Bricks are obtained by moulding clay in rectangular blocks of uniform
size and then by drying and burning these blocks. As bricks are of
uniform size, they can be properly arranged, light in weight and hence
bricks replace stones.
Composition – Manufacture Process.
Composition – Following are the constituents of good brick earth.
Alumina: – It is the chief constituent of every kind of clay. A good brick
earth should contain 20 to 30 percent of alumina. This constituent
imparts plasticity to earth so that it can be moulded. If alumina is present
in excess, raw bricks shrink and warp during drying and burning.
Silica-A good brick earth should contain about 50 to 60 percent of silica.
Silica exists in clay either as free or combined form. As free sand, it is
mechanically mixed with clay and in combined form; it exists in
chemical composition with alumina. Presence of silica prevents crackers
shrinking and warping of raw bricks. It thus imparts uniform shape to the
bricks. Durability of bricks depends on the proper proportion of silica in
brick earth. Excess of silica destroys the cohesion between particles and
bricks become brittle.
Lime – A small quantity of lime is desirable in finely powdered state to
prevents shrinkage of raw bricks. Excess of lime causes the brick to melt
and hence, its shape is last due to the splitting of bricks.
Oxide of iron– A small quantity of oxide of Iron to the extent of 5 to 6
percent is desirable in good brick to imparts red colour to bricks. Excess
of oxide of iron makes the bricks dark blue or blackish.
Magnesia– A small quantity of magnesia in brick earth imparts yellow
tint to bricks, and decreases shrinkage. But excess of magnesia decreases
shrink leads to the decay of bricks.
The ingredients like, lime, iron pyrites, alkalies, pebbles, organic
matter should not present in good brick earth
Manufacture of bricks:
The manufacturing of brick, the following operations are involved
- Preparation of clay
(i) Preparation of clay :- The preparation of clay involves
a) Unsoiling :- Top layer of 20cm depth is removed as it contain
b) Digging: – Clay dug out from ground is spread on level
ground about 60cm to 120cm heaps.
c) Cleaning:-Stones, pebbles, vegetable matter etc removed and
converted into powder form.
d) Weathering:- Clay is exposed to atmosphere from few weeks
to full season.
e) Blending:- Clay is made loose and any ingradient to be added
to it is spread out at top and turning it up and down in vertical
f) Tempering:- Clay is brought to a proper degree of hardness,
then water is added to clay and whole mass is kneaded or
pressed under the feet of men or cattle for large scale,
tempering is usually done in pug mill as shown in the fig 2.1
Process:- Clay with water is placed in pug mill from the top. When the
vertical staff is rotated by using electric pair, steam or diesel or turned by
pair of bullocks. Clay is thoroughly mixed up by the actions of
horizontal arms and knives when clay has been sufficiently pugged, hole at the bottom of tub, is opened cut and the pugged earth is taken out from
ramp for the next operation of moulding.
Moulding: Clay, which is prepared form pug mill, is sent for the next
operation of moulding. Following are the two ways of moulding.
Hand Moulding: Moulds are rectangular boxes of wood or steel, which
are open at top and bottom. Steel moulds are more durable and used for
manufacturing bricks on large scale as shown in fig 2.2. Bricks prepared
by hand moulding are of two types.
(a) Ground moulded bricks: ground is first made level and fine sand is
sprinkled over it. Mould is dipped in water and placed over the
ground to fill the clay. Extra clay is removed by wooden or metal
strike after the mould is filled forced mould is then lifted up and raw brick is left on the ground. Mould is then dipped in water every time
lower faces of ground moulded bricks are rough and it is not possible
to place frog on such bricks.
Ground moulded bricks of better quality and with frogs on their
surface are made by using a pair of pallet boards and a wooden block
(b) Table-moulded bricks: Process of moulding these bricks is just
similar to ground bricks on a table of size about 2m x 1m.
