The process of manufacturing of building bricks.

Manufacture of Bricks:

The following distinct operations are required in the process of manufacturing of bricks.

1. Selection of site

2. Preparation of brick clay

3. Moulding of bricks

4. Drying of bricks

5. Burning of bricks

1. Selection of site:

Site should be so selected that soil of suitable properties is available in sufficient quantity. The workers employed on the job should have all the facilities nearby. Fuel in abundance should be available as near the site as possible. The water table during burning session is atleast 1m below the floor of the kiln.

2. Preparation of brick clay: 

IS 2117 suggest that the brick clay should be prepared in two stages.

(a) Weathering: The soil is dug out and exposed to weather for softening or mellowing for a period of about 4 weeks. The soil is turned at least twice during the above period and is kept wet. The big clods under the atmospheric agencies action disintegrate to make a uniform mass. In the process the impurities also get eliminated by oxidation process. 

(b) Tempering: After weathering is completed, water is mixed in required quantity with the soil to obtain plastic consistency for moulding the bricks. If required, other materials including sand could be added at this stage to get soil of proper composition. The mixed soil is then pugged under the feet of men/ or animals. Pug mills both hand or mechanically operated could also be used.

3. Moulding of bricks:  

Bricks can be molded to required shaped and size by any of the two methods:

(a) Hand Moulding: This method is also known as Soft Mud Process. This is called so because the water used is 25% to 30% by weight of clay. The lump of tempered caly is dashed and pressed into wet moulds by hand. Extra clay if any is removed by strike. If wet mould is used the bricks are called slop moulded bricks. Instead of water sand can also be sprinkled before dashing the clay. Such bricks are called sand moulded.

Moulds used are either of wood or preferably of steel. Wooden moulds should be prepared from well seasoned wood. Handles are attached to the sides. Steel moulds are made from 3 to 6mm thick plates. Whatever be the material, the moulds are of an allowance for shrinkage is kept so that the bricks of appropriate size are manufactured.

The hand moulded bricks can further be classified into two types viz, (i) ground moulded (ii) table moulded, depending upon the surface being used for moulding the bricks. In case of ground moulded bricks, a pallet is placed on the top and whole thing is then turned upside down. The mould is taken out. The bricks are made to stand on the longer sides for drying. this also ensures use of less space for quick and better drying. 

In case of table moulded bricks, the process adopted is similar except that the moulder has to keep standing while moulding the bricks, which decreases its output, affecting thereby the cost of bricks.

(b) Machine Moulding: It proves to be economical and less time consuming. This method is also known as stiff mud process and semi mud process.

(i) Stiff mud process: 12 to 18% water is mixed up with the proportioned brick clay mix. The final mixing, kneading and tempering is done in pug mill formed by extruding stiff clay through a mould or orifice in the machine. The stiff clay is cut into bricks with the help of wire and as such are also called wire bricks. The bricks obtained have a smooth finish having rectangular faces and correct size.

(ii) Semi dry process: Only 7 to 10% water is added in this process so as to get damp mix which is pressed under a pressure of 100 to 120N/sqmm in dry caly machines to form bricks. The material is automatically measured off. The pressed moulded brick is then transferred to another machine for receiving the frog. These bricks are called pressed bricks and do not require any drying. These bricks are called pressed bricks and do not require any drying. These bricks too are well shaped with smooth rectangular faces and correct size. They are stronger and more durable than the bricks manufactured from stiff mud process.  

4. Drying of bricks: 

Drying the moulded bricks is necessary to prevent cracking and didtortion and to allow even shrinkage. The bricks are laid longitudinally on edge in stacks of width of 2 bricks and eight to ten tiers. These are stacked across each tier. The bricks are dried to almost bone dry condition and may have a moisture content of 3 to 5%.

Two methods are employed for drying of bricks.

(i) Natural drying: The bricks are positioned and released from the mould, then turned on side with frog on the side and then transported and arranged in rows in hacks with space left in between the bricks for circulation of air. Hack is a yard with its floor raised above the ground and provided with a roof and movable side screens. Generally the period required for bricks to dry is 7-10 days depending upon the weather conditions.

(ii) Artificial drying: If the bricks are required early, artificial drying is resorted to cutting down the drying time to 1 - 3 days. Artificial drying can be done either by hot floor drier or by tunnel drier. The moulded bricks are passed through this drier and are heated either by furnaces or by flues. In case of tunnel driers heat energy fromkilns can be utilized and, therefore, prove to be econimical. Tunnel drier can be either periodic or continuous. In periodic ones, the bricks are filled, dried and taken out in rotation whereas in the continuous ones the bricks are fed at one end and taken out at the other. The tempreature is maintained at less than 120 degree C. The shrinkage that takes place during drying is 5 to 10%.

5. Burning of Bricks: 

Burning is necessary to impart hardness, strength and durability to the bricks. The bricks should neither be over burned to avoid to avoid brittleness nor underburnt to avoid make them soft/katcha.

When the temperature of dull red heat, about 650°C, is attained, the organic matter contained in the brick is oxidized and also the water of crystallization is driven away. But heating of bricks is done beyond this limit for the following purposes:

(i) If bricks are cooled after attaining the temperature of about 650°C, the bricks formed will absorb moisture from the air and get rehydrated.

(ii) The reaction between the mineral constituents of clay are achieved at higher tempreature and these reactions are necessary to give new properties such as strength, hardness, less moisture absorption, etc. to the bricks.

When the temperature of abour 1100°C is reached, the particles of two important constituents of brick clay, namely, alumina and sand, bind themselves together resulting in the increase of strength and density of bricks. Further heating is not desirable and if the tempreature is raised beyond 1100°C, a great amount of fusible glassy mass is formed and the bricks are said to be vitrified. The bricks begin to loose their shape beyond a certain limit of vitrification.

The burning of bricks is done either in clamps or in kilns. The clamps are temperatory structures and they are adopted to manufacture bricks on a small scale to serve a local demand or a specific purpose. The kilns are permmanent structures and they are adopted to manufacture bricks on a large scale.