Bricks - Oolitic Middle School

Oolitic Middle School Industrial Technology The Story of Bricks INTRODUCTION | WINNING | PREPARATION | MOLDING | DRYING | FIRING | MODERN BRICKS | SUMMARY Words to know: adobe, architecture, cavity, clinker, compression, course, cycle, die, exterior, extrusion, firing, green, interior, kiln, laminated, manual, oxide, porosity, pug mill, vitrify

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Introduction It has been said that the study of bricks is the study of civilization. Bricks made of mud and straw have been used for thousands of years. Some bricks over 10,000 years old have been discovered. One of the first building materials created by man, bricks are still a highly desired choice to this day. Some of the newest homes and modern buildings use bricks in their construction. The simple brick has maintained its

reputation as a superior building product longer than perhaps any other product in the modern world. Plain mud bricks like the ones pictured on the left are called 'adobe' or sometimes 'slump bricks.' Examples of adobe bricks have survived throughout history and are found in many ancient buildings. Adobe was used in the ziggurat temples of Mesopotamia and by the emperors of ancient Rome. Since trees were scarce in the Old Southwest, the American Indians and the pioneer settlers alike chose adobe brick as the logical material for buildings. Adobe bricks are still used in certain parts of the world today.

The cliff dwellings of Mesa Verde, Colorado were built with adobe bricks around 1200 A.D.

Adobe bricks are made from a mixture of mud and small pieces of straw or reeds. The bricks are formed by hand and left in the sun to dry. But there is a problem with plain mud bricks. Over time, rain water will dissolve them and cause them to crumble and break apart.

Early brick makers eventually learned that if they would "burn" the bricks by baking them in a very hot oven called a kiln, the bricks would become very hard and durable. The basic brick making process has survived for thousands of years. To better understand the complete story, let's take a look back into history and discover how the brick making process evolved into the modern brick making industry of today. There are five basic steps to making bricks. BACK TO TOP

1. MINING. The first step is called mining or "winning" the clay.

The original color of clay does little to indicate the color of the finished brick. The color of a finished brick is generally determined by how much iron mineral is in the clay. Clay with a high iron content will produce a deep red brick after firing (See step 5). Clay with less iron produces a buff or light brown colored brick. The early brick makers relied on their own experience when they chose their clay. They chose it primarily by it's color and texture. They didn't have the use of sophisticated laboratories to analyze the clay back then. Because the steam shovel was not invented until 1879, early brick makers had to dig for the clay with hand shovels. They obviously wanted to find their clay just under the topsoil in order to minimize much of the hard digging. The digging was usually done in the autumn. That way, the clay could be left exposed to the freeze-thaw cycles of the coming winter. A cycle is a series of events that happen over and over again. The freezing and thawing over and over again would help break down the clay. That made the clay softer and easier to work

by hand in the Spring. The winter exposure not only made the clay soft but also removed unwanted oxides. Oxides are minerals that have combined with oxygen and are considered undesirable impurities in brick making. BACK TO TOP

2. PREPARATION. The second step is preparation of the clay By spring the clay was suitable to be worked by hand. It was sometimes ground into a powder and sifted through a screen to remove stones. Sometimes the clay was placed into a soaking pit where it was mixed with water to obtain the proper consistency for molding.

A horse driven pug mill. This was a great improvement over doing this process by hands and feet.

Next, the clay was kneaded with the hands and feet to mix all the elements together. This step was called tempering or pugging and was the hardest work of all. In the mid-1800's horse driven pug mills (above) were invented to make this task easier. Pug mills are still used today, but of course they are now powered by

huge electric motors instead of horses. The clay was then removed from the soaking pit or pug mill by a temperer who carried it to the molding table to begin the next step in the process. BACK TO TOP

3. MOLDING. The third step is molding. The assistant brick molder was called the clot molder. That job was to prepare a lump of clay (a clot) and give it to the brick molder. The brick molder was the key to the operation and the head of the team. He or she would stand and work at the molding table for twelve to fourteen hours a day. With the help of the assistants, a brick molder could make 3,500 to 5,000 bricks in a day. They would take the clot of clay, roll it in sand and "dash" it (toss it forcefully) into a mold. A mold is a wooden frame that is the shape of a brick. The clay was then pressed into the mold with the hands. The excess clay was scraped from the top of the mold with a flat stick called a strike that had been soaking in water. This excess clay was returned to the clot molder to be reformed into later bricks.

