Clay Products

Clay Products
Building Construction Material

Introduction
• Clay is naturally occurring mineral that is
found almost every where on the surface of
the earth making the soil cover or the soft
ground.
• It is so widespread in its occurrence that in
common language it is often referred as
earth.

Introduction
• Miner logically pure clay may be composed
of one or more minerals of clay group such
as Kaolinite, Montmorillonite, illite,
vermiculite and Allohane, etc. Kaolite is the
most important mineral component of common
clays.
• Clay occurs universally. Man has used them
since ancient times for making earthware of
great variety.

Classification of Clays
• Clays are classified in two ways: Genesis or
their mode of formation and their dominant
characteristics.
• All clays are classified in two ways:
• Genesis or their mode of formation and their
dominant characteristics
• The Residual Clays
• The Transported Clays

• The Residual Clay includes all varieties of
clays that are found covering the rocks from
which they are formed by natural processes.
Such clays are pure in their chemical
composition that is related broadly to the
parent rock. China Clay is the best example
of residual Clay.

The Transported Clays.
• These are the most common clays spread on the
earth. They are formed by the disintegration and
decomposition of the pre-existing rocks by the
natural agencies followed by removal and
transportation of broken pieces to far off places
where they are finally deposited. Hence any such
deposit of transported clay will have all the particles
transported from many places and different source
materials. The transported clay are, therefore quite
heterogeneous in mineralogical and chemical
composition.

• The transported clays are sometimes
further distinguished into glacial clays,
marine clays, alluvial clays and lacustarine
clay when glaciers, seas, rivers, and lakes
respectively have played dominant roles in
their formation.

• On the Basis of their dominant
Characteristics, Clays are classified as into
four groups:
• China Clay
• Fire Clay
• Vitrifying Clay
• And Brick Clay

China Clay
• It is the purest type of clay containing very
high percentage of mineral Kaolinite or
Kaolin Al2Si2O5(OH)4. It is Considered high
Grade clay and is used for the manufacturing
of crockery and other proclain ware

Fire Clay
• They are also called refractory clay and
contain, besides Kaolinite, Silica and alumina
in very high temperature without suffering any
deformation off shape given to them at the time
of moulding.
• They are mainly used in manufacturing of
refractory bricks that are of great importance
in many metallurgical, mechanical and
chemical operations

Vitrified Clays
• These clays are rich in fluxing compounds like iron oxide
and carbonates of calcium and magnesium. These are
easily fusible. i.e. cannot withstand high temperatures
above 1100 0 C the fluxes get softened and form glass like
material that acts as a strong bonding agent between the
other clay particles. This results in the formation of a smooth
surfaced hard burnt clay products.
• The Vitrifying clays called hard burnt clay are specially
used for manufacturing of facing brick, flooring tiles,
sewer pipe and other similar ornamental and dense clay
products.

The Brick Clays
• These are low grade clays used most for the
manufacturing of building bricks and similar clay
products. Brick clay are rich in silica, alumina,
oxide of iron, calcium, magnesium and organic
matter. It is mutual proportion of these components
that define their suitability or otherwise for making
good quality bricks for building construction.
• The oxides of iron, calcium and magnesium act as
fluxes that fuse easily at brick-making temperature and
bind the alumina and silica particles thoroughly giving
the brick desired properties of cohesion and strength.

Bricks
• Building bricks may be defined as
“Structural units of rectangular shape and
convenient size that are made of suitable
clays by different processes involving
moulding, drying, and burning.

Bricks
• It is now established that even in remote
ancient civilizations bricks were common
material of construction. It is believed that it
was in Egypt that bricks were used some 6000
years ago. Excavation in prehistoric sites in
our country has revealed that bricks were
used abundantly in Indus valley civilization
at Mohan jo daro and Harappa.

Bricks
• Even at present bricks is the most basic and
favorable material for common construction
throughout the world. This may be attributed
to no of factors:

Bricks
Availability of clay
• Clay suitable for making bricks are available
almost universally. Hence brick making can be
adopted anywhere in the world. This is not true
either for stones or for concrete that are material
which compete with clay.

