The proportion of various ingredients in a good brick earth are as follows: a. SILICA: Brick earth should contain about 50 to 70 % 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. b. 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. c. 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. d. 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 in colour. e. 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