Manufacturing process of bricks: Preparation, Moulding, Drying and Burning

Manufacturing process of bricks

In the process of manufacturing bricks, the following four distinct operations are involved:

(1) Preparation of clay

(2) Moulding

(3) Drying

(4) Burning. 

(1) Preparation of clay - 

The clay for bricks is prepared in the following order:

(¡) Unsoiling

(ii) Digging

(iii) Cleaning

(iv) Weathering

(v) Blending

(vi) Tempering.

(i) Unsoiling -

The top layer of soil, about 200 mm 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.

(ii) 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 of ground. The height of heaps of clay is about 600 mm to 1200 mm.

(iii) Cleaning - 

The clay, as obtained in the process of digging, should be cleaned of stones, pebbles, vegetable matter, etc. 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. The lumps of clay should be converted into powder form in the earth crushing roller.

(iv) Weathering - 

The clay is then exposed to atmosphere for softening or mellowing. The period of exposure varies from few weeks to full season. For a large project, the clay is dug out just before the monsoon and it is allowed to weather throughout the monsoon. This imparts plasticity and strength to the soil.

(v) Blending - 

To increase the quality of soil, additional soil such as sandy or calcareous clays may be added in suitable proportions with a small quantity of coal, ash, etc. The whole mass is mixed uniformly, and water is added. The blending makes clay fit for the next stage of tempering.

(vi) Tempering -

It is the process of kneading the soil under the feet of men or cattle after adding the necessary quantity of water in order to make the soil stiff and homogeneous. In general, a soft plastic clay could be prepared by using about 25 to 30% of water. This procedure is adopted for the majority of common handmade bricks. For making superior bricks on a large scale, the earth is tempered in a pug mill.

For manufacturing good bricks on a large scale, the tempering is usually done in a pug mill. A typical pug mill capable of tempering sufficient earth for a daily output of about 15000 to 20000 bricks. The process of grinding(पिसाई) clay with water and making it plastic is known as the pugging.

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 right angles to each other. The bottom of tub is covered except for the hole to take out pugged earth. The diameter of pug mill at bottom is about 800 mm and that at top is about one meter. A long vertical shaft is placed at the center. The central shaft is provided with a number of horizontal arms attached with cutting knives for breaking clay lumps, if any. A long horizontal arm is fitted at the top of the vertical shaft. The shaft is rotated with the help of bullocks or sometimes by electric power. Clay and water in the ratio of 1:3/4 are fed into the vessel from the top. The tempered clay is collected from the hole provided at the bottom of the vessel. 

(2) Moulding - 

Moulding is a process of giving a required shape to the brick from the prepared brick earth. Bricks are made in traditional size (in inch) and in metric size (cm) as prescribed by the Bureau of Indian Standards. Metric size bricks are called modular bricks. Nominal size of bricks is the size including thickness of the mortar. Hence, the actual size of modular bricks is 19 cm × 9 cm × 9 cm, and the nominal size of modular bricks is 20 cm × 10 cm × 10 cm.

A brick mould is a rectangular box of steel or wood. Both ends, the top and bottom, of the box are open. The inside dimensions of the mould are 20 cm × 10 cm × 10 cm. The bricks shrink during drying and burning. Hence the moulds are to be larger than the size of fully burnt bricks. The moulds are therefore made longer by about 8 to 12 percent in all directions. Moulding of bricks is carried out either by hand or by machine.

1. Hand Moulding - 

• In hand moulding, bricks are moulded manually. This is preferred in areas where the manpower is cheap and available readily and where only a small quantity of bricks is needed. The moulds are rectangular boxs which are open at top and bottom. They may be of wood or steel. 

• Hand moulding are of two types:

(a) Ground-moulding

(b) Table-moulding

(a) Ground-moulding - 

The process of moulding bricks on the ground by manual labour is called ground moulding. 

