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Thursday, 1 February 2018

Manufacturing of Brick

Abstract: This Technical Note presents fundamental procedures for the manufacture of clay brick. The types of clay used,
the three principal processes for forming brick and the various phases of manufacturing, from mining through storage, are
discussed. Information is provided regarding brick durability, color, texture (including coatings and glazes), size variation,
compressive strength and absorption.

Key Words: absorption, clays, color, cooling, compressive strength, de-hacking, drying, durability, firing, forming, hacking,
manufacturing, mining, preparation, shales, size variation, texture.

• Brick is made of clay or shale formed, dried and fired into
a durable ceramic product.
• There are three ways to form the shape and size of a
brick: extruded (stiff mud), molded (soft mud) and drypressed.
The majority of brick are made by the extrusion
• Brick achieves its color through the minerals in the fired
clay or through coatings that are applied before or after
the firing process. This provides a durable color that never
fades or diminishes.
• Brick shrink during the manufacturing process as
vitrification occurs. Brick will vary in size due to the
manufacturing process. These variations are addressed by
ASTM standards.
• The method used to form a brick has a major impact on
its texture. Sand-finished surfaces are typical with molded
brick. A variety of textures can be achieved with extruded
• Brick manufacturers address sustainability by locating
manufacturing facilities near clay sources to reduce
transportation, by recycling of process waste, by
reclaiming land where mining has occurred, and by taking
measures to reduce plant emissions. Most brick are used
within 500 miles of a brick manufacturing facility.

The fundamentals of brick manufacturing have not changed over time. However, technological advancements
have made contemporary brick plants substantially more efficient and have improved the overall quality of the
products. A more complete knowledge of raw materials and their properties, better control of firing, improved kiln
designs and more advanced mechanization have all contributed to advancing the brick industry.
Other Technical Notes in this series address the classification and selection of brick considering the use, exposure
and required durability of the finished brickwork.

Clay is one of the most abundant natural mineral materials on earth. For brick manufacturing, clay must possess
some specific properties and characteristics. Such clays must have plasticity, which permits them to be shaped or
molded when mixed with water; they must have sufficient wet and air-dried strength to maintain their shape after
forming. Also, when subjected to appropriate temperatures, the clay particles must fuse together.
Types of Clay
Clays occur in three principal forms, all of which have similar chemical compositions but different physical
Surface Clays. Surface clays may be the upthrusts of older deposits or of more recent sedimentary formations. As
the name implies, they are found near the surface of the earth.
Shales. Shales are clays that have been subjected to high pressures until they have nearly hardened into slate.
Fire Clays. Fire clays are usually mined at deeper levels than other clays and have refractory qualities.
Surface and fire clays have a different physical structure from shales but are similar in chemical composition.All
three types of clay are composed of silica and alumina with varying amounts of metallic oxides. Metallic oxides
act as fluxes promoting fusion of the particles at lower temperatures. Metallic oxides (particularly those of iron,
magnesium and calcium) influence the color of the fired brick.

Although the basic principles of manufacture are fairly uniform, individual manufacturing plants tailor their
production to fit their particular raw materials and operation. Essentially, brick are produced by mixing ground
clay with water, forming the clay into the desired shape, and drying and firing. In ancient times, all molding was
performed by hand. However, since the invention of brick-making machines during the latter part of the 19th
century, the majority of brick produced in the United States have been machine made.

Phases of Manufacturing
The manufacturing process has six general phases: 1) mining and storage of raw materials, 2) preparing raw
materials, 3) forming the brick, 4) drying, 5) firing and cooling and 6) de-hacking and storing finished products (see
Figure 1).
Mining and Storage. Surface clays, shales and some
fire clays are mined in open pits with power equipment.
plant storage areas (see Photo 1).
Continuous brick production regardless of weather
conditions is ensured by storing sufficient quantities
of raw materials required for many days of plant
operation. Normally, several storage areas (one for
each source) are used to facilitate blending of the
clays. Blending produces more uniform raw materials,
helps control color and allows raw material control for
manufacturing a certain brick body.

