How Does Clay Turn Into Bricks?

Clay is