Composition of good Brick 

The ingredients of good brick soil are:

Alumina: A good brick earth should contain 20-30% alumina. It gives plasticity to the soil. An excess of alumina causes shrinkage and warping of the brick during drying and firing, and it becomes very hard when fired.
Silica: Silica makes up 50-60% of good brick earth. It is seen either in free or combined state. In free state, it is mechanically mixed with clay and in combined form it exists in chemical composition with alumina. Cracking, shrinkage and warping of raw bricks are being prevented due to the presence of silica. Durability of bricks depends on silica ratio. An excess of silica destroys the cohesion between the particles and the bricks become brittle.

Lime: A good brick soil should contain lime not exceeding 5%. It should be present in a very fine powdery state to prevent cracking of the bricks. Lime prevents shrinkage of bricks. Excess of lime causes the bricks to melt and hence lose their shape.
Iron oxides: A small amount of iron oxide in the range of 5-6% is desirable in good brick earth. It gives red color to the bricks. But excess of lime turns the color dark blue or black. On the other hand, if the amount of lime is less, the color of the bricks will turn yellow. It also helps in fusing the sand and thus increases the hardness of the bricks.
Magnesia: The presence of small amounts of magnesia imparts yellow color to bricks and reduces shrinkage. But if excessive, it causes rotting of bricks.