refractory clays, and the average of a number of analyses of these 

 shows it to be 1.3 per cent. Brick clays, which are usually easily 

 fusible, contain from 3 to 7 per cent, of iron oxide. 



Effect of iron oxide on absorptive power and shrinkage of clay. 

 — So far as the writer is aware no experiments have been made 

 to discover the increased absorptive power of a clay containing 

 limonite, although the clay soils show that the quantity of water 

 absorbed is greater with limonite present. This greater absorp- 

 tive power may be accompanied by an increased shrinkage. The 

 fire shrinkage might also be great because of the increased loss 

 of combined water due to the presence of limonite. 1 



Lime. 



Lime is found in many clays, and in the low-grade ones may 

 be present in large quantities at times. Quite a large number of 

 minerals may serve as sources of lime in clays, but all fall into 

 one of the three following groups : 



1 See tests, under Fire shrinkage, Chap. IV. 