Bricks
Size Shape and handling
• Bricks are made in ready –to-use sizes and
shapes. This affords very convenient
handling and use. For stones, however, some
dressing is absolutely essential. In case of
concrete, an elaborate formwork and shuttering
are basic requirements. Moreover transporting
and lying of concrete also requires great care,
caution and expertise.

Bricks
Cost
• Bricks as also brickwork are cheaper
compared to stone masonry and
construction with concrete in most cases.
This is because of the factors mentioned under
availability. Size and shape and construction
methods

Manufacturing of Bricks
• The process of manufacturing of bricks is
carried out in number of stages It Is
essential a sequential process. That is, next
stage is reached only when the previous stages
have been completed in all respect. No
jumping over or omission of stage is
possible. Each stage has its own significance
in the process.

Manufacturing of Bricks
The Stages are listed below:
• Selection of suitable type of clay
• Preparation and tempering of mud
• Moulding of brick units
• Drying of moulded bricks
• Loading of dried bricks in kilns;
• Firing or burning of dried bricks;
• Cooling of the units;
• Unloading of the kiln;

Selection of Suitable Brick Earth
• A good type of bricks cannot be made from every
type of clay. A suitable brick earth should have the
following composition in the desired proportions:
• 1) Alumina (20- 30 %) All clays are chemically
hydrous aluminum silicates. The alumina content is
responsible for giving the plastic character to the clay
in wet conditions. When alumina content is higher than
30 % the bricks becomes more plastic and also shrink
on drying. But if Alumina Content is Less than 20 %,
clay may be difficult to mould to proper shapes.
Hence desired content of Alumina is about 20 – 30
%.

• Silica ( 50- 60 %)
• Silica present in Ideal Proportion i.e. 50-
60% imparts the qualities of hardness and
strength to the bricks. It is also responsible
for resistance against shrinkage and durability
of the brick to weather. However while the
proportion of silica are exceptionally high in
the clay such bricks when burnt would be
quite brittle and porous.

• Iron Oxide (4-6%)
• This Oxide act as a flux, i.e. it lowers down
Softening temperature of Silica and other
clay components during firing. Further, The
Iron Oxide in the clays may make their
burning difficult and give them yellowish
appearance.

• Lime (4-6 %)
• This Components make the burning of bricks
quicker. Provided,
• It should not be more than 4 %, because in this
case may result in excessive softening of the clay
on heating.
• It must be present only in fine powder form
otherwise it may give rise to slaking, which is
harmful and may cause slow disintegration of
bricks.

• Magnesia
• Which is invariably associated with lime, has
similar effect

Constituents &
Desirable quantity
Significance Effects if Excess quantity Effects if lesser
quantity
Alumina (Al2O3) (20-30
%):
plastic character to the clays in wet
conditions and resistance against shrinkage
on drying
more plastic and shrink more
on drying, which may develop
cracks in on drying.
clay may be difficult
to mould in proper
shape
Silica (SiO2) (50-60 %)
free or combined form
Impart hardness and strength to the brick.
Prevents the shrinkage, cracking and
warping of raw bricks, so imparts uniform
shape to the bricks. Durability - depends
upon proper composition of silica.
not be mouldable easily and
may not burn easily. If burnt,
would be quite brittle and
porous.
-
Iron oxides (CaCO3) (4-
6%)
Acts as flux. Red colour to the burnt
bricks
causes brick too soft during the
burning stage, so shape is lost
cause burning difficult
and also gives
yellowish appearance
Lime (Fe2O3) (4-6%): Makes burning and hardening of the
bricks quicker. It must be present only in
powdered and thoroughly dispersed form. If
lime is present as nodules, it may give rise to
slaking when brick comes in contact with
moisture.
not be more than 5%, because
it may result in excessive
softening of clay on heating
-
Magnesia (1% or less) Similar effect like lime so total percentage
should be consider, provides a yellow tint to
the bricks
- -

Undesirable Components
brick clay should be free from the following
components:
The lime modules:
• obstruct the proper burning, when bricks are
placed in kiln.
• If it present, then burnt bricks are likely to cause
disintegration of bricks by their slaking action.
The organic matter:
• Roots of grasses, leaves and other organic matter,
which produces carbon on burning within the body
of brick.
• Brick darker in appearance and too porous in
nature, remove in preparation stage.

