1. Alumina
This is the main constituent of every kind of brick earth. This imparts plasticity to the earth so that it can be moulded. If alumina is present in excess, it leads to shrinking and warping during drying and burning. This can be prevented by the addition of sand.
2. Silica
It exists in brick earth as free sand or in a combined form as silicate of alumina. The presence of silica prevents cracking, shrinking and warping of raw bricks. Thus, it imparts uniform shape to bricks. The addition of silica also increases hardness, durability and resistance to heat. Excess of silica removes the cohesion between particles and makes the brick brittle.
3. Lime
It enables the silica to melt during burning and bind the particles together. Lime should be in the form of powder, otherwise lumps of lime are converted into quick lime after burning. Quicklime slakes and expands in the presence of moisture and results in the splitting of bricks into pieces. Excess of lime causes the brick to melt too readily, and the shape will be lost.
4. Iron Oxide
It acts as a flux and helps the grains of sand to melt and bind the particles of clay together. It is responsible for imparting a red colour to the brick. It improves the durability of the brick. Excess of iron oxide makes the bricks dark blue or blackish, and less quantity of iron oxide makes the brick yellowish in colour.
5. Magnesia
A small quantity of magnesia in brick earth imparts a yellow tint and decreases shrinkage. Excess of magnesia causes decay of bricks.
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