PROPERTIES OF CLAYS 



139 



per cent, silica is not incompatible with the formation of a good 

 brick. Uncombined silica, if not in excess, is beneficial, as it 

 preserves the form at high temperatures ; in excess it destroys 

 cohesion and makes the brick brittle and weak. Fire clays 

 should not contain over three to four per cent, of fusible 

 impurities. 



The fusibility of a clay usually increases with the amount of 

 impurities present. Kichter* found that potash rendered a clay 

 more fusible than any of the other impurities ; iron is said to 

 come next, then lime and lastly magnesia. Phosphates also 

 increase the fusibility of clay slightly. 



Iron is usually present in clay in the form of a lower oxide, 

 and in burning is converted to a higher one, coloring the brick 

 red. According to Segerf the shade of color produced by iron 

 is influenced by the amount of iron oxide present, the chemical 

 composition of the clay, and the mechanical division of the color- 

 ing substance, the degree of burning, and whether the fires of 

 the kiln are reducing or oxidizing. The intensity of the colora- 

 tion increases with the amount of peroxide of iron from four to 

 eight per cent. From this up to 21 per cent, no change in the 

 color was noticed. 



Carbonate of lime counteracts the color produced by iron due 

 to the formation of a light colored double silicate of iron and 

 lime. This fact is often made use of in the manufacture of cream- 

 colored brick, a certain proportion of lime being added to the 

 clay. Clays rich in carbonate of lime burn yellow in a reducing 

 atmosphere. If there is an excess of lime the bricks get flesh 

 colored in the oxidizing flame. 



If iron is present in the form of sulphate it usually decomposes 

 at the point of fusion, giving a lower oxide which fluxes and pro- 

 duces a distortion of the brick. A reducing flame is detrimental 

 to the formation of sulphate. 



It has been stated that a clay containing over three per cent, 

 of lime is not fit for making brick, but this limit is too low, as 

 clays with five and six per cent, of lime will often make a very 

 good product and the celebrated Milwaukee brick have 23 per 

 cent. 



* Brickmaker, Oct. 1892. 

 t Brickmaker, Oct. 1692. 