Undesirable Components
Sulphides and sulphates:
• Iron sulphide in form of pyrite and alkalkies in form
of potash and soda are often present in some clays.
• Iron sulphide causes disintegration of the brick during
burning stage itself.
The alkali salts:
• Act as fluxes during burning and create additional
softening.
• If it is present in burnt bricks, it absorbs the moisture
from the atmosphere and form the solution within the
body of brick.
• On evaporation, these solution form white patches. This
effect is called efflorescence.

Field Tests For Brick Earth
• When the manufacture of brick is on large scale,
it is always advisable to go through survey of
deposits of clays which include mapping of area
for considerable depth Analysis of chemical
composition testing the engineering properties of
the specimen made from such earth.
• Such survey assure the total quantity and
quality of the clay.

Test for consistency
• Small sized balls are made from the soils by mixing it
with appropriate quantities of sand and water.
• Balls are allowed to air dry under a shaded place.
• When they are completely dry, each ball is observed for
its shape, size and appearance of any cracks.
• If soil is of suitable type, it will not show any
deformation or crack in them.
• If some shrinkage is there, we may vary the mutual
proportions of soil, sand and water and observed again.
• By varying these proportion, a right proportion is
found for making good quality bricks.
• If negative results are obtained in all the trials, it means
the brick clay is not suitable.

Test For Moulding Properties
• Test is performed on the soil which have passed the consistency
test.
• Soil is thoroughly mixed after adding some more water for
preparing homogeneous mud.
• Thin threads, about 3-4 mm thick are made by rolling between
the palms of two hands from small amount of mud paste.
• Length of such threads indicates good plastic nature of the soil.
• Longer and thinner the threads, the soil is of good plasticity
and threads break quickly on rolling in non-plastic type of
clays.
• Test bricks are made from such paste and allowed to dry. If the
corners, edges and surface shape remain intact even after
drying, then clay is described as satisfactory.

Test to determine deformation on burning
• Test helps in finding out approximate ratio of fluxes in the
clays.
• Test bricks are made from the mud prepared in first two cases.
• This bricks are air-dried.
• Burnt in a potter’s kiln for three to five days and cooled in air
• The clays are of satisfactory quality when bricks show
• Typical red colour
• Have maintained their dimensions at corners and edges
• Have burnt uniformly
• If burnt bricks shows defects like warping of surfaces, twisting
at edges and swelling at places, the brick clay is considered
defective and unsuitable for making good bricks.

Test to determine deformation on
burning

Strength tests:
• If clays passes above all the tests, then only this
test is carried out.
• Field test involves dropping the properly burnt
bricks, one by one, from a height of 2-3 meters
on hard dry ground below.
• If brick is of good quality, then it should easily
withstand this shock without breaking.
• If brick is of poor quality, then it break easily
on falling from such heights.

Preparation Of Mud
Winning:
• Obtain brick earth from its natural deposit.
• Clear off from vegetation, pebbles and other
organic matter.
• Manual digging or mechanical excavation are used
to obtain dry soil or brick matter.
• Clay is spread on even ground for seasoning so
that clay is exposed to atmosphere for good time.
• At this stage, earth is further cleaned off any
pebbles, stones lime nodules and visible organic
matter.
• The seasoned clay is ready for making mud by
mixing with adequate amount of water.

Preparation Of Mud
• Tempering: Converting the brick earth to mud
of proper consistency by thoroughly mixing
with desired quantities of water.
• Manual tempering: Clay is spread on a
platform and thoroughly mixed under feet of
either man or cattle.
• Water is added gradually in small quantities till
desired homogeneity and plasticity are
obtained.
• Pug mill tempering (pugging): mechanical
device called pug mill.