(1) Machine moulding: This method proves to be economical when
bricks in huge quantity are to be manufactured at the same spot. It is
also helpful for moulding hard and string clay. These machines are
broadly classified in two categories
(a) Plastic clay machines
(b) Dry clay machines
a) Plastic clay machines: This machine containing rectangular opening
of size equal to length and width of a brick. 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, so there bricks are called wire cut
b) Dry clay machines: In these machines, strong clay is first converted
into powder form and then water is added 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 behavior than ordinary hand moulded bricks. They carry distinct frogs and exhibit uniform
(2) Drying: The damp bricks, if burnt, are likely to be cracked and
distored. Hence moulded bricks are dried before thay are taken for
the next operation of burning. Bricks are laid along and across the
stock in alternate layers. The drying of brick is by the following
(i) Artificial drying – drying by tunnels usually 1200C about 1 to 3
(ii) Circulation of air- Stacks are arranged in such a way that
sufficient air space is left between them free circulation of air.
(iii)Drying yard- special yards should be prepared slightly higher
level prevent the accumulation of rain water
(iv)Period for frying – usually about 3 to 10 days to bricks to
(v) Screens – screens are necessary, may be provided to avoid direct
exposure to wind or sun.
(3) Burning: This is very important operation in the manufacturing of
bricks to impart hardness, strength and makes them dense and
durable. Burning of bricks is done either in clamps or in kilns.
Clamps are temporary structures and they are adopted to manufacture bricks on small scale. Kilns are permanent structures and they are
adopted to manufacture bricks on a large scale
(1) A trapezoidal shape in plan with shorter is slightly in excavation and
wider end raised at an angle of 150 from ground level
(2) A brick wall with mud is constructed on the short end and a layer of
70cm to 80cm thick fuel (grass, cow dung, ground nuts, wood or
coal) laid on the floor
(3) A layer consists of 4 or 5 courses of raw bricks laid on edges with
small spaces between them for circulation of air
(4) A second layer of fuel is then placed, and over it another layer of raw
bricks is putap. The total height of clamp in alternate layers of brick
is about 3 to 4 m
(5) When clamp is completely constructed, it is plastered with mud on
sides andtop and filled with earth to prevent the escape of heat
(6) The period of burning is about one to two months and allow the same
time for coding
(7) Burnt bricks are taken out from the clamp
(i) The bricks produced are tough and strong because burning and
cooling are gradual
(ii) Burning in clamps proves to be cheap and economical
(iii) No skilled labour and supervision are required for the
construction of clamps
(iv) There is considerable saving of clamps fuel
(i) Bricks are not of required shape
(ii) It is very slow process
(iii) It is not possible to regulate fire in a clamp
(iv) Quality of brick is not uniform
Kilns: A kiln is a large oven, which is used to burnt bricks by
1) Intermittent kilns
2) Continuous kilns
1) Intermittent kilns: These intermittent in operation, which means that
they are loaded, fired, cooled and unloaded.
a) Intermittent up-draught kilns
b) Intermittent down-draught kilns
a) Intermittent up-draught kiln: This is in the form of rectangular with
thick outside walls as shown in the fig 2.4. wide doors are provided at
each end for loading and unloading of kilns. A temporary roof may be
installed to protect from rain and it is removed after kiln is fired. Flues
are provided to carry flames or hot gases through the body of kiln.
(i) Raw bricks are laid in row of thickness equal to 2 to 3 bricks and
height 6 to 8 bricks with 2 bricks spacing between rows
(ii) Fuels are filled with brush wood which takes up a free easily
(iii) Loading of kiln with raw bricks with top course is finished with
flat bricks and other courses are formed by placing bricks on
(iv) Each door is built up with dry bricks and are covered with mud or
(v) The kiln is then fired for a period of 48 to 60 hours draught rises
in the upward direction from bottom of kiln and brings about the
burning of bricks.
(vi) Kiln is allowed to cool down and bricks are then token out
(vi) Same procedure is repeated for the next burning
Bricks manufactured by intermittent up drought kilns are better than
those prepared by clamps but bricks burnt by this process is not uniform,
supply of bricks is not continuous and wastage of fuel heat.
(b) Intermittent down-draught kilns:
These kilns are rectangular or circular in shape. They are
provided with permanent walls and closed tight roof. Floor of the kiln
has opening which are connected to a common chimney stack through
flues. Working is same as up-draught kiln. But it is so arranged in this
kiln that hot gases are carried through vertical flues upto the level of roof
and they are then released. These hot gases move down ward by the
chimney draught and in doing so, they burn the bricks.
