In order to get a good quality brick, the brick earth should contain the following constituents.
- Silica
- Alumina
- Lime
- Iron oxide
- 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.
- 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.
- 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
- 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.