Manufacturing of Bricks
Four distinct operations are involved:
- Preparation of clay
1. Preparation of clay
Clay of bricks is prepared in the following order:
(i) Unsoiling: The top layer of soil, about 20 cm in depth, is taken out and thrown away.
(ii) Digging: Clay is then dug out from the ground. It is spread on the levelled ground. The height of heaps of clay is about 60 cm to 120 cm.
(iii) Cleaning: Soil should be cleaned of stones, pebbles, vegetable matter, etc.
(iv) Weathering: Clay is then exposed to the atmosphere for softening or mellowing. The period of exposure
varies from a few weeks to a full season.
(v) Blending: Clay is made loose and any ingredient to be added to it is spread out at its top. It is carried out by taking a small portion of clay every time and by turning it up and down in vertical
(vi) Tempering: In the process of tempering, clay is brought to a proper degree of hardness and it is made fit for the next operation of moulding.
Water in the required quantity is added to clay and the whole mass is kneaded or pressed under the feet of men or cattle. Tempering should be done exhaustively to obtain a homogeneous mass of clay of uniform consistency. For manufacturing good bricks on a large scale, tempering is usually done in a Pug mill. The diameter of the pug mill at the bottom is about 80 cm and that at the top is about one metre.
NOTE: The process of grinding clay with water and making it plastic is known as pugging.
The clay which is prepared as above is sent for the next operation of moulding. Two types of moulding are hand moulding and machine moulding.
(i) Hand Moulding
- In hand moulding, the bricks are moulded by hand i.e. manually.
- It is adopted where manpower is cheap and is readily available for the manufacturing process of bricks on a small scale.
- There are two types of moulds (a) wooden mould and (b) steel mould.
- Steel mould is better than wooden mould.
- The bricks shrink during drying and burning. Hence the moulds are to be made larger than the size of fully burnt bricks.
- The moulds are therefore made larger by about 8-12% in all directions.
(a) Ground Moulded Bricks
- The ground is first made level and fine sand is sprinkled over it.
- The mould is dipped in water and placed over the ground.
- The lump of tempered clay is taken and it is dashed in the mould.
- The clay is pressed or forced into the mould in such a way that it fills all the corners of the mould.
- The bricks prepared by dipping mould in water every time are known as slope-moulded bricks.
- Fine sand or ash may be sprinkled on the inside surface of the mould instead of dipping mould in bricks. water is known as sand-moulded bricks.
(b) Table Moulded Bricks
- The process of moulding these bricks is just similar to hand moulded bricks.
- In this case, the moulder (personnel) stands near a table of size about 2 m x 1m.
- The bricks are moulded on the table and sent for the further process of drying.
- The efficiency of the moulder decreases gradually because of standing in the same place for a long duration
- The cost of moulding is more than hand moulding.
(ii) Machine Moulding
- The moulding may also be achieved by machines.
- This process is economical when bricks in huge quantities are to be manufactured at the same spot in a short time.
- Moulding machines are broadly classified into two categories viz. Plastic clay machine and Dry clay machine.
(a) Plastic Clay Machine
- Such machines contain a rectangular opening of size equal to the length and width of a brick.
- The pugged clay is placed in the machine and as it comes out through the opening it is cut into strips by wires fixed in frames.
- As the bricks are cut by wire they are also known as wire-cut bricks.
(b) Dry Clay Machines
- In these machines, the strong clay is first converted into powder form.
- A small quantity of water is then added to form a stiff plastic paste.
- Then paste is placed in a mould and pressed by machine to form hard and well-shaped bricks.
- These bricks are known as pressed bricks and they do not practically require drying.
- They can be sent directly for the process of burning.
NOTE: The wire-cut and pressed bricks have a regular shape, sharp edges and corners. They have smooth external surfaces. They are heavier and stronger than ordinary hand-moulded bricks. They carry distinct frogs and exhibit uniform dense textures.
- Moisture content is brought down to 2% for the burning operation.
- The damp bricks, if burnt, are likely to get cracked and distorted.
- For drying, bricks are laid longitudinally in stacks of width equal to two bricks.
- A stack consists of eight or ten tiers. Bricks are laid along and across the stack in alternate layers. All bricks are placed on the edge.
- The bricks are generally dried by natural process.
- When bricks are to be rapidly dried on a large scale then artificial drying may be adopted.
- In such a case, the moulded bricks are allowed to pass through special dryers which are in the form of tunnels or hot channels or floors.
- The temperature is usually less than 120°C and the process of drying of bricks takes about 1-3 days depending upon the temperature maintained in the drie
- When the temperature of dull red heat of about 650°C is attained, the organic matter contained in the bricks 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 reactions between the mineral constituents of clay are achieved at higher temperatures and these reactions are necessary to give new properties such as strength, hardness and low moisture absorption.
- When the temperature of about 1100°C is reached, the particles of two important constituents of bricks clay, viz. alumina and sand fuse themselves together resulting in an increase in strength and density.
