Manufacturing of bricks consists of the following 4 operations or steps.
Preparation of brick clay or brick earth
Moulding of bricks
Air drying of bricks
Burning of bricks
1. Preparation of brick clay or brick earth
In this step the soil is excavated in steps and then laid on leveled ground. Then the soil is cleaned of impurities such as vegetation matter, stones or pebbles etc. After removing impurities it is exposed to weather for few months. This is called the process of weathering. After completion of weathering process the soil is blended with other material to prepare good brick earth. Then the mixed soil is tempered by being thoroughly broken up, watered and kneaded. The tempering is usually done in pug mill.
2. Moulding of Bricks
Bricks are moulded in many ways depending on the quality of the product to be made. Generally the moulding is done in the following two ways
For hand moulding the tempered clay is forced in the mould in such a way that it fills all the corners of the mould. Extra clay is removed either by wooden strike or frame with wire. Mould is then lifted up and raw brick is left on ground.
Machine moulding is is used where large numbers of bricks are to be made. Machines used for moulding is generally of two types.
Plastic clay machines
Dry clay machines
In plastic clay machine the clay in plastic state is forced to rectangular openings of a size equal to the length and breadth of the bricks and are then cut into strips of thickness of the brick with wires in frames.
In dry clay machines, dry clay is reduced to powder, filled dry into mould by the machine and then are subjected to high pressure to form hard and well shaped bricks.
3. Drying of Bricks
Drying is usually done by placing the bricks in sheds with open sides so as to ensure free circulation of air and protection from bad weather and rains. The bricks are allowed to dry till they are left with 5 to 7 percent moisture content. The drying period usually varies from 7 to 14days. The moulded bricks are dried because of the following reasons.
If damp bricks or green bricks are directly taken to burning then, they are likely to be cracked and distorted
To remove maximum moisture from the brick so as to save time and fuel during burning
To increase the strength of raw bricks so that they can be handled and stacked in greater heights in the kiln for burning without damage.
4. Burning of the Bricks
It is the very important step in manufacture of bricks. Bricks may be burnt by two distinct methods given below.
Burning in a clamp or Pazawah known as clamp burning
Burning in a flame kiln or Bhatta known as kiln burning
In clamps, one batch of green bricks is heaped along with firewood, coal etc. and sealed with clay. It is then fired slowly to intense heat which may take many days. Modern kilns, however, permanent structures consisting of many chambers. There are intermittent and continuous kilns. Moulded clay is stacked in the chambers. They are then slowly dried and burned to high temperature and cooled. One cycle of loading, drying, burning, cooling and emptying may take as much as two weeks. These processes are carried out intermittently in intermittent kilns and in cyclic order in continuous kilns.