Manufacturing of Bricks
In the process of manufacturing of bricks, the following “Preparation of clay, Moulding, Drying and Burning“distinct operations are involved.
Each of the above operations of the manufacturing bricks will now be studied at length.
Preparation of clay
The clay for brick is prepared in the following order.
Unsoiling: The top layer of the soil, about 200mm in depth, is taken out and thrown away. The clay in top soil is full of impurities and hence it is to be rejected for the purpose of preparing bricks.
Digging: The clay is then dug out from the ground. It is spread on the levelled ground, just a little deeper than the general level. The height of heaps of clay is about 600mm to 1200mm.
Cleaning: The clay as obtained in the process of digging should be cleaned of stones, pebbles, vegetable matters. If these particles are in excess, the clay is to be washed and screened. Such a process naturally will prove to be troublesome and expensive.
Weathering: The clay is then exposed to the atmosphere for softening and mellowing. The period varies from few weeks to full season.
Blending: The clay is made loose and any ingredient to be added to it, is spread out at its top. The blending indicates intimate or harmonious mixing. It is carried out by taking a small amount of clay every time and turning it up and down in a vertical direction. The blending makes clay fit for the next stage of tempering.
Tempering: In the process of tempering, the clay is brought to a proper degree of hardness and it is made fit for the next operation of moulding . Kneaded or pressed under the feet of man or cattle. The tempering should be done exhaustively to obtain the homogeneous mass of clay of uniform character. For manufacturing good bricks on a large scale, tempering is done in a pug mill. A typical pug mill capable of tempering sufficient earth for daily output of about 15000 to20000 bricks.
A pug mill consists of a conical iron tub with cover at its top. It is fixed on a timber base which is made by fixing two wooden planks at a right angle to each other. The bottom of the tub is covered except for the hole to take out pugged earth. The diameter of pug mill at the bottom is about 800mm and that at the top is about 1 m. The provision is made in the top cover to place clay inside pug mill. A vertical shaft with horizontal arms is provided at the centre of the iron tub. The small wedge-shaped knives of steel are fixed at arms. The long arms are fixed at the vertical shaft to attach a pair of bullocks. The ramp is provided to collect the pugged clay. The height of the pug mill is about 2m. Its depth below ground is 600m to800mm lessen the rise of the barrow run and to throw out the tempered clay conveniently. In the beginning, the hole for pugged clay is closed and clay with water is placed in pug mill from the top. When the vertical shaft is rotated by a pair of the bullock, the clay is thoroughly mixed up by the action of horizontal arms and knives and homogeneous mass is formed.
The rotation of the vertical shaft can also be achieved by using steam, diesel or electrical power. When the clay has been sufficiently pugged, the hole at the bottom of the tub is opened out and pugged earth is taken out from the ramp by barrow i.e. a small cart with wheels for next operation of moulding. The pug mill is then kept moving and feeding of clay from the top and taking out of pugged clay from the bottom are done simultaneously. If tempering is properly carried out, the good brick earth can then be rolled without breaking in small threads of 3mm diameter.