History of Brick Masonry
1) Development Of Brick.
2) Steps In The Manufacturing Process.
3) Brick Today
4) Properties Of Brick
5) Characteristics And Shapes
1. Development Of Brick
Brick is one of the oldest manufactured building materials.
Ancient records and excavations show that brick construction dates
back more than 5000 years. Brick making was a highly respected skill
in early civilizations. Many old masonry walls bore evidence of
royal support for the craftsman who made the bricks. Royal seals and
the names of kings were stamped in some of the bricks.
One of the earliest types of brick was adobe brick. These
brick contained straw for greater strength, just as reinforcing wire
is used in concrete to give it strength in modern construction.
After the adobe brick were molded into shape, they were placed in
the sun to dry. Adobe brick are still used in a few countries today.
After the sundried adobe brick had been in use for some time, it
was discovered that a brick subjected to fire in a closed area such
as a kiln, or oven, for a definite period of time became very hard
and highly fire resistant. The fired brick resisted weather and the
passage of time far better than unfired bricks. Some of the brick
were coated with a thick enamel or glaze. The glazes were commonly
red, yellow, green or a combination of these colors. When subjected
to heat in the kiln, the color hardened and possessed a glass-like
finish. Some of these glazed brick, recovered from old buildings,
still retain their original color after 2000 years. Glazed brick are
made today but have limited use since they are costly to
In the past, the mason usually made the brick he was later to lay
in the wall. Many of the brick made in ancient times had a recessed
panel known as a frog. When mortar was forced into the frog, the
brick had a stronger bond in the wall being constructed. The
craftsman who made the brick also used the frog as a place to mold
letters or inscriptions to identify himself or the job on which the
brick was to be used.
The making of brick was regarded by many of the old world
craftsmen as a secret process. Brick masons were one of the first
groups of craftsmen to form an organization to keep these processes
secret and confined to their own group. These specialized
organizations were called guilds and were the forerunners of modern
In 1666 a great fire changed London, England from a city of
wooden buildings to a city of brick construction. The manufacture of
brick attained a high degree of excellence and dominated the
building field in this period of history.
Early records indicate that the first brick manufactured in the
United States were made in Virginia in 1611 and in Massachusetts in
1629. The brick were made by hand using very simple methods and
tools. Many of the brick used in construction in the early American
settlements were brought from England as ballast in sailing ships.
Some of these brick can still be found in the foundations and walls
of the remaining original houses in the eastern part of the United
The invention of the steam engine in England in the year 1760,
and the subsequent Industrial Revolution, brought a change from
manual labor to the use of power-driven machinery to make brick.
This change started the true development of the brick in dustry in
America. The first brick-making machine was patented in 1800.
Technological developments during the last century have helped to
make the manufacture of brick a very efficient and highly productive
process. More complete knowledge of the characteristics of the raw
material, improved kiln designs, controlled heat in the kilns, and
extensive mechanization have all played an important part in
modernizing brick manufacturing.
The modern brick plant meets the challenge of increasing
production, while retaining a high quality for the final product, by
using computerized manufacturing methods and highly skilled workers.
Basically, brick are made by mixing water with finely ground clay
or shale or a combination of both. The mixture is then formed into
the desired shape, predried, and burned in a kiln for a
While the basic steps of brick manufacturing are standard
throughout the industry, each brick-making plant has minor
variations to these steps due to local conditions. For example, one
brick plant may be near the source of the raw material, while
another plant may have to truck or rail the material in from a
distant source. These two plants will have different ways of
obtaining and stocking their raw materials.
Manufacturing process diagram
2. Steps In The Manufacturing Process
The manufacturing process has eight maior steps taking the raw
material from the ground, or winning; preparing the raw
material for use; forming the material into brick units; predrying
the units; burning the brick in the kiln under controlled heat; and
drawing and storing the brick. All other operations of making brick
stem from these eight major steps.
2.1. Taking the Material from the Ground. The
removal of the raw material from the ground is called winning. Power
equipment is used to mine surface clay and shale in open pits.
Trucks or railways bring the material to the storage piles
Enough raw material is stored to assure plant operations for
several days in the event that bad weather halts mining and/or
shipping operations. Several storage areas are provided so that the
clay and shale can be blended to yield material with a better
composition. Blending produces more uniform raw material, helps
control the color of the finished product, and permits some control
over providing raw material suitable for manufacturing a given type
of brick unit.