A brick molding table

A single cavity wooden brick mold

Single, double, four, or six cavity brick molds were used. A cavity is the hollow area inside the mold where the wet clay is placed to form the shape of the brick. A single cavity brick mold made one brick at a time, a double cavity mold made two bricks at a time, and so on. Making one brick at a time had an advantage since even a child could carry a single brick to the drying area. Beech wood was the preferred material for constructing the mold because it was believed that the clay would not stick to it. The top of the mold was often laminated (covered by a thin layer) with iron to prevent wear. The mold was also coated each time with sand so the bricks would slide out easily. These types of bricks are referred to as "sand struck bricks." The next person on the team was called the off-bearer. The off-bearer's job was to move the filled mold from the molding table to the drying area using either a pallet or a wheelbarrow. There it would be placed on a level bed of sand. The off-bearer would remove the brick from the mold, stack the brick to dry,

then return the empty mold to the brick molding table. At the molding table he or she would wet the mold and coat it again with sand. It was now ready for the brick molder to form the next brick. BACK TO TOP

4. DRYING. The fourth step is drying.

Bricks being wheeled to the hackstead and stacked for drying. Notice the color of the bricks at this stage.

The bricks were left stacked in the drying area for a few days. After about two days they were turned over. This was done to facilitate uniform drying and to prevent warping. During this time tools called dressers or clappers were used to straighten any crooked bricks and to obtain a smooth surface. After about four days in dry, hot weather the bricks were

sufficiently hard to allow them to be stacked on end. A finger's width space was left between each one to allow air to flow between them. This helped them dry faster. The stacking area was called a hack or a hackstead. The bricks were covered by a roof or with straw to protect them from the rain or harsh sun. After about two weeks the bricks were ready to be fired. BACK TO TOP

5. FIRING. The fifth step is firing, or burning. Simply heating bricks by placing them in an open fire is not sufficient for firing bricks. They would not get hot enough. In order to obtain the required temperature they must be baked in a kiln. Since early brick making usually took place on the site where the clay was mined, a kiln had to be constructed each time a batch of bricks was made. If fired bricks were left over from a previous batch, they were used to construct the outer walls of the next kiln. The surface was daubed (sealed by smearing over the cracks) with mud to help hold in the heat. If no previously fired bricks were available, the kiln was constructed entirely of green bricks. Green bricks are raw, unbaked bricks. The bricks were stacked in such a way that they acted as their own kiln. Wood and coal were used for fuel.

Stacking and firing the kiln was part of the "art" of brickmaking. Green bricks were used to construct the kiln shown above. The holes in the walls are the fire holes. The green bricks were then stacked inside, about a finger's width apart so the heat would flow between them easily.

After the structure was daubed and sealed with clay, wood or coal was placed in the fire holes, and the fires were lit. The fires were kept low for the first 24-48 hours until the bricks were completely dry. When the steam from the bricks cleared, the intensity of the fire was increased.

The fires burned for about six days – day and night. The brick makers remained on site during the entire burn period, getting little sleep and keeping the fires burning. Near the end of the burn, the kiln reached a temperature of approximately 1,850 degrees Fahrenheit.

Even after drying for more than two weeks in the air, the green bricks still contained 9-15% water. For this reason the fires were kept low for the first 24-48 hours while the bricks continued to dry. During this time steam would rise from the top of the kiln in huge white clouds. They called this steam "water smoke."

When the steam gases finally cleared it was the signal to increase the intensity of the fires. If this was done too early, steam would form inside the bricks and cause them to explode. The brick shown below on an old school house in Leesville, Indiana is an example of this condition. This brick was fired too green and the internal gasses caused it to burst open.

Intense fires were maintained in the fire holes around the clock for about a week. It took that long to reach the required temperature of around 1,850 degrees F. Reaching the correct temperature was very important. When a brick reaches the correct temperature it begins to 'vitrify.' Vitrification happens when sand and other materials in clay melt and fuse together. This changes the clay into a glasslike material. Properly fired bricks are very strong in compression strength. Compression is the squeezing force on a material. In the case of bricks, most of the compression force comes from their own weight on top of each other. Bricks that do not get hot enough during firing are weak in compression strength and will crumble under load. Bricks that become too hot will become

too glass-like and brittle. The trick for the early brick maker was to get them just right. The correct temperature was just at the point when the vitrification process began. Since thermometers were not available in those days, the brick maker had to rely on his knowledge and experience to guess when the bricks were done. At that time the fire was put out, the fireholes of the kiln were bricked over, thus ending the firing process. It then took more than a week for the kiln to cool down sufficiently to remove the bricks.