Pug Mill Tempering
• A simple pug mill consist of steel cylinder
covered at top and hole at or near the bottom.
• Most important part of pug mill is a central
vertical shaft provided at the base which can
be rotated with the help of long arm through
animal or motor power.
• Central shaft is attached with the horizontal
blades, each carries some knives.
• Seasoned clay and water are added from an
opening provided at the top.
• The churning effect to the clay-water mixture,
that is converted into mud of desired
plasticity and consistency.
• The mud is then taken out from the hole at the
base and new charge is field.

Moulding Of Bricks
• Moulding: is the process of making green
bricks of proper shape and size from thoroughly
tempered clay.
• Two Main Methods: hand moulding and
machine moulding.

Moulding Of Bricks
• Hand Moulding: Using skilled manpower.
• Most common method in India.
• The quality of tempered clay is kept soft.
• More water content (18-25% by weight). So, we
can call this method as soft mud process.
• Ground moulding: bricks are shaped from such
a soft mud by hand on a specially prepared
ground.
• Table moulding: bricks are shaped from such a
soft mud by hand on a specially designed table.

Tools For Hand Moulding
• A brick mould:
• wood or steel.
• Its inside dimensions are kept slightly bigger
than the desired dimensions of finished brick
because bricks on drying are liable to shrink in size.
• It may be a single unit or a multiple unit type.
• The stock board (moulding board):
• Small wooden board with raised central
projection carrying the identification marks
(frog) of the manufacturer.

The Stock Board
(Moulding Board)

• The Pallets:
• Thin wooden plates used for handling the green bricks
from the moulding boards to the drying field.
• The Strike:
• Wood or metal
• Has its one edge quite thin to slash surplus mud from
the top of the moulded brick, while it is in the mould.
• Sometimes a thin wire strung in a wooden block for
holding is used for the same purpose, it is called cutting
wire.

Wooden mould & Steel mould
Stock board

Ground Moulding
• Common method of
moulding bricks in India
• A stretch of land is first
flattened, levelling and
cleaned and made smooth
by mud plastering.
• Some sand is sprinkled
uniformly over it to make
it non-sticky.

Ground Moulding
(a) For making ordinary bricks:
• Mould is either dipped in water (slop moulding) or
some sand is sprinkled on its inside surface (sand
moulding).
• Mould is placed on the ground at desired spot.
• Lump of mud is dashed into the mould by hand. Mud
should reaches to the sides and corners of the mould.
• Any surplus mud is removed by using the strike or the
cutting wire.
• The mould is then lifted up with a jerk and leaves
behind the moulded brick on the ground below.

Ground Moulding
• Moulding bricks with frog (pallet moulding)
• By using a stock board and pallets.
• The stock board is provided with a raised projection
carrying the identification mark.
• The mould is placed on the stock board (instead of
ground) and a brick is moulded.
• The brick is then taken away using two pallet to the
drying field.

Ground Moulding
• Frog: an identification mark during the
moulding stage with the help of a stock
board. It serves following purposes:
• The name of the manufacture of the brick is easily
found and can be known for the quality of the
brick.
• The frog-faced side is placed upward during
use, which accommodates some extra mortar,
which develops structural grip between upper
and lower brick.

Table Moulding
• The skilled worker – moulder carries out all
the moulding operations on a specially
designed table of suitable dimensions.
• Table is large enough to accommodate all the
materials required in hand moulding such as
stock board, moulds, cutting edge, water,
sand and tempered mud.

Table Moulding
• Process is similar to pallet moulding on ground.
• The moulder is placed on the stock board and then
sprinkles some sand inside the surface of mould.
• Dashes a lump of mud into it, press it thoroughly and
skillfully.
• Cuts away any surplus mud with the strike or the cutting
edge.
• Places a pallet over the mould and turn it over.
• The moulded brick is transferred to the pallet, which is
carried away by a helper.
• This process is repeated for each brick.
• Initial cost is slightly higher than ground moulding but
more efficient and economical in the long run because
production is better in quality and quantity.

Machine Moulding
• Essential process in all mechanized brick
making plants.
• Cheaper in longer run and gives uniform
quality brick.
• Main two methods: Stiff mud method and
Dry process method.
• Stiff mud method: Small quantity of water (8-
12 % by volume) is added to the clay to create
plasticity.
• This stiff mix is made to pass out under
pressure from moulding machine.