2.2. Preparing the Material. If the raw material is
in large lumps, it may be crushed before it is placed on the storage
pile. The crushing breaks up the large pieces and removes the
stones. Following this, 4- to 8-ton grinding wheels revolving in a
circular pan grind and mix the material. It then passes through an
inclined vibrating screen which controls the particle sizes. The
finely ground material is taken by a conveyor belt to the site where
it is formed into single brick.
Raw material is stored to assure operations for
2.3. Forming the Clay or Shale into a Brick Shape.
Three methods of forming are us ed in the production of bricks: the
stiff-mud process, the soft-mud process, and the
Stiff-Mud Process. The most frequently used process at
present is the stiff-mud process. It produces a harder and denser
brick than is obtainable from the other processes. A greater volume
of brick can be manufactured by this method to meet the growing
demands of the construction industry.
The first step in the stiff-mud process is to add water to the
raw material to make a plastic, workable mass suitable for molding.
The mixing is done in a machine called a pug mill. The pug
mill has a mixing chamber which contains one or two revolving shafts
which thoroughly mix the raw material and a measured amount of
The stiff mud process is done in a brick
machine which consists of a pug mill,
a de-airing chamber, and an auger extruder
After the mixing is completed, the pugged clay is forced through
an opening called a die, a process much like toothpaste being
forced from a tube. The long, formed ribbon of brick being extruded
through the die is called the column. As the column moves
away from the die, it is cut into lengths which are either the
height (side cut) or length (end cut) of the brick.
The cutting is done automatically by a large circular wire cutter
which cuts each brick to the same size. These green brick (brick in
their soft condition before they are burned in a kiln) are then
placed on dryer cars.
Soft-Mud Process. This is the oldest way of making brick
and was used before brick-making machines were developed. Automated
machinery is now used in this process.
The soft-mud process is suited for clays which contain too much
natural water for the stiff-mud process. The clay is mixed with
twice as much water as in the stiff-mud process and is pressed into
wooden molds. The molds are lubricated with sand or water so the
clay does not stick to the mold. When sand is used to lubricate the
molds, the bricks are sand-struck and have a sandy finish.
When water is used, the bricks are water-struck and have a
very smooth finish.
Dry-Press Process. In this process, the clay is mixed with
a small amount of water as compared to stiff mud and is then forced
into steel molds under very high pressure. This method is not in
wide use now in the U.S.A., but some are produced in the Pacific
Northwest. There is less demand for pressed brick because of its
selected use and the high cost of manufacturing.
2.4. Predrying Brick Before Burning In the Kiln.
Excess moisture must be removed from the brick before they are
burned in the kiln. Years ago, brick were allowed to dry in the open
air before they were placed in the kiln. The modern method is to
predry the brick in the forward section of the kiln using the waste
heat from the hot section of the kiln. The heat and humidity must be
regulated carefully to prevent sudden changes in the temperature
which will cause excessive cracking and deformation of the brick.
The drying time in the kiln is greatly reduced from that of the open
air process. In other words, with the brick being dried in the kiln,
the weather is not a factor affecting the moisture content of the
brick. The brick are placed in the drying chamber on special,
rolling steel cars and are left there for a predetermined drying
time before they move into the hot section of the kiln. The
temperature in the drying area is 100 to 400 degrees in Fahrenheit.
2.5. Burning the Brick in the Kiln.
The burning process is one of the most specialized steps in the
manufacture of brick. Kilns have changed drastically over the years.
Since brick making emerged as an industry, three basic kilns have
come into use: the scove kiln, the beehive kiln, and
the tunnel kiln.
The Scove Kiln. The scove kiln, was the first kiln to be
used to burn brick. There are very few of these kilns in use today.
The unburned brick were stacked in piles inside the kiln which was
then plastered on the outside with mortar to lock in the heat during
the burning process. Openings were left in the bottom walls of the
kiln where hardwood fires (in later years, gas and oil) provided
heat for the kiln. This method was not very efficient since the
brick nearest the fires were burned hard, while those near the top
of the kiln were soft and could be used only for filler walls or
The Beehive Kiln. An improved kiln, the beehive kiln, came
into use after the scove kiln. The beehive kiln is a round brick
structure wrapped with steel bands to control the expansion caused
by heating the kiln. The brick to be burned are stacked in the kiln
with narrow spaces between them so the heat will pass completely
around them. The kiln is sealed by walling the doors shut with brick
and mortar. The heat may be applied either from the bottom or the
top. When these kilns first came into use, wood and coke were the
main sources of heat. Gradually these fuels were replaced by gas and
oil because they could be controlled and regulated more efficiently.
