Constituents of good brick earth

   In order to get  a  good quality  brick, the  brick earth should contain the  following  constituents. 

1. Silica 
2. Alumina 
3. Lime 
4. Iron oxide 
5. Magnesia 

1. Silica

• Brick  earth should contain about  50 to 60 %  of  silica. 
• It  is  responsible  for  preventing  cracking, shrinking and warping  of  raw  bricks. 
• It  also  affects  the  durability  of  bricks. 
• If  present  in  excess,  then  it  destroys  the  cohesion  between  particles  and  the brick becomes  brittle.

2. Alumina 

• Good brick earth should contain about  20%  to 30%  of  alumina. 
• It is  responsible  for  plasticity  characteristic  of  earth,  which  is  important in  moulding operation. 
• If  present  in excess, then the  raw  brick shrink and  warp during  drying. 

3. Lime

• The percentage of  lime  should be  in the  range  of  5%  to 10%  in a  good brick earth. 
• It  prevents  shrinkage  of  bricks  on drying. 
• It  causes  silica  in clay  to melt  on burning  and thus  helps  to bind it. 
• Excess  of  lime  causes  the  brick to melt  and brick looses  its  shape. 

4. Iron  oxide 

• A good brick earth should contain about  5%  to 7%  of  iron oxide. 
• It  gives  red colour  to the  bricks. 
• It improves  impermeability  and  durability. 
• It  gives  strength and hardness. 
• If  present  in excess, then the  colour  of  brick becomes  dark blue  or  blakish. If  the  quantity  of  iron oxide  is  comparatively  less,  the  brick becomes  yellowish colour 

5. Magnesia

• Good brick earth should contain less  a  small  quantity  of  magnesia  about 1%
• Magnesium in  brick  earth  imparts  yellow  tint to  the  brick. 
• It  is  responsible  for  reducing  shrinkage 
• Excess  of  magnesia leads  to  the decay  of  bricks.