(i) Bricks are evenly burnt
(ii) Performance of this kiln is better than that of up-draught
(iii) This kiln is suitable for burning of structural clay tiles,
terra cota because of close control of heat.
- Continuous kilns:
These kilns are continuous in operations. This means that
loading, firing, cooling and unloading are carried out simultaneously in these kilns. There are three types of continuous kilns.
a) Bull’s trench kiln
b) Hoffman’s kiln
c) Tunnel kiln
a) Bull’s trench kiln: This kiln may be of rectangular, circular or oval
shape in the plan as shown in fig 2.5. It is constructed in a trench
excavated in ground either fully under ground partially projecting above
ground openings is provided in the outer walls to act as flue holes.
Dampers are in the form of iron plates and they are used to divide the
kilns in suitable sections and most widely used kiln in India.
The bricks are arranged in such a way that flues are formed. Fuel
is placed in flues and it is ignited through flue holes after covering top
surface with earth and ashes to prevent the escape of heat usually two
movable iron chimneys are employed to form draught. These chimneys
are placed in advance of section being fired. Hence, hot gases leaving the
chimney warm up the bricks in next section. Each section requires about
one day to burn. The tentative arrangement for different sections may be
Section 1 – loading
Section 2 – empty
Section 3 – unloading
Section 4 – cooling
Section 5 – Burning
Section 6 – Heating
b) Haffman’s kiln: this kiln is constructed over ground and hence, it is
sometimes known as flame kiln. Its shape is circular to plan and it is
divided into a number of compartments or chambers. A permanent roof
is provided; the kiln can even function during rainy season. Fig 2.6
shows plan and section of Hoffman’s kiln with 12 chambers
Chamber 1 – loading
Chamber 2 to 5 – drying and pre-heating
Chambers 6 and 7 – burning
Chambers 8 to 11 – cooling
Chamber 12 – unloading
The initial cost in stalling this kiln is high, the following advantages
(i) Good quality of bricks are produced
(ii) It is possible to regulate heat inside the chambers through fuel
(iii) Supply of bricks is continuous and regular
(iv) There is considerable saving in fuel due to pre heating of raw
bricks by flue gases
c) Tunnel kiln: This type of kiln is in the form of tunnel, which may be
straight, circular or oval in the plan. Raw bricks are placed in trolleys
which are then moved from one end too the other end of tunnel. Raw
bricks get dried and pre-heated as they approach zone of fire. In zone of
fire, bricks are burnt to the required deque and they are then pushed
forward for cooling. When bricks are sufficiently cooled, they are
unloaded. The kiln proves to be economical when the bricks are
manufactures on a large scale. As temperature is under control, uniform
bricks of better quality are produced.
Bricks can broadly be divided into two categories.
(i) Unburnt or sundried bricks
(ii) Burnt bricks
(i) Unburnt or Sun dried bricks- UN burn or sun dried with the
help of heat received from sun after the process of moulding.
These bricks can only be used in the constructions of temporary
and cheap structures. Such bricks should not be used at places
exposed to heavy rains.
(ii) Burnt Bricks: The bricks used in construction works are burnt
bricks and they are classified into the following four categories.
a. First Class bricks: These bricks are table moulded and of
standard shape. The surface and edges of the bricks are
sharp, square, smooth and straight. The comply all the
qualities of good bricks and used for superior work of
b. Second class bricks: These bricks are ground moulded
and they are burnt in kilns. The surface of bricks is some
what rough and shape is also slightly irregular. These
bricks are commonly used at places where brick work is
to be provided with a coat of plaster.
c. Third class bricks: These bricks are ground moulded and
they burnt in clamps. These bricks are not hard and they
have rough surfaces with irregular and distorted edges.
These bricks give dull sound when struck together. They
are used for unimportant and temporary structures and at
places where rainfall is not heavy.
d. Fourth class bricks: These are over burnt bricks with
irregular shape and dark colour. These bricks are used as
aggregate for concrete in foundation, floors, roads, etc
because of the fact that the over burnt bricks have
compacted structure and hence, they are some times
found stronger than even first class bricks.
Qualities of Good Brick:
(i) Bricks should be table moulded, well burnt in kilns, copper
coloured, free from cracks and with sharp and square edges.