The beehive kiln requires quite a bit of time to burn the brick. As
a result, this type of kiln yields a limited amount of brick.
The Tunnel Kiln. The most modern kiln is the tunnel kiln
which is built of brick and lined on the inside with fire brick. The
brick to be burned are stacked on flat cars which move very slowly
through the kiln. The cars move from the predrying section of the
kiln into the burning section.
The most modern kiln is the tunnel kiln where a
temerature of 1950 degrees F or more can be reached
The average time for the unit to pass completely through the kiln
is about 36 hours and is controlled by computers. The numbers of
brick on a car is rather small compared to the number of brick
stacked in the older types of kilns. The cars, however, pass through
the tunnel kiln continuously and the brick all receive the same heat
treatment, resulting in a more uniform product.
The heat in the kiln is supplied by gas, oil or powdered coal.
The heat gradually increases as the brick pass from the kiln inlet
to the firing zone in the center of the kiln. This is where the
greatest temperature is reached, an average of 1,950 degrees
Fahrenheit. From the center of the kiln, the brick move to the
outlet. As they move from the center, the heat is reduced slowly to
eliminate cracking, pitting, and other problems which are due to a
rapid reduction of temperature.
It is very important that the rate of temperature change in the
kiln be controlled during the burning operation. Tunnel kilns are
equipped with recording instruments which provide a constant check
on the temperatures in the kiln.
Control center for the tunnel kiln
While the brick are still in the kiln, they can be given a
treatment called flashing. Flashing means that the amount of
oxygen used in the burning is reduced. The flame changes from red to
blue. This changes the oxidation rate of the kiln which, in turn,
causes the brick to take on varying shades of color. The type of
clay or shale in the brick determines how the brick reacts to
In the stiff-mud process cutting is done
automatically by a large circular wire cutter
From extruder to the automatic wire cutter
2.6. Removing the Brick from the Kiln.
Drawing is the process of removing the brick from the
kiln. The brick leave the kiln on the cars which are placed in a
holding area for a period of time un til the brick are cool. Then
the brick are removed from the cars and placed on pallets for
It is true that modern masons no longer manufacture the brick
they use as their forerunners did many years ago. The modern brick
is a superior product due to the high degree of manufacturing
quality control that is used. Much of the guesswork and trial and
error methods of the past have been replaced by scientific processes
and computerized manufacturing.
3. Brick Today
The term brick as used today denotes a solid masonry rectangular
unit formed in a plastic state from clay and shale and burned in a
kiln. The United States Federal Trade Commission has ruled that no
product made from materials other than clay or shale can be called
brick, unless the name includes the material from which the unit is
manufactured, such as cinder brick, sand brick, sand lime brick, or
Raw Materials. Clay and shale are the principal materials
used to make brick. Usually concentrated in large deposits, these
materials are found all over the world.
Clay is a natural product which is formed by the weathering of
rocks. Shale is made in very much the same way from the same
material but is commpress ed into layers in the ground. Shale is
very dense and is harder to remove from the ground than clay. As a
result, shale is a more costly raw material.
Two or more kinds of clay and shale may be mixed together to
obtain a material having the proper consistency and composition.
There are several forms of clay that have a similar chemical
composition but different physical characteristics.
Surface clays are found near the surface of the earth. They may
be offshoots of old deposits or the result of more recent weathering
Shales are clays that have been formed under high pressure, by
natural conditions, until they resemble slate.
Fire clays are mined from a greater depth than are the other
clays, resist higher temperatures and have a greater resistance to
heat shock. They contain fewer impurities than shales or surface
clays and have more uniform chemical and physical properties.
