ZIG ZAG BRICK KILN

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.
SUMMARY:
TECHNICAL NOTES on Brick Construction 9
December
2006
• 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
method.
• 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.
• 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.
© 2006 Brick Industry Association, Reston, Virginia Page 1 of 7
INTRODUCTION
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.
RAW MATERIALS
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
characteristics.
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
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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.
The manufacturer minimizes variations in chemical composition and physical properties by mixing clays from
different sources and different locations in the pit. Chemical composition varies within the pit, and the differences
are compensated for by varying manufacturing processes. As a result, brick from the same manufacturer will have
slightly different properties in subsequent production runs. Further, brick from different manufacturers that have the
same appearance may differ in other properties.
MANUFACTURING
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.
Then the clay or shale mixtures are transported to
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.
Figure 1
Diagrammatic Representation of Manufacturing Process
Photo 1
Clay or Shale Being Crushed
and Transported to Storage Area
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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. 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
Photo 2
Clay is Thoroughly Mixed with Water
in Pug Mill Before Extrusion
Photo 3
After Mining, Clay is Extruded Through a Die and
Trimmed to Specified Dimension Before Firing
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some influence on appearance. Brick placed face-toface
will have a more uniform color than brick that are
cross-set or placed face-to-back.
Firing. Brick are fired between 10 and 40 hours,
depending upon kiln type and other variables. There
are several types of kilns used by manufacturers.