Machine Moulding
• Two versions of machines can be used such as:
• Worm gear moulding machine
• A vacuum press for brick moulding.
Worm gear moulding machine: consists of
• A feeding chamber provided with a worm gear to
apply pressure.
• A hopper at the top to receive the clay mix from
the pug mill
• A fixed die provided at the front narrow end
• A conveyor belt on a set of rollers
• Cutting wire device adjusted in front of the die

Worm Gear Moulding Machine
The operation steps on machine consist of
• Feeding the properly mixed stiff mud into the chamber
through the hopper
• Forcing forward the mud charge using the worm gear.
The pressed mud comes out through the die in the form of
continuous rectangular ribbon having the height and
width of the bricks.
This ribbon gets cut into brick lengths by pressing down the
cutting wire device, when the conveyor belt is under the
device.
Capacity of machine: 1000 – 2000 units per hour or more
depending upon the machine.

Machine Moulding
• Dry press process: a very small quantity of water is
added to finely crushed and thoroughly cleaned batch
of clay.
• Mix is almost dry, at best damp.
• Such damp clay is fed through hoppers to special brick
moulds.
• Pressure - 50-150 Kg/cm2 through hydraulic presses.
• This pressure is sufficient enough to convert the loose
damp clay mass into dense and very compact brick unit.
• Brick is removed from the mould.
• These bricks are having perfect shapes on all the sides,
edges and corners.
• Such bricks are also called as pressed bricks.

Drying Of Bricks
• Reasons To Dry The Green
Bricks:
• To Make Green Brick
Strong Enough To Bear
Rough Handling
During Stacking In The
Kiln For Burning.
• To Allow Loss Of
Moisture At A Slow Rate
To Avoid Disintegration.
• To Save Fuel During
Burning Stage.
Drying
Artificial
method
drying in the
chamber
drying in the
tunnel
Natural
method
Air drying
and Sun
drying

Drying Of Bricks
• Natural method (stacking method): there are two
different stages involved in the drying process
such as:
1) Pre stacking stage: Moulded bricks are laid
sidewise and flat wise for 2 – 3 days in the drying
fields. So, that they become hard enough to
handle for stacking in heaps.
2) Stacking stage: Bricks are arranged in the layers
(100 cm wide and 10 brick layer height) by skilled
workers.
• Staking is done in specially prepared drying
ground.
• Enough space is left in between the individual
bricks and between layers.

Natural method (stacking method)

Drying Of Bricks
• Artificial drying: essential in mechanized
brick making unit.
• Independent of weather condition.
Chamber Drying:
• Arrange in stacks in drying chamber by keeping
sufficient spaces for free circulation of hot air
around them.
• Hot air is circulated under control condition of
temperature and humidity.
• 2 – 4 days or more, and then next batch is stacked.

Drying Of Bricks
Tunnel drying:
• Bricks are stacked on mobile cars that are made on
travel on rails within a specially designed drying
tunnel.
• The tunnel is divided into compartments and each
car is made to stay in a particular compartment for
pre-fixed duration.
• The cars come out from other end.
• 2 – 3 days for a car load of bricks to dry to desired
extent.

Burning Of Bricks
• Absolutely essential to develop in brick
making the desired building properties
such as sufficient strength, hardness,
durability and resistance to decay and
disintegration.

Burning Of Bricks
• 3 chemical changes take place during
burning process:
• Dehydration: complete removal of water from
the pores of the bricks.
• Completed at the temperature range of 425 –
7650C.
• Losses all the free water.

Burning Of Bricks
• Oxidation: it start taking place during heating
of the bricks at the above temperature and
gets completed at about 6500C-9000C.
• All the organic matter in the brick earth gets
oxidized. Carbon and sulphur are eliminated as
oxides.
• Fluxes (lime, magnesia, iron) become reactive at
these temperature.
• Brick acquires the red colour due to the oxidation
of iron in the clay.