Medora Brick Plant A 50-man workforce once produced 54,000 handmade bricks a day at the Medora Brick Plant in nearby Jackson County, Indiana. The plant opened in 1906. The round "beehive" shaped buildings in the picture are nine of the twelve kilns that were used. The low tin-roofed buildings in the center of the picture are the hack-steads. With its aging brick making technology, the old plant became unable to compete with more modern plants. The Medora Brick Plant closed on January 31, 1992. Almost all brick that was used in this area before 1992 came from this brick plant.

When cool,

the kiln was disassembled and the sorting process begun. If green bricks had been used to make the kiln, those bricks from the outermost walls were saved to be fired again in the next kiln. Some bricks which were closest to the fire received a natural wood ash glaze from the sand that fell into the fires, became vaporized, and deposited on the bricks. These bricks were used in the interior (inside) courses of the walls because of their attractive, slick and shiny appearance. A course is one row of laid bricks. Bricks that were severely over-burned, cracked, or warped were called 'clinkers' and were occasionally used for garden walls or garden paths. They were probably called clinkers because of the clinking sound they made when they were struck together. They were too glassy and much too brittle to be used in buildings. Those that were only slightly under-fired had a salmon color and early brick layers knew that the porosity (tiny air holes) in these bricks would help to insulate a structure. These bricks were placed on the innermost courses of a wall. This is an example of a salmon colored brick. Finally, the best bricks were chosen for the exterior (outside) walls of buildings because they were the strongest and most durable.

A famous brick building from Colonial times:

Independence Hall, Philadelphia, Pennsylvania was built with hand made bricks. It was started in 1732 and completed in1753.

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Modern Brick Making Of course, brick clay is no longer mined by hand. Huge trachoes and other earth-moving equipment can now do in minutes what it once took days to do with hand shovels and wheelbarrows.

Clay can now be transported easily from the mining site to permanent brick plants. There, sophisticated testing labs, pug mills, temperature controlled kilns, and robotic equipment are used to transform it into hundreds of styles and textures of brick products. The making of bricks is no longer a guessing game dependent upon the knowledge and experience of only a few people. Modern high output brick factories have replaced the hand molding process with a much faster method of forming bricks from clay. Today's factories use what is referred to as the extrusion process. A large machine called an extruder forces

a soft clay mixture through a small opening called a die. In this case the die is a block of steel with an opening the size and shape of a brick. This forms a long continuous rectangle of clay which can then be cut into individual bricks. The extrusion process can be illustrated by the cookie press shown below. Soft cookie dough is extruded through one of the dies shown on the left to form the desired shape of cookie. The long 'extrusion' of dough is then cut off into individual cookie sections.

The brick extruder being operated by the woman above right works on the same principle. The long extrusion is first cut into about 6-foot long sections called slugs. The slug next passes through a row of wires that slices it into individual brick sections, creating several bricks at a time. This process is obviously much faster than the ancient method of molding bricks one by one by hand. A cookie extrusion press

A brick extrusion press

The extrusion process is used to form shapes in other materials as well. Aluminum extrusions are quite common.

Aluminum is heated to make it soft so it can be extruded through a die. Some familiar uses for aluminum extrusions are in residential storm doors and storm windows. Those are aluminum extrusions around the chalkboard in our classroom. The Krispy-Kreme Donut Company uses the extrusion process to make their popular donuts. They would be unable to keep up with the huge demand otherwise. Cutting donuts from a flat sheet of dough as most companies do is much too slow. All of the Krispy-Kreme stores together can extrude a stack of donuts as high as the Empire State Building in only 22 seconds. Can you think of other products that are extruded? BACK TO TOP

Summary Bricks have been around since man first began building structures. Some say architecture, the art and science of designing and constructing buildings, began when the first two bricks were laid together well.

A robot handles bricks in a modern brick plant.

The history of the brick industry is a good example of the benefits of mass production. Modern brick making uses at least some variation of every one of the five original steps we have discussed. Today, much of the difficult, manual (hand) labor is performed by high-tech machinery and computer controlled robots. Laboratories ensure consistent clay mixtures. Temperature and atmospheric controlled kilns produce uniform, high quality bricks in hundreds of varieties, shapes, and sizes.

The warm solid feeling of a brick house is greatly preferred by a wide variety of home builders. Ask the Three Little Pigs!

Bricks are attractive and relatively inexpensive products with good mechanical properties. The natural resource (clay) is plentiful and readily available. These are necessary qualities of any good building material.

Nevertheless, bricks are essentially just burnt clay. . . and, they have been around for thousands of years. Amazingly, they continue to serve as the backdrop of the modern age.

We have all been taught since childhood to follow the yellow brick road . . . to follow our dreams into the future. When we do, we will no doubt discover that the age-old brick is still with us when we get there. When it comes to bricks, perhaps modern isn't as modern as we think it is. BACK TO TOP

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