Manufacturing of Bricks

MANUFACTURING OF BRICKS

The following operations are involved in the process of manufacturing bricks:

Preparation of Clay

The clay for brick is prepared in the following order:

Unsoiling: The clay in top soil is full of impurities and hence it is to be removed for the purpose of preparing bricks. The top layer of the soil, about 200mm in depth, is to be taken out.

Digging: The clay is then dug out from the ground and is spread on the levelled ground. 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.

Weathering: The clay is then exposed to atmosphere for softening and mellowing. The period varies from few weeks to full season.

Blending: The clay is made loose and blending is carried out by taking a small amount of clay every time and turning it up and down in 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. The tempering should be done exhaustively to obtain homogeneous mass of clay of uniform character. For manufacturing good bricks on a large scale, tempering is done in pug mill.

Moulding

Moulding is the next process after preparation of clay. Moulding is of two types namely Hand Moulding and Machine Moulding.

(i) Hand moulding:

The bricks are moulded manually using hands in hand moulding. This process is followed for small scale production, where cost of manpower is low. The moulds are of wood or steel rectangular boxes which are open at top and bottom. Small projections are made on the longer side of the mould to act as handles. Sometimes strips of brass or steel are fixed on the edges of wooden moulds to make them more durable. Generally steel moulds are preferred over wooden moulds since they are more durable and bricks manufactured will be of uniform size. The bricks shrink during drying and burning. Therefore the moulds are made larger than burnt bricks upto 8-12%. The bricks prepared by hand moulding are of two types namely

❖ Ground moulded and

❖Table moulded

Ground moulded bricks: In this process, the ground is first made level and fine sand is sprinkled over the ground. The mould is immersed in water before placing over the ground. A stack of tempered clay is taken and is placed in the mould. Then the clay is pressed in the mould in such a way that it fills all the corners of mould. Using a wooden strike or a frame with wire, the surplus clay is removed. Wooden strike or a frame with wire thus used is to be dipped in water every time. Finally the mould is then lifted up and raw brick formed is left on the ground for drying. Again the mould is dipped in water and the process is repeated to prepare another brick. The process is replicated till the required number of raw bricks is made. Even though the ground is made level, the lower faces of ground moulded bricks are rough. Hence it is not possible to place frog on such bricks. A frog is a mark of depth about 10mm to 20mm which is placed on raw brick during moulding, since.

❖ It indicates the trade name of the manufacturer

❖ In brick work, the bricks are laid with frog uppermost. Thus it affords a key for mortar when the next brick is placed over it.

The ground moulded bricks of good finishing and with frogs on their levelled surface are prepared by using a pair of pallet boards and a wooden block.

Table Moulded Bricks: In this process, a table is used for moulding of bricks instead of ground. The process remains the same. Raw bricks are moulded on a table of size 2m x 1m and sent for further process of drying. Since bricks are table moulded, the end product will be of better quality with levelled surface and finishing. Challenges faced in table moulding are

❖ The efficiency of the moulder decreases gradually due to standing for a longer duration.

❖ The cost of brick is also more when compared to ground moulding.

(ii) Machine Moulding:

In this process, the bricks are machine moulded and is carried out by two methods namely:

❖ Plastic clay machine moulding

❖ Dry clay machine moulding

Plastic Clay Moulding: This machine consists of a rectangular opening with length and width equal to an ordinary brick. The processed clay is placed in the machine and it comes out through the rectangular opening. Then it will be cut into strips by the wire fixed at the frame. The wire frame is placed in such a way that the strips thickness is equal to standard size of the bricks. The bricks thus manufactures are also called as wire cut bricks.

Dry Clay Machine Moulding: In dry clay machines, the processed clay is dried and converted in to powered form. Then a small quantity of water is added to form a stiff plastic paste. Such paste is placed in mould and pressed by machine to form dry and well-shaped bricks. They do not require the process of drying.

Drying

After moulding the damp bricks are likely to be cracked and distorted while burning. Hence the moulded raw bricks are dried before they are taken for the next process of burning. For the drying process the bricks are laid longitudinally in batches of width equal to two bricks. A batch consists of eight to ten tiers. The bricks are laid along and across the batch in alternate layers and are placed on edges. The bricks are allowed to dry until it becomes leather hard of moisture content about 2%.

Burning

This is the final process in the manufacturing of bricks. Dried bricks are burnt at high temperature to gain the strength, durability and appearance. All the water present is removed at 6500C but they are further burnt till 11000C, since fusing of sand and lime takes place at this temperature. Chemical bonding takes place after the temperature is cooled down resulting in the hard and dense mass. Also bricks are not burnt above this temperature because over burning results in the melting of the bricks leading to distorted shape and a very hard mass when cooled which will not be workable while brickwork. Bricks can be burnt using the following methods:

❖ Clamp Burning

❖ Kiln Burning

Clamp Burning: Clamp is a temporary structure generally constructed over the ground with a height of about 4 to 6 m. It is employed when the demand of the bricks is less and when it is not a monsoon season. This is generally trapezoidal in plan whose shorter edge among the parallel sides is below the ground and then the surface rising constantly at about 15 degrees to reach the other parallel edge over the ground. First layer of fuel for burning is laid as the bottom most layer with the coal, wood and other locally available materials. Another layer of about 4 to 5 rows of bricks is laid and then again a second fuel layer is laid over it. The thickness of the fuel layer goes on with the height of the clamp. After these alternate layers of the bricks and fuel, the top surface is covered with the mud so as to preserve the heat. Fire is ignited at the bottom, once fire is started it is kept under fire by itself for one or two months and same time period is needed for the cooling of the bricks.

Kiln Burning: Kiln is a large oven used for the burning of bricks. Generally coal, wood and other locally available materials like cow dung, husk can be used as fuel. They are of two types:

❖ Intermittent Kilns.

❖ Continuous Kilns.

Intermittent Kilns: These are like the cyclic kind of kilns, because in these kilns only one process can take place at one time. Various processes which take place in the kilns are loading, burning, cooling and unloading of bricks. There are two kind of intermittent kilns namely

❖ Up-draught Intermittent Kilns

❖ Down draught Intermittent Kilns

Continuous Kilns: These kilns are called continuous because all the processes take place simultaneously. They are used when the bricks demand is more in a short period of time. It is a fast method of burning and bricks burning are completed in one day itself. There are two well-known continuous kilns namely

❖ Bull's Trench Kiln

❖ Hoffman's Kiln