MANUFACTURE OF BRICKS
The manufacture of bricks is carried out in a number of
stages. It includes the following:
1. Selection and
preparation of clay
2. Shaping and moulding of
units
3. Drying
4. Burning
SELECTION AND PREPARATION OF CLAY
As
a practise, suitable deposits of clay are first located and thoroughly tested
for the quality of brick making. Clay for bricks is prepared in the following
order.
a. Unsoiling: The top layer of the soil is taken out. This is because
the clay in the top layer is full of impurities and, hence, it is to be
rejected for the purpose of preparing bricks.
b. Digging: The clay which is dug out is spread on level ground,
just a little deeper than the general ground level. The height of the heap of
clay is about 60–120 cm.
c. Cleaning: The clay should be made free from stones, pebbles and
vegetable matter. If these particles are in excess, the clay is to be washed
and screened, which is considered to be uneconomical.
d. Weathering: The softening of clay is done by exposing it to the
atmosphere. The period of exposure varies from weeks to full seasons.
e. Blending: The clay is made loose and any ingredient to be added
is spread out at its top. Blending indicates intimate mixing. A small portion
of clay is taken every time for mixing.
f. Tempering: In this stage, the clay is brought to a proper degree
of hardness and it is made fit for moulding. Water in the required quantity is
added and the whole mass is mixed so as to form a mass of uniform character. A
large-scale tempering is usually done in a pug mill. The process of grinding
clay with water and making it plastic is known as pugging.
A
pug mill consists of a conical iron tub with a cover at the top. It is fixed on
a timber base which is made by fixing two wooden planks at right angles. The
diameter of the pug mill at the bottom is about 80 cm and at the top is about 1
m. A vertical shaft with horizontal arms is provided at the centre of the iron
tub. The small wedge-shaped knives of steel are fixed on the horizontal arms.
Openings are provided at the top and bottom for charging clay and water and
removing the mix respectively. The height of the pug mill is about 2 m.
Moulding
Moulding is the process of
making rectangular-shaped brick units from properly tempered clay. The two
types of moulding are
a. Hand moulding
b. Machine moulding.
Hand
moulding
This
is presently the most common method for brick manufacture. This is adopted
where manpower is cheap and readily available. The moulds used for hand
moulding are rectangular boxes made from well-seasoned wood or steel open at
the top and bottom. Hand moulding is of two types:
i. Ground moulding
ii. Table moulding
Ground
moulding
In
this method, the ground is first levelled and fine sand is sprinkled over it. The
mould is dipped in water and placed over the ground. The clay is pressed in the
mould in such a way that it fills all the corners of the mould. Any surplus
earth from the top of the mould is removed using a cutting wire or a metal with
a sharp edge, which has to be dipped in water every time it is used. The mould
is then lifted up and the brick is left on the ground. The mould is dipped in
water and placed close by and another brick is moulded in the same way. If the
mould is dipped in water every time, such preparation of bricks is known as
slope moulded bricks. If fine sand or ash is sprinkled on the inside surface of
the mould instead of dipping the mould in water, such bricks are called sand
moulded bricks.
In
pallet moulding, bricks of higher quality and with frogs are produced. The
frogs are made using a pair of pallet boards and a wooden block. A frog is a
mark of depth about 10–20 mm made on raw bricks during moulding. The frog is
provided for mainly two purposes:
i. It serves as a key
of mortar when other bricks are placed over it.
ii. It indicates the trade
name of the manufacturer.
Table
moulding
The
process of moulding operations are carried out on a specially designed moulding
table. The clay, the mould, water pots, stock board, etc. are placed on this
table. The bricks are moulded similar to the ground moulding on the table. The
cost of brick moulding increases when table moulding is adopted.
Machine
moulding
The
moulding can also be achieved by using machines. It is quite economical when
bricks are produced in huge amounts. The machine moulding is broadly classified
into two categories:
i. Plastic clay machines
ii. Dry clay machines
Plastic
clay machines
The machines contain rectangular
openings 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 openings it is cut into
strips by wires fixed in frames. Hence, it is known as wire cut bricks.
Dry
clay machines
In this machine, the strong clay is first converted to powder form. A small
quantity of water is added to the stiff plastic paste. Such paste is placed in
the mould and pressed by machines to form well-shaped hard bricks. These bricks
are known as pressed bricks.
The
machine moulded bricks have regular shape, sharp edges and corners; they are
heavier and stronger than hand-moulded bricks.
DRYING
The
drying of bricks is necessary, firstly to make them strong enough for rough
handling during subsequent stages and secondly to save fuel during burning. For
drying the bricks are laid longitudinally in stocks of bricks with the width equal
to two bricks. The drying of bricks is achieved by either natural or artificial
methods.
The important facts to be remembered
while drying of bricks is as follows:
a.
The bricks are generally dried by natural process. But when bricks are to be
rapidly dried, artificial drying may be adopted. In artificial drying, bricks
are made to pass through driers in the form of tunnels or hot channels or
floors. The tunnel driers are more economical than hot floor driers.
b.
The brick in stocks should be arranged in such a way that sufficient air space is
left between them for circulation of air.
c.
Special drying yards should be prepared and accumulation of rainwater should be
prevented.
d.
The period of drying depends upon the prevailing weather conditions.
BURNING
Burning
of dried bricks is essential to develop the desired engineering properties,
like hardness, durability and resistance to decay. Three chemical changes are
known to take place in the brick earth during burning, namely dehydration,
oxidation and vitrification.