Burning Of Bricks
• Vitrification: last reaction takes place at
temperature range of 9000C to 11000C.
• The alumina and silica start softening in the
presence of the fluxing compounds.
• The constituent grains get bound firmly.

Burning Of Bricks
• Bricks can be burnt using
the following methods:
(a) Clamp Burning
(b) Kiln Burning

Burning Of Bricks
Clamp Burning:
• (Open kiln or Pazawah)
• Temporary structure, constructed over the ground
• Height - about 4 to 6 m.
• Used for lower scale bricks, not suitable in monsoon
season.
• Trapezoidal in plan, whose shorter edge among the
parallel sides is below the ground and slope angle of
about 15 ⁰
• A vertical brick and mud wall is constructed at the
lower edge to support the stack of the brick.

Burning Of Bricks
• Clamp Burning:
• (Open kiln or Pazawah)
• First layer - fuel is laid as the bottom most
layer with the coal, wood and other locally
available material like cow dung and husk.
• Another layer - 4 to 5 rows of bricks
• Then again a fuel layer is laid over it.
• Thickness of the fuel layer decreases with the
height of the clamp.
• Top surface - covered with the mud so as to
preserve the heat.
• Fire is ignited at the bottom, once fire is started
it is kept under fire by itself for one or two
months and same time period is needed for
the cooling of the bricks.
• In a clamp of 10 m * 7.5 m sides, about one-
lakh bricks can be burnt in two months.

Burning Of Bricks
• Advantages of Clamp Burning:
• Easy to erect and operate
• Any type of fuel can be used
• Not requires skilled labour and least supervision after
burning
• Economical
• Clamp is not liable to injury from high wind or
rain

Burning Of Bricks
• Disadvantages of Clamp Burning:
• Burning of bricks is not uniform, such as bricks at the
bottom are over-burnt while at the top are under-burnt.
• Bricks loose their shape, and reason may be their
descending downward once the fuel layer is burnt.
• This method can not employed for the manufacturing
of large number of bricks and it is costly in terms of fuel
because large amount of heat is wasted.
• It can not be employed in monsoon season.
• Time required for burning is too long.
• No possibility to regulate heat, once it start burning.

Burning Of Bricks (Kiln Burning)
• Kilns are permanent structures, used for burning.
• Fuel - Coal and other locally available materials like
wood, cow dung etc.
• Two types based on their principle of construction:

Burning Of Bricks (Kiln Burning)
Intermittent Kilns: periodic kilns, only one process can take place at one
time.
• The brick supply from such kilns is intermittent and not
continuous.
• e.g. The Allahabad kiln
Continuous Kilns: possible to get supply of bricks almost continuously.
• Used when the bricks are demanded in larger scale and in short
time.
• Operations in these chambers are so controlled that at any given
time, when one chamber is in the loading process, another chamber
may be in burning stage, third chamber in the preheating stage,
fourth chamber in the cooling stage and fifth chamber in the
unloading or supply stage.
• e.g. The Bull’s trench kiln, The Hoffman’s continuous kiln, The
Tunnel kiln.

Burning Of Bricks
(Kiln Burning)
Intermittent Kiln
• Rectangular, Four Permanent Walls.
• Totally Underground Or Partly Underground And Partly Over
Ground.
• The Longer Walls Are Raised Perpendicular To The Direction Of
Prevailing Winds.
• Provided With Number Of Opening Called Flues (F), Exactly Opposite To
Each Other For Charging Fuels And Controlling Air.
• Openings Are Provided With Door Sheets, Which Can Be Raised Or
Lowered.
• The Shorter Walls Are Provided With Doors For Loading And Unloading
The Bricks.
• There Are Four Stages In The Working Of Kiln:
• Loading
• Firing
• Cooling
• Unloading

Burning Of Bricks
(Kiln Burning)
Loading: dried bricks are laid in rows on the
raised portions.
• Each row - two to three brick thick.
• Height - 8 to 10 brick layers.
• Brick laying in such a manner that
make arch-shaped opening for,
a) supply of fuel b) supply of air
• While staking, some space is provided
between individual bricks for circulation
of air around each brick.
• Top layer - covered with thick layer of
dry earth, which helps in preventing
escape of heat from the kiln during the
burning process.

