GENERAL OVERVIEW OF BRICK MANUFACTURING

INTRODUCTION
Brick is one of man’s oldest building materials. Although equipment and technology has improved the quality and appearance of the brick produced, the basic raw material and manufacturing steps remain the same.

RAW MATERIAL
Clay is one of the most abundant mineral materials on earth. However, not all clay possesses the properties to be used in modern brick production. There are three general types of clay used in brick manufacturing.

1. Surface Clay: May be the up thrusts of older deposits or of more recent sedimentary formations. They are formed near the surface.
2. Shale: Clays that have been subjected to high pressures until they have hardened almost to the form of slate.
3. Fire Clay: Usually mined at deeper levels than the other two clays and have refractory qualities.

MANUFACTURING
There are six general phases in the manufacture of brick.

1. Winning - the mining of clay or shale and transportation of it to the manufacturing site.
2. Preparation - crushing, grinding and mixing the clay into a uniform raw material.
3. Forming - mixing water with the prepared clay in a plug mill. The amount of water used will determine the plasticity of the clay. The process used to form the brick will determine the plasticity required.
4. Drying - after the wet clay is formed it is dried to remove the moisture through evaporation. The drying time is usually 24 to 48 hours.
5. Firing and Cooling - firing, one of the most specialized steps in the manufacturing of brick, requires from 40 to 150 hours depending upon kiln type and other variables. Firing may be divided into six general stages: 1) water-smoking (evaporating free water), 2) dehydration, 3) oxidation, 4) vitirification, 5) flashing and 6) cooling. All except flashing and cooling are associated with rising temperatures in the kiln. After the temperature has reached the maximum and is maintained for a prescribed time, the cooling process begins. Because the cooling has a direct effect on color and because excessively rapid cooling will cause cracking and checking of the ware, cooling is an important stage in the flashing process.
6. Drawing - the process of unloading a kiln after cooling and packing the brick for shipment.

COLOR
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 burn red when exposed to an oxidizing fire. A general rule of thumb is the darker the brick, the higher the firing temperature.

TEXTURE AND COATINGS
Many brick have smooth or sand-finished textures produced by the dies or molds used in forming, in the stiff-mud process, many textures may be applied by attachments which cut, scratch, roll, brush or otherwise roughen the surface as the clay column leaves the die. Many plants apply engobes (slurries) of finely ground clay, coloring agents and water to the roughened Column. Sands can be rolled into the slurry coating to create interesting and distinctive patterns in the finish products.


BRICK CLASSIFICATIONS
ASTM
(American Society for Testing and Materials)
ASTM is the world’s leader in the development of voluntary
consensus standards for materials, products, systems and services.
Its 140 standards writings committees annually publish over 7,000
standards to meet the needs of numerous industries and services.

ASTM C216-97
ASTM C216-97 is the standard specification for Facing Brick.

EXPOSURE (WEATHERING)
Because of the varying climates throughout the country, and a different application of brick, specific grades of brick are required. Brick must meet a grade of SW or MW based on the weathering index and the exposure of the brick. The weathering index is the product of the average annual number of freezing cycle days and the average annual winter rainfall in inches. The exposure is related to either a vertical or horizontal surface and whether the unit will be in contact with the earth. A higher weathering index or a more severe exposure will require a face brick to meet the SW requirements of the brick. The grade is typically based on physical properties of the brick.

Appearance
Brick types are related to the appearance of the unit, and specifically to limit on dimensional tolerances, distortion tolerances and chippage of the unit. The brick type can be selected depending upon whether: a high degree of precision is necessary; a wider range of color or size is permitted; or a characteristic architectural effect is desired. The three appearance grade for ASTM C216 brick are: FBX, FBS and FBA. (Note: FBA grades is as specified by the purchaser)


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