COMPOSITION OF GOOD BRICK EARTH

Silica ( 50-60 %) :-

• It prevents cracking , shrinking and warping of raw bricks
• It imparts durability and uniform shape to bricks. 
• Excess of silica destroys the cohesion between particles and the brick become brittle
• A large  percentage of sand or uncombined silica in clay is undesirable . However . It is added to decrease shrinkage in burning and to increase the refractoriness of low alumina clays. 

Alumina (20-30%)

• It absorbs water and imparts plasticity to the earth such that it can be moulded 
• It alumina is present in excess , it causes cracks in bricks on drying and becomes too hard when burnt. 
• Clayes having high alumina are found to be very refractory 

Lime (<10%)

• Prevents the shrinkage on drying 
• It causes silica in clay to melt on burning and hence helps to bind it. 
• In carbonates form , lime lower the fusion 
• Red bricks are obtained on burning at considerably high  temperature ( more then 800°c ) and buffburning bricks are formed by increasing the lime content.
• Excess of lime causes the brick to melt and hence brick looses its shape 

Magnesia ( <1% ) 

• A small quantity of magnesia in brick earth imparts yellow tint to the bricks and decreases shrinkage 
• But excess of magnesia leads to the decay of bricks .

Iron oxide (< 7% ) 

• It gives red colour on burning when excess of oxygen is available and dark brown or even black colour when oxygen available is insufficient, but excess of ferric oxide makes the brick dark blue .
• It improves impermeability and durability 
• It lower the fusion point of the clay , especially if present as ferrous oxide 
• It given strength and hardness