- Heating is not desirable and if the temperature is raised beyond 1100°C, a great amount of fusible glassy mass is formed and the bricks are said to the vitrified.
- The burning of bricks is done either in clamps or in kilns.
- The clamp is a temporary structure and is used for the small-scale production of bricks.
- Kilns are permanent structures and they are adopted to manufacture bricks on large scale.
- Burning imparts hardness and strength to bricks and makes them dense and durable. Bricks should be burnt properly. If bricks are overburnt, they will be brittle and hence, break easily. If they are burnt, they will be soft and hence, cannot carry loads.
- A piece of ground is selected. Its shape in the plan is generally trapezoidal. The floor of the clamp is
- prepared in such a way that the short edge is slightly in the excavation and the wider edge is raised at an angle of about 15° from ground level.
- Fuel may consist of grass, cow dung, litter, husks of rice or ground nuts etc. The thickness of this layer is about 70 cm to 80 cm. Wood or coal dust may also be used as fuel.
- A layer, consisting of 4 or 5 courses of raw bricks, is then put up. Bricks are laid on edges with small spaces between them for the circulation of air.
- The second layer of fuel is then placed and over, it another layer of law bricks is put up. The total height of a clamp is about 3 m to 4 m.
- When the clamp is completely constructed, it is plastered with mud on the sides and top and filled with earth to prevent the escape of heat. The clamp is allowed to burn for a period of about one to two months. It is then allowed to cool for more or less the same period as burning.
Advantages of Clamp Burning:
- The burning and cooling of bricks are gradual in clamps. Hence the bricks produced are tough and strong.
- The burning of bricks by clamp proves to be cheap and economical.
- No skilled labour and supervision is required for the construction and operation of clamps.
- The clamp is not liable to injury from high wind or rain.
- There is a considerable saving of fuel.
Disadvantages of Clamp Burning:
- Bricks are not of regular shape.
- It is a very slow process.
- It is not possible to regulate fire in a clamp.
- The quality of bricks is not uniform.
- A kiln is a large oven which is used to burn bricks. The kilns which are used in the manufacture of bricks
are of the following two types:
(1) Intermittent kilns (2) Continuous kilns
(1) Intermittent kilns:
These may be over ground or underground and they are classified into two types:
(a) Intermittent up-drought kilns
(b) Intermittent down-draught kilns
(a) Intermittent up-drought kilns
- These kilns are in the form of rectangular structures with thick outside walls.
- Flues are provided to carry flames of hot gases through the body of the kiln.
- The top course is finished with flat bricks. Other courses are formed by placing bricks on the edge.
- Strong fire is maintained for a period of 48 to 60 hours.
- (i) The quality of burnt bricks is not uniform. The bricks near the bottom are over burnt and those near the top are burnt.
- (ii) The supply of bricks is not continuous.
- (iii) There is a waste of heat as the kiln is to be cooled down every time after burning.
(b) Intermittent down-draught kilns:
- These kilns are rectangular or circular in shape.
- They are provided with permanent walls and closed tight roofs.
- The working of this kiln is more or less similar to the up-drought kiln.
- But it is so arranged in this kiln that hot gases are carried through vertical flues up to the level of the roof and they are then released.
- These hot gases move downward by the chimney drought and in doing so, they burn the bricks.
- Bricks are evenly burnt.
- The performance of this kiln is better than that of an up-drought kiln.
- There is close control of heat and hence such kilns are useful for burning structural clay tiles.
These kilns are continuous in operation. This means that loading, firing, cooling and unloading are carried out simultaneously in these. There are various types of continuous kilns.
(a) Bull’s Trench kiln (b) Hoffman’s kiln (c) Tunnel kiln
Bull’s Trench Kiln:
Section 1-Loading Section 2-Empty
Section 3-Unloading Section 4-Cooling
Section 5-Burning Section 6-Heating
- This kiln may be of rectangular, circular or oval shape in plan
- It may be fully underground or partly projecting above ground.
- This is the most widely used kiln in India and it gives a continuous supply of bricks.
- Bricks are arranged in sections. They are arranged in such a way that flues are formed. Fuel is placed in flues and it is ignited through flue holes after covering the top surface with earth and ashes to prevent the escape of heat. Flue holes are provided in sufficient numbers on top to insert fuel when burning is in progress.
Hoffman’s Kiln: This kiln is constructed over ground and hence, it is sometimes known as a flame kiln. Its
the shape is circular in plan and it is divided into a number of compartments or chambers.
Since it is provided with a permanent roof, the kiln can even function during the rainy season.
The figure shows the plan and section of Hoffman’s kiln with 12 chambers. Each chamber is provided with the following:
- The main door for loading and unloading bricks.
- Communicating doors which would act as flues in open condition.
- A radial flue connected with a central chimney
- Fuel holes with covers drop fuel, which may be in the form of powdered coal, into the burning chamber.
The main doors are closed with dry bricks and covered with mud when required.