Preparation. To break up large clay lumps and stones, the material is processed through size-reduction machines
before mixing the raw material. Usually the material is processed through inclined vibrating screens to control
particle size.
Forming. Tempering, the first step in the forming process, produces a homogeneous, plastic clay mass. Usually,
this is achieved by adding water to the clay in a pug mill (see Photo 2), a mixing chamber with one or more
revolving shafts with blade extensions. After pugging, the plastic clay mass is ready for forming. There are three
principal processes for forming brick: stiff-mud, soft-mud and dry-press.
Stiff-Mud Process - In the stiff-mud or extrusion process (see Photo 3), water in the range of 10 to 15
percent is mixed into the clay to produce plasticity. After pugging, the tempered clay goes through a deairing
chamber that maintains a vacuum of 15 to 29 in. (375 to 725 mm) of mercury. De-airing removes
air holes and bubbles, giving the clay increased workability and plasticity, resulting in greater strength.
Next, the clay is extruded through a die to produce a column of clay. As the clay column leaves the die,
textures or surface coatings may be applied (see PROPERTIES, Textures, Coatings and Glazes). An
automatic cutter then slices through the clay column to create the individual brick. Cutter spacings and die
sizes must be carefully calculated to compensate for normal shrinkage that occurs during drying and firing
(see PROPERTIES, Size Variation). About 90 percent of brick in the United States are produced by the
extrusion process.

Soft-Mud Process -
The soft-mud or molded process is particularly suitable for clays containing too
much water to be extruded by the stiff-mud process. Clays are mixed to contain 20 to 30 percent water
and then formed into brick in molds. To prevent clay from sticking, the molds are lubricated with either
sand or water to produce “sand-struck” or “water-struck” brick. Brick may be produced in this manner by
machine or by hand.
Dry-Press Process
This process is particularly suited to clays of very low plasticity. Clay is mixed with
a minimal amount of water (up to 10 percent), then pressed into steel molds under pressures from 500 to
1500 psi (3.4 to 10.3 MPa) by hydraulic or compressed air rams.
Drying. Wet brick from molding or cutting machines contain 7 to 30 percent moisture, depending upon the forming
method. Before the firing process begins, most of this water is evaporated in dryer chambers at temperatures
ranging from about 100 ºF to 400 ºF (38 ºC to 204 ºC). The extent of drying time, which varies with different clays,
usually is between 24 to 48 hours. Although heat may be generated specifically for dryer chambers, it usually is
supplied from the exhaust heat of kilns to maximize thermal efficiency. In all cases, heat and humidity must be
carefully regulated to avoid cracking in the brick.

Hacking is the process of loading a kiln car or kiln with brick. The number of brick on the kiln car is
determined by kiln size. The brick are typically placed by robots or mechanical means. The setting pattern has
some influence on appearance. Brick placed face-toface
will have a more uniform color than brick that are

All properties of brick are affected by raw material composition and the manufacturing process. Most
manufacturers blend different clays to achieve the desired properties of the raw materials and of the fired brick.
This improves the overall quality of the finished product. The quality control during the manufacturing process
permits the manufacturer to limit variations due to processing and to produce a more uniform product.
The most important properties of brick are 1) durability, 2) color, 3) texture, 4) size variation, 5) compressive
strength and 6) absorption.

The durability of brick depends upon achieving incipient fusion and partial vitrification during firing. Because
compressive strength and absorption values are also related to the firing temperatures, these properties, together
with saturation coefficient, are currently taken as predictors of durability in brick specifications. However, because
of differences in raw materials and manufacturing methods, a single set of values of compressive strength and
absorption will not reliably indicate the degree of firing.

The color of fired clay depends upon its chemical composition, the firing temperatures and the method of firing
control. Of all the oxides commonly found in clays, iron probably has the greatest effect on color. Regardless of its
natural color, clay containing iron in practically any form will exhibit a shade of red when exposed to an oxidizing
fire because of the formation of ferrous oxide. When fired in a reducing atmosphere, the same clay will assume a
dark (or black) hue. Creating a reducing atmosphere in the kiln is known as flashing or reduction firing.
Given the same raw material and manufacturing method, darker colors are associated with higher firing
temperatures, lower absorption values and higher compressive strength values. However, for products made from
different raw materials, there is no direct relationship between strength and color or absorption and color.

This Technical Note on manufacturing brick is the first in a series covering the manufacturing, classification and
selection of brick. It provides a synopsis of the manufacturing process and discusses the various properties that
are a function of this process. More detailed descriptions of the ceramic properties of brick are not within the
purview of the Brick Industry Association. This type of information is more readily available through the National

Brick Research Center, ceramic engineers and educators.
The information and suggestions contained in this Technical Note are based on the available data
and the combined experience of engineering staff and members of the Brick Industry Association.
The information contained herein must be used in conjunction with good technical judgment and a
basic understanding of the properties of brick masonry. Final decisions on the use of the information
contained in this Technical Note are not within the purview of the Brick Industry Association and
must rest with the project architect, engineer and owner.

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