Dehydration
is completed within 425–750°C temperature range and it results in the expulsion of
most of the water from the bricks.
During
oxidation, carbon and sulphur are eliminated as oxides, whereas the fluxes are
also oxidized.
Oxidation
starts at the range of dehydration temperatures and is completed at about
900°C.
Vitrification
is the extreme reaction and occurs when heating is carried out beyond 900°C.
This is commonly not required in building bricks although in other clay
products like sewer pipes it is necessary.
Burning
of bricks is either done in clamps or kilns. Clamps are temporary structures
while kilns are permanent structures. Clamps are adopted to manufacture bricks
on a small scale while kilns are adopted to manufacture bricks on a large
scale.
CLAMPS
The
shape of the clamp is generally trapezoidal. The brick wall is constructed on
the short end and a layer of fuel is placed on the prepared floor. The fuels
generally used are cow dung, litter, husk of rice, wood, coal, etc. The
thickness of the fuel layer varies from 70 to 80 cm. The layer consisting of 4 or 5
courses of raw brick are then put up. Sufficient space for the circulation of air is
provided. Alternate layers of fuel and bricks are placed over this. The total
height of the clamp is around 3-4 m. When nearly one-third of the height is
reached, the lower portion of the clamp is ignited so as to burn the bricks in the
lower part when the construction of bricks in the upper part is in progress.
After construction is complete, it is completely plastered with mud in order to
prevent the escape of heat. The clamp is allowed to burn for 1 or 2 months and
cooling is also done for 2 months and later the burnt bricks are taken out.
KILNS
The kilns used for the
manufacture of bricks are of two types:
i. Intermittent kilns
ii. Continuous kilns
Intermittent kilns
These
kilns may be underground or overground in the model. They are classified in two
ways:
Intermittent up-draught kilns These kilns are in the form of rectangular structures
with thick outside walls. 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 the kiln. A roof is provided to
protect the raw bricks from rain.
The
quality of the bricks is not uniform; the bricks at the bottom are overburnt
and at the top are under- burnt. The supply of bricks is not continuous and
there is considerable wastage of fuel in the kiln.
Intermittent down-draught kilns These kilns are rectangular or circular in shape. They
are provided with permanent walls and a closed tight roof. The floor of the kiln
has openings that are connected to a common chimney stacked through flues. They
are so arranged that in this kiln the hot gases are carried through the
vertical flues up to the level of the roof and then released. As a result, the bricks
are evenly burnt and the performance is much better than intermittent
up-draught kilns. Here, there is close control of heat and the bricks obtained
are evenly burnt.
Continuous kilns
These
kilns are continuous in operation were loading, firing, cooling and unloading
are simultaneously carried out.
Bull’s trench kiln This is one of the continuous type kilns. These kilns
are rectangular, circular or oval-shaped in the plan. These kilns are constructed
in a trench excavated on the ground. It may be fully underground or partially
projecting above the ground. The outer and inner walls are to be constructed in
bricks. Openings are provided on the outer walls to act as flue holes. Iron
plates are used to divide the kiln into suitable sections. The fuel is placed
in flues and is ignited after covering the top surface with earth to prevent the
escape of heat. Usually, two movable iron chimneys are employed to form
draught. The chimneys are placed in advance of the firing sections so that the
warm gases leaving the chimney warm up the bricks in the next section. As the
section has burnt, the flue holes are closed and allowed to cool down. Later the
fire is advanced to the next section.
Hoffmann kiln This kiln is constructed underground is circular in
plan and consists of a number of chambers. A permanent roof is provided so that
the kiln can function even in the rainy season. The chamber in Hoffmann’s kiln
is provided with the main door for loading and unloading bricks. Communicating
doors should act as flues in the open condition. A radial flue connected with a
central chimney and fuel holes are also provided. The advantages are that the
bricks are uniformly, equally and evenly burnt and that there is no air
pollution in the locality. Also, there is a saving in fuel and a high percentage
of good bricks are produced.
Tunnel kiln This type of
kiln is in the form of a tunnel, which is oval, circular or straight in the plan.
It contains a stationary source of fire. The raw bricks are placed in trolleys
which are then moved from one end to another end of the tunnel. The raw bricks
get dried and preheated as they approach the zone of fire and in the zone of fire, the bricks are burnt and pushed forward for cooling. Later, after cooling, they
are unloaded.
shows the comparison between clamp and
kiln burning.
|
ITEM |
CLAMP
BURNING |
KILN
BURNING |
|
Structure |
Temporary |
Permanent |
|
|
|
|
|
Initial
cost |
Very
low as no structures are to be built |
Very
high as permanent structures are to be built |
|
Cost
of fuel |
Low
as grass, cow dung etc. is being used |
High
as coal dust is being used |
|
Quality
of bricks |
Percentage
of good bricks is less, around 60% |
Percentage
of good bricks is more, around 90% |
|
Supervision |
Not
necessary throughout the process |
Skilled
supervision required |
|
Wastage
of heat |
more |
less |
|
capacity |
About
20,000–1,00,000 bricks at a time |
25,000
bricks per day |
|
suitability |
For
small scale |
For
large scale |
|
Time
for burning and cooling |
2-6
months |
24
hours for burning and 12 days for cooling |
 |
| Manufacturing Process Of Bricks |
Must read: Method
of Laying Brickwork in Masonry Construction
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