THE MANUFACTURE OF BUILDING BRICK. 235 



into a solid mass, which is hard and rock-like when cool. Other 

 changes, due to the presence of certain ingredients or certain 

 physical characteristics of the clay, occur in specific cases. 



The amount of heat required for burning brick will vary with 

 the clay, and the color, density and degree of hardness desired, the 

 same clay giving different results, when burned at different tem- 

 peratures. Common bricks are rarely burned higher than cone 05 

 or 03, while pressed brick are frequently fired to- cone 7 or 8, be- 

 cause the clays generally used have to' be burned to that point to 

 render them hard. 



General effects. — In burning, the last traces of moisture are 

 driven off. This vapor, which is termed water-smoke or steam 

 by the brickmaker, is simply the moisture which has been retained 

 in the pores of the clay. Its expulsion results in a slight loss of 

 weight. With further heating to very dull redness the chemically 

 combined water disappears. 



If the clay contains considerable carbonaceous matter, this will 

 burn off at a low red heat, provided in the first place sufficient air 

 is present to insure an oxidizing atmosphere. In this case carbon 

 in the clay uniting with the oxygen of the kiln atmosphere, burns 

 off as carbon dioxide. If the heat is raised too rapidly the clay 

 contracts before all the carbonaceous matter has burned off, and 

 the result is a black centre to the brick, which may also- be accom- 

 panied by a swelling of the clay. In calcareous clays the carbonate 

 of lime present also loses its carbon dioxide. The driving off of 

 all these substances will, therefore, tend to' make the brick very 

 porous. Further heating, however, after the volatilization of these 

 substances, causes a drawing together of the clay particles, or 

 shrinkage, and this is accompanied by an increase in density and 

 hardness, the maximum density and shrinkage being reached when 

 the brick is vitrified. 



These effects of heating a clay can be summarized as follows : 



1. Loss of volatile substances present, such as water, carbon 

 dioxide and sulphur trioxide, the volatilization of these leaving 

 the clay more or less porous. 



2. A shrinkage of the mass, by further heating. 



3. Hardening of the clay due to fusion, of some at least, of the 

 particles. 