Although surface clays and fire clays differ in physical
structure from shale, the three types of clay are chemically
similar. All these are made of silica and alumina with varying
amounts of metallic oxides and other impurities. Metallic oxides act
as fluxes and promote fusion at lower temperatures. The amount of
iron, magnesium, and calcium oxides in the clays influences the
color of the finished product. The material from each deposit of
clay and shale has chemical characteristics which may be uniform for
that deposit but may differ from the characteristics of material in
other deposits. The changes in characteristics from deposit to
deposit are due to differences in the relative amounts of the
chemical components. As a result, brick made from the material in
one deposit will have one set of characteristics for color, finish,
and texture, while brick made from material in another location may
look different because the chemical composition of the material
varies slightly from that at the first location. In addition, all
clay and shale do not react in the same manner to processing
4. Properties Of Brick
All properties of structural clay products such as brick are
affected by the composition of the raw materials used and the
manufacturing process. Basically, important properties of brick are
color, texture, size variation, absorption, compressive strength,
and durability, Most burned brick is fireproof regardless of the
other properties mentioned in this unit. Due to the technical nature
of brick properties, they will be generally discussed.
4.1. Color. The chemical composition of the natural
clay and the minerals which may be added to the natural clay
determine the color of the finished brick. Another factor which
affects color is the temperature at which the clay is burned and how
well the temperature is controlled in the kiln.
Of all the natural oxides found in clays, that of iron has the
greatest effect on color. Regardless of its natural color, clay that
has any iron in it will burn red due to the formation of ferrous
oxide, If the finished product is to have a darker color, the clay
must be burned at a higher temperature.
Architects and builders can add to the architectural beauty of
modern brick buildings by choosing from the range of colors in which
brick are available.
Since there may be some differences in the bricks from
different burnings, it is advisable to buy enough brick for a job at
one time.. In this way, the mason has a sufficient quantity of brick
in the proper color to complete the job. Any excess brick can be
used for repairs.
4.2. Texture. Texture in brick is the arrangement
of the particles of raw materials in the brick and the appearance
and finish of the brick. For example, a hard, smooth brick has a
fine finish or texture, while brick that has a sand finish is said
to have a coarse texture. These finishes are the result of the type
of process used to make the brick. In the stiff-mud process, many
textures can be obtained by using attachments which cut, scratch,
roll, brush, or otherwise roughen the clay column as it leaves the
die. Shown are several different brick textures commonly available,
Wire cut, deformed, and rug face brick are all brick that have a
rough face of irregular design to suit any job. These brick are
popular for fireplaces and buildings in which the builder wants to
show a bold course design in the finished lob. Old brick are often
used to obtain this special effect. However, the quality of the old
brick is an important factor, since they may deteriorate rapidly
when used on an outside wall.
Figure 1 - Brick textures available in the
The treatment of mortar joints is a definite form of texture and
affects the final appearance of the wall. The same kind of brick can
be used to build a wall but different ways of finishing the mortar
joints will create various textures in the wall. A wall using
recessed and projected brick achieves a different texture due to the
resulting shadow and highlighting effects. The use of colored mortar
gives the wall added texture and warmth. The texture of the wall
should always be considered so that it will blend in with the
architectural design of the building and its surroundings.
4.3. Size Variation. Clays shrink during the drying
and burning processes. Therefore, allowances for shrinkage must be
made. Since air shrinkage and fire shrinkage vary for the different
clays, the burning process requires careful control based on testing
Fire shrinkage increases as the temperature of the burning area
increases. Since darker colors are obtained at a higher burning
temperature, some differences between the sizes of dark and light
brick should be expected. To assure uniform size in their brick,
manufacturers work very hard to control the factors which cause
shrinkage. Because of the differences in raw materials and
temperature variations in the kilns, absolute uniformity is
impossible. However, the specifications (written details or a
description) for brick types indicate the desired size as a range of
permissible to allow economical manufacture.
4.4. Absorption. Absorption is the weight or
amount of water a masonry unit absorbs at certain conditions for a
stated length of time. This weight is expressed as a percentage of
the weight of the dry unit.
4.5. Compressive Strength. Compressive Strength
is defined as the resistance a brick has to increasing loads or
stress placed on it before it breaks. Run on brick used must be
first tested to determine its strength.
Both the compressive strength and the absorption of a brick are
affected by the properties of the clay, the methods of
manufacturing, and the burning process. Although there are
exceptions, brick produced by the stiff-mud process have higher
compressive strength ratings and lower absorption rates than units
produced by the soft-mud or dry-press methods. These are additional
reasons why the stiff-mud process is used more than any other method
of manufactu ring
The beehive kiln yields a limited amount of brick