THE MANUFACTURE OF BUILDING BRICK. 239 



At one factory it had formerly been the practice to burn with 

 an oxidizing fire to a high temperature, 1 viz., from cone 11-12, 

 and then to cause reducing conditions to take place in the kiln 

 during the last 5 or 6 hours of the burn. This practice, however, 

 was changed, it being found that by maintaining a reducing fire 

 during the entire period following water smoking, a lower tem- 

 perature was sufficient. 



3. The oxidation which causes the flashing probably takes place 

 in the first twelve hours after closing the kiln, and can be regulated 

 by a proper handling of the dampers. 



In the experiments of Bleininger already referred to, it was 

 found that a reduction of air, equal to 20 per cent, below that re- 

 quired for ideal oxidation, and considered as 100, is usually suf- 

 ficient to produce flashing. 



By this is meant that "100 per cent, of air represents theoret- 

 ically ideal conditions, in which just enough air is present to con- 

 sume all the combustible gases forming C0 2 ; less than 100 per 

 cent, of air corresponds to- reducing conditions. For instance, if 

 an analysis on calculation represents 90 per cent, of air, it tells us 

 that the gases are reducing to the extent of 10 per cent, of air; 

 similarly no per cent, shows an excess oi air to the amount of 

 10 per cent." 



While 100 per cent, represents theoretically the amount of air 

 required for perfect combustion, still in actual practice with coal 

 fuel, the mixture of gases is not perfect, and it may be necessary 

 to have more than 100 per cent, of air present to bring about thor- 

 ough oxidation. 



4. As regards the rate of cooling, it was found that the longer 

 the period of cooling from the maximum' temperature down to 

 approximately 700 C, the darker the flash under given condi- 

 tions. 



Kilns. — Bricks are burned in a variety of kilns, ranging from 

 temporary structures, which are torn down after each lot of brick 

 is burned, to patented or other permanent forms of complicated 

 design. They are built on one of two principles, either up-draft 

 or down-draft. In the former the heat from the fire boxes at the 



1 Ibid., p. 79. 