At first a level ground is prepared, and a thin layer of fine sand is spread(बिखेरना) over the ground. The mould is wetted and placed on the ground firmly. The tempered brick earth is dashed into the mould. The earth is pressed in the mould such that the earth fills all the corners of the mould without leaving any air gap within the brick. The excess earth is removed using a wooden or metal strike dipped in water. The mould is then lifted, and the raw brick is left on the ground. The mould is cleaned, dipped in water and placed near the previous brick; the process is repeated till the ground is completely covered with the prepared raw bricks. On an average, a moulder can mould about 750 bricks per day. When the bricks have sufficiently dried, they are taken to the drying shed and placed in an ordered manner.

Brick prepared by dipping moulds in water every time called slop-moulded brick, and if sand is sprinkled(छिड़का) on the sides of the mould, the brick is called a sand-moulded brick.

(b) Table-moulding - 

Table moulding is done on a table of size 2 m × 1 m instead of on the ground. The process of moulding is almost similar to ground moulding except for a few changes. Invariably, table-moulded bricks are provided with a frog. A frog is a mark of depth of about 10 to 20 mm provided in a mould. This serves two purposes, viz., it provides a key for the mortar when the next brick is placed with its flat surface over this and to place the trademark of the manufacturer.

A stock board of the same site as the inside dimensions of the mould with a projection for the frog with the trademark of the manufacturer is kept on the moulding table, and the moulder stands behind the table.

The mould is placed to fit the stock board and the tampered earth is dashed against the mould, carefully filled, and excess earth is removed. Then a thin board called the pallet board is placed on the mould; the mould and pallet board are lifted together followed by lifting the mould leaving the brick on the pallet board. Another pallet board is kept on the brick and carried to the drying yard where it is placed on its edge and the pallet boards are removed. This procedure is repeated.

2. Machine Moulding -

Machine moulding may also be achieved by machines. Moulding machines are used when a large number of bricks are to be manufactured within a short time. Machine-moulded bricks are heavier and stronger than the hand-moulded ones. These bricks have a sharp regular shape and size, a smoother surface and sharp edges.

These machines are broadly classified in two categories:

(a) Plastic clay machines

(b) Dry clay machines

(a) Plastic clay machines : 

Such machines contain a rectangular opening of size equal to 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. The arrangement is made in such a way that strips of thickness equal to that of the brick are obtained. As the bricks are cut by wire, they are also known as the 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 added to the powder to form a stiff plastic paste. Such paste is placed in mould and pressed by machine to form hard and well-shaped bricks. These bricks are known as the pressed bricks, and they do not practically require drying. They can be sent directly for the process of burning.

3. Drying of Bricks - 

Moulded bricks cannot be burnt directly as they may get cracked or distorted. Hence, before burning they are dried. Natural drying or artificial drying may be resorted to. The bricks are left to dry for about two weeks. 

1. Natural Drying -

Moulded bricks contain about 7–30% moisture depending upon the method of manufacture. The object of drying is to remove the moisture to control the shrinkage and save fuel and time during burning. Bricks are normally dried in natural open-air driers. They are stacked on raised ground and are protected from bad weather and direct sunlight.

It is also called hack drying. It comprises placing moulded bricks in rows on their edges on a slightly raised ground called a hack. A small space is given between bricks for the circulation of air. The air- and sun-dried bricks are strong enough and can be used for the construction of small structures.

2. Artificial Drying -

When bricks are needed continuously and to a large scale, artificial drying is resorted to. The bricks are dried in special dryers which receive heat from special furnaces that are made especially for this purpose. Hot flue gases from the chambers of a kiln and waste steam from engines may also be used for the artificial drying of bricks.

(4) Burning -

This is a very important operation in the manufacture of bricks. It imparts hardness and strength to the bricks and makes them dense and durable. The bricks should be burnt properly. If bricks are over-burnt, they will be brittle and hence break easily. If they are under-burnt, they will be soft and hence cannot carry loads.