(ii) Bricks should be uniform shape and should be of standard
(iii) Bricks should give clear ringing sound when struck each
(iv) Bricks when broken should show a bright homogeneous and
compact structure free from voids.
(v) Bricks should not absorb water more than 20 percent by
weight for first class bricks and 22 percent by weight for
second class bricks, when soaked in coldwater for a period of
(vi) Bricks should be sufficiently hard no impression, should be
left on brick surface, when it is scratched with finger nail.
(vii) Bricks should be low thermal conductivity and they should be
(viii) Bricks should not break when dropped flat on hard ground
from a height of about one meter.
(ix) Bricks, when soaked in water for 24hours, should not show
deposits of white salts when allowed to dry in shade.
(x) No brick should have crushing strength below 55kg/cm2
2.4 Special Types: Bricks are made in a wide range of shapes and to
suit the requirements of the location where they are to be used.
Special form of bricks may be needed due to structural
consideration or for ornamental decoration as defined by the
architect. Specially moulded bricks avoid the cumbersome
process of cutting and rounding the rectangular bricks to the
desired shape. Some of the special types of bricks commonly
used are given below.
a. Squint Bricks: These bricks are made in a variety of shapes and
are used to the construction of a cute and obtuse squint quoins as
shown in the fig2.7.
b. Bull Nosed Bricks: These bricks are used to form rounded
c. Perforated Bricks: These bricks may be standard size bricks
produced with perforations running through their thickness.
Perforated bricks are easy to burn and their light weight makes it
possible to cut down the weight of the structure and effect in
foundations. The aperture of the perforations is such that it gives
maximum amount of ventilation. But does not permit the entry of
rats or mice. These bricks are used for constructing load bearing
walls of low buildings, panel walls for multistoried buildings and
for providing partition walls.
d. Hallow Bricks: These bricks are made of clay and are provided
with one or more cavities. Hallow bricks are light in weight and
are used to increase insulation against heat and dampness. They
are used for the construction of load bearing walls, partition walls
or panel walls to multistoried buildings.
e. Circular Bricks: These bricks have internal and external faces
curved to meet the requirement of the particular curve and radius
of the wall. These bricks are used for wells, towers etc
f. Plinth cornice and String Course Brick: These bricks are
moulded in several patterns with the object of adding
architectural beauty to the structure and at the same time to
helping to throw the rack water off the face of the walls.
g. Coping Bricks: These bricks are manufactured in a variety of
shapes to set the thickness of the wall and are throated on the
underside to throw off rain water as shown in the fig2.7
h. Paving Bricks: These bricks are specially made for paving the
surface of streets and highways. These bricks are usually made
from shale, fire clay on a mixture of the two. They are unaffected
by weather and ordinary traffic wear. They are loaded on the bed
of sand which in term rests on foundation of stone or concrete.
The bricks are laid by grouting with cement mortar or asphalt.
They are machine moulded and are burnt in a continuous kiln to
ensure high degree of vitrification.
Tests for bricks :
A brick is generally subjected to following tests to find out its
suitability of the construction work.
iii. Crushing strength or compression strength
v. Presence soluble salts
vi. Shape and size
1) Absorption: A good should not absorb not more than 20 percent
of weight of dry brick
2) Compressive strength: crushing or compressive strength of
brick is found out by placing it in compression testing machine. It
is pressed till it breaks. Minimum crushing strength of brick is 35kg/cm2 and for superior bricks, it may vary from 70 to 140
3) Hardness: No impression is left on the surface the brick is
treated to be sufficiently hard
4) Presence of soluble salts: The bricks should not show any grey
or white deposits after immerted in water for 24 hours
5) Shape and size: It should be standard size and shape with sharp
6) Soundness: The brick should give clear ringing sound struck
7) Structure: The structure should be homogeneous, compact and
free from any defects
Grading of Bricks
As per IS 10719557 and 1970 code specifications,
a. Bricks with compressive strength not less than 140kg/cm2 –
Grade A-A class.
b. Bricks with compressive strength not less than 105kg/cm2 – First
class bricks – Grade A.
c. Bricks with compressive strength not less than 70kg/cm2 –
Second class bricks – Grade B.
d. Bricks with compressive strength not less than the average value
35kg/cm2 – class III bricks – Grade C.