Burning Of Bricks
(Kiln Burning)
Firing: After loading, fuel is supplied in the opening at the
base, is fired through the arch shaped opening provided
for this purpose.
• Low fires: 1 – 2 days for removing moisture left in the
brick during the process of drying.
• Firing is brought to full strength.
• The flames spread upward baking and burning the
bricks stacked within the body of the kiln in 2 – 3 days
time.

Burning Of Bricks
(Kiln Burning)
Cooling: After visual inspection and
ensuring that all the fuel has been burnt,
the kiln openings are closed for 7 – 10
days, which helps in natural slow
cooling of the burnt lot of bricks.
• Slow cooling is essential for
avoiding development of cracks in
the cooling bricks.
• If cooling at fast rate, cracks are
likely to occur and spoil the brick.
Unloading: after sufficient cooling, the
top layer of the dust is removed.
• Burnt bricks are removed from the
top layer to downwards.
• The entire kiln is emptied and
thoroughly clean for loading next
batch.

Burning Of Bricks
(Kiln Burning - The Bull’s Trench Kiln)
• Principle: continuous type of kiln.
• Number of compartments, which can be operated
in sequential process.
• Possible to obtain a regular supply of burnt
bricks.

Burning Of Bricks
• Construction: rectangular, circular or oval shape in the
plan.
• Below the ground level by excavating a trench of the required
width for the given capacity of brick manufacturing.
• Range of dimension of trench depending upon the desired
capacity of the bricks:
– Length: 50 – 78m
– Width: 6 – 8 m
– Depth: 1 – 2 m
• A typical trench kiln has two walls:
– Inner wall - continuous and close contact with the inner land part.
– Outer wall - number of openings or gates provided with dampers or
doors which can be opened or closed.

Burning Of Bricks

Burning Of Bricks
• This Trench is divided generally in 12 chambers so that 2
numbers of cycles of brick burning can take place at the
same time for the larger production of the bricks.
• The structure is under-ground so the heat is conserved to a
large extent so it is more efficient.
• Once fire is started it constantly travels from one chamber
to the other chamber, while other operations like loading,
unloading, cooling, burning and preheating taking place
simultaneously.
• Such kilns are generally constructed to have a manufacturing
capacity of about 20,000 bricks per day.
• Drawback: not a permanent roof, so it is not easy to
manufacture the bricks in the monsoon seasons.

Burning Of Bricks
• Working:
• first prepare to keep all compartments of the
kiln in operation.
• Each compartment has to pass through following
six stages such as loading, unloading, cooling,
burning, preheating and cleaning.

Burning Of Bricks
• Loading:
• stacking of brick is done carefully within the kiln
boundaries
• Enough space is left between any two bricks in a layer
for free circulation of hot gases.
• Fuel galleries are made in the lower regions that are
continuous with the flues in the outside wall.
• Vertical flues (holes) are left connecting the fuel galleries
and horizontal flues. These holes should be left at
approximately every meter during loading.
• Arched openings are left at the end for placing the chimney.

Burning Of Bricks
• The top of the loaded section is thoroughly
covered with 20 – 30 cm deep layer of ash and
dust.
• Metal plates may be placed over them during
the operation to cover the top.

Burning Of Bricks
• Preheating:
• hot and waste gases from a preceding burning chamber
are made to pass to the loaded chamber by raising the
dampers between the two chambers.
• Gases are not very hot.
• To make the bricks completely dry and make them ready for
final burning.
• Burning:
• required volume of air is supplied by regulating the
opening by raising of the gates provided in the outer wall.
• Additional quantities of fuel may be added from the flues
provided at the top.
• 24 – 30 hours for perfect burning.

Burning Of Bricks
• Cooling:
• All the outer gates are closed by lowering the dampers.
• The interdepartmental gates are opened up for leading
the hot gases to the preheating section.
• 3 – 4 days to cool down completely before unloading.
• Unloading:
• The top layer of ash and dust are first removed from the top
of the section.