The burning of clay may be divided into three main stages:

• Dehydration(400-650°C) -

Heating brick earth up to about 640°C produces only physical changes. This is also known as water smoking stage. During dehydration, the water which has been retained in the pores of the clay after drying is removed and some of the carbonaceous matter is burnt.

• Oxidation period(650-900°C) - 

When brick earth is heated up to 700–1,000°C, it undergoes chemical changes. During chemical action alumina and silica in brick earth fuse together resulting in a compound which is strong and stable. After this chemical transformation, it does not turn back to break earth on cooling. During the oxidation period, the ferrous iron is oxidized to the ferric form. 

• Vitrification - 

 To get a good quality brick it has to be heated to the required temperature.

To convert the mass into glasslike substance — the temperature ranges from 900–1100°C for low melting clay and 1000–1250°C for high melting clay. On heating the brick earth beyond 1,300°C, the above materials get vitrified.

Burning of bricks is done in a clamp or kiln. A clamp is a temporary structure whereas kiln is a permanent one.

Burning in Clamp or Open Kiln or Pazawah - 

Clamp or open kiln is a temporary structure where dried bricks are stacked in alternate layers of fuel, i.e., both bricks and fuels are placed in alternate layers. Locally available materials such as grass, rice, husk, wooden chippings, cheap quality woods and dried cow dung are used as fuel. The amount of fuel is reduced successively in the top layers. Some space is left between bricks for free circulation of hot gasses. The top and sides of the clamp are plastered with mud. About 20,000 to 1,00,000 bricks are available after burning and cooling. It takes around 3–6 months to complete the burning and cooling of the bricks in the clamp.

The main advantages of this method are that its initial cost is low, fuel cost is low and there is no need of a permanent structure and skilled labourers. The regulation and circulation of heat are not possible, and hence only 60% of good-quality bricks can be expected. Further, only a small quantity of bricks can be manufactured at a time.

Burning in Kiln -

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 - 

When a large quantity of good-quality bricks are needed, intermittent kilns have to be used.

In intermittent kilns, the process of burning is discontinuous which means that they are loaded, fired, cooled and unloaded. Such kilns may be either rectangular or circular in plan. They may be overground or underground.

These kilns are in the form of rectangular structures, with thick outside walls. The wide doors are provided at each end for loading and unloading of kilns. The flues are channels or passages which are provided to carry flames or hot gases through the body of kiln. Dampers are provided in the flue openings to regulate the air supply. 

Circulation of flues is kept up for 3–4 days. The bricks are cooled for a week’s time. A temporary roof may be installed of any light material. Such roof gives protection to the raw bricks from rain while they are being placed in position. This roof is to be removed when the kiln is fired.

2. Continuous Kilns -

In continuous kilns, the process of burning is continuous. This means that loading, firing, cooling and unloading are carried out simultaneously in these kilns. In a continuous kiln, bricks are stacked in various chambers wherein the

bricks undergo different treatments at the same time. When the bricks in one of the chambers

is fired, the bricks in the next set of chambers are dried and preheated while bricks in the other set of chambers are loaded and in the last are cooled. 

There are three types of continuous kilns:

    (i) Bull’s Trench Kiln

    (ii) Tunnel Kiln

    (iii) Hoffman’s Kiln

(i) Bull’s Trench Kiln -

This type of kiln is usually oval in plan and is constructed in a trench excavated in the ground. It may be fully underground or partly projecting above ground. The depth of trench is about 2 meters. The outer and inner walls are constructed of bricks, and the flue holes are provided in the outer walls. Dampers are provided to conveniently divide the kiln into sections.

Bricks are arranged in sections in such a way that flues are formed. Fuel is placed in the flues, and the top surface is covered with earth and ashes to prevent the escape of heat. The fuel is burnt through the flue holes. Additional flue holes are provided at the top to insert fuel when burning is in progress. Two movable chimneys are used to form a draught. These chimneys are placed before the section is fired. This arrangement makes the hot gases leaving the chimney warm up the bricks in the next section. When the burning is over in a section the flue holes are closed and the bricks are cool down gradually. The fire is advanced to the next section and the chimneys are moved forward.

