218 STABLES. 



are made from the granite clay of South Devon, and which 

 have the following composition : 



Silica .... 82.90 



Alumina . . . 14.11 



Peroxide of iron . . . . 2 -39 



Lime and magnesia . . . Only a trace. 



Potash and soda .... O-54 



Loss ..... 0.06 



Total . . . 100 



The chief deleterious substances which are found in brick- 

 clay are : organic matter (vegetable mould for instance), 

 lime, magnesia, soda, and potash. As organic matter will 

 not stand heat, its presence in the clay of which bricks are 

 made, would evidently spoil them when they are put into the 

 furnace. The more bricks are heated in manufacture, the less 

 porous do they become ; hence the best temperature is that 

 which is just short of fusion. If lime, magnesia, potash or 

 soda be present, it will form a silicate which will act as a flux 

 in causing the clay to melt before the required temperature 

 is attained. Another important reason for the desirability 

 of the absence of lime or limestone (CaCO 3 ), is that if it be 

 present, a nodule of quicklime (CaO) may be formed in the 

 brick when the clay is heated. The chemical action with 

 limestone would be as follows : 



CaCO 3 = CaO+CO 2 (carbonic acid gas). 



If a brick wall which contained one or more nodules of 

 quicklime became damp, the quicklime would be liable to 

 unite with the absorbed water, and the resulting chemical 

 action would probably be sufficiently violent to crack the brick. 

 Here we have the following chemical equation : 



CaO-f H 2 O (water) = Ca(OH) 2 (slaked lime). 



Soda and potash not only form fluxes, but their silicates 

 are to some extent soluble in water, and consequently their 

 presence is objectionable. 