Burning Of Bricks
• Advantages :
1) Cost of fuel is less as heat of hot gases is fully
utilized for pre-heating of bricks
2) Continuous supply
3) High percentage of first class bricks
• Disadvantages :
1. High initial cost
2. Constant skilled supervision is essential

Burning of bricks
(Kiln Burning - The Hoffman’s continuous kiln)
• Principle: modern and more refined
type of brick kiln.
• For large number of bricks.
• Control condition of temperature.

Burning of bricks
Construction: over ground
• It is sometimes known as flame kiln.
• Circular in plan and it is divided into a 12 number of chambers.
• Permanent roof - can be worked throughout the year.
• The adjacent chambers are inter connected by doors which
can be open or closed by raising or lowering dampers.
• Central chimney is provided, which is connected to all twelve
chamber.
• Each chamber is also provided with separate gate in the outer
wall through which it can be loaded, unloaded and fired.

Burning Of Bricks
(Kiln Burning - The Hoffman’s Continuous Kiln)
• Working: at any time, some chambers can be in
burning, other in the preheating, still others in
cooling and some in the unloading stage.
• The most important condition for the working is
establishment of upward current of air within the kiln,
which can be done by closing all the outer gates except
of the chamber, which is being unloaded.
• Natural air enters the kiln through this gates.

Burning Of Bricks

Burning Of Bricks
• It is made to pass through different chambers by
opening their interconnecting doors.
• The flue at the back of each chamber is kept closed.
• The air will do the job of cooling, burning and pre-
heating.
• The air is then enter the chimney for pre-heating stage.
• A scheme of cyclic operation is prepared in advance for
working.
• There are many possibilities. Based on time requirement
for each stage, we can decide the operation of the kiln.

Burning of bricks
• Advantages :
1. High percentage of first class bricks
2. Regular out-turn of bricks
3. Bricks are evenly burnt
4. Economy in fuel as all the heat of combustion is
utilized
• Disadvantages :
1. High initial cost
2. Constant skilled supervision is essential
3. Economy demands regular consumption of its
output, which may not be possible everywhere.

Burning Of Bricks
(Kiln Burning - The Tunnel Kiln)
• In form of tunnel.
• Continuous type and
highly efficient.
• The tunnel is provided with
rail tracks for cars.
• Raw bricks are placed in
trolleys which are then
moved from one end to the
other end of tunnel.

Burning Of Bricks
(Kiln Burning - The Tunnel Kiln)
• Three sections: pre-heating section, the burning section and
the cooling section.
• The car loaded with raw bricks is moved into the pre-
heating chamber. The bricks are heated by the waste
gases coming from the burning section.
• After few hours stop, the car is moved into the burning
chamber for 20 to 24 hrs.
• Then the car is moved to the cooling chamber.
• 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.

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 Percentage of good quality bricks is small about
60% or so.
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
Fire is under control throughout the process of
burning.
6. Skilled
supervision
Not necessary throughout the process of burning. Continuous skilled supervision 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.
Hot flue gas is used to dry and pre-heat raw
bricks. Hence wastage of heat is the least.

Comparison Between Bull’s Trench And
Hoffmans Kiln
No. Item Bull’s Trench kiln Hoffman’s kiln
1. Continuity Semi-continuous in strict sense
because it has no roof and can not
burn bricks during monsoon.
Perfect, because it has a
permanent roof and burning
operations can be carried out
through out the year.
2. Initial cost Low, because construction is
ordinary type.
High, because construction
requires perfect designing and
high quality material.
3. Operating
cost
Higher, because quantity of fuel
consumed is more and
conservation of heat is less.
Lower, because maximum
conservation of heat is possible
in this type of kiln
4. Quality of
bricks
Ratio of best quality to total bricks
is less.
Ratio of best quality to total
bricks is quite high.
5 Suitability Suitable for low investment
production of bricks
Most suitable for semi
mechanized and mechanized
production

References
• Building Construction : Dr B.C. Punmia
• Civil Engineering Material : Prof. Singh
• Internet Web Sites

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