As loading, burning, cooling and unloading are carried out simultaneously, a continuous supply of bricks is available. This is the mostly used kiln in India.

(ii) Tunnel Kiln -

This kiln is in the form of a tunnel which may be of any shape in plan, viz., straight, circular or oval. The zone of fire is at one place. The moulded bricks are loaded on a trolleys, which are moved from one end of the tunnel to the other end. During this process when they approach the zone of fire that are completely dried and pre-heated.

These bricks are burnt in the zone of fire and are then moved for cooling. After adequate cooling, the bricks are unloaded. As the temperature can be controlled, better quality bricks are produced. The bricks from this kiln are said to be economical.

(iii) Hoffman’s Kiln -

This is constructed over the ground to produce a continuous supply of bricks on a large scale.

This is circular in plan and is provided with a chimney at the centre. Around the chimney are 12 chambers that are in an annular shape. Each chamber comprises of the following parts:

(i) A main door for the loading and unloading of bricks (e.g. D1).

(ii) Communicating doors for the flow of flue gases between the chambers (e.g. A1 and B1).

(iii) A radical flue from each chamber to the chimney (e.g. F1).

(iv) Fuel holes for providing fuel, and powdered coal is used as fuel.

Functions that occur in the chambers are listed below:

Chamber 1 – Loading

Chambers 9–12 – Drying and per-heating

Chambers 7 and 8 – Burning

Chambers 3– 6 – Cooling

Chamber 2 – Unloading

 The working of a kiln is as follows:

(i) Cool air enters through Chambers 1 and 2 as they are open doors.

(ii) It crosses the cooling Chambers 3–6 and enters the burning Sections 7 and 8 in a heated condition.

(iii) It moves to Chambers 9–12 to dry and pre-heat the raw bricks.

(iv) It escapes into the atmosphere through the damper of Chamber 12 and the chimney.

 The flow of air and fuel gas are shown by arrows in each chamber. Although the initial cost is more, Hoffman’s Kiln claims several advantages:

(i) High-quality bricks with uniform burning are obtained with regulation of heat.

(ii) Supply of bricks are continuous in all seasons because the top of the kiln is closed, and the working is not stopped.

(iii) Considerable saving in fuel due to pre-heating of raw bricks by fire gases.

(iv) No air pollution in the locality, as the exhaust gases do not contain black smoke or dust particles.

COMPARISON BETWEEN CLAMP-BURNING AND KILN BURNING -

No.	Item	Clamp-burning	Kiln-burning
1.	Capacity	About 20000 to 100000 bricks can be prepared at a time.	Average 25000 bricks can be prepared per day.
2.	Cost of fuel	Low as grass, cow dung, litter, etc. may be used.	Generally high as coal dust is to be used.
3.	Initial cost	Very low as no structures are to be built.	More as permanent structures are to be constructed.
4.	Quality of bricks	The percentage of good quality bricks is small about 60% or so.	The percentage of good quality bricks is more about 90% or so.
5.	Regulation of fire	It is not possible to control or regulate fire during the process of burning.	The fire is under control throughout the process of burning.
6.	Skilled supervision	Not necessary throughout the process of burning.	The continuous skilled super- vision is necessary.
7.	Structure	Temporary structure.	Permanent structure.
8.	Suitability	Suitable when bricks are to be manufactured on a small scale and when the demand of bricks is not continuous.	Suitable when bricks are to be manufactured on a large scale and when there is continuous demand of bricks.
9.	Time of burning and cooling	It requires about 2 to 6 months for burning and cooling of bricks.	Actual time for burning of one chamber is about 24 hours and only about 12 days are required for cooling of bricks.
10.	Wastage of heat	There is considerable wastage of heat from top and sides and hot flue gas is not properly utilized.	The hot flue gas is used to dry and pre-heat raw bricks. Hence the wastage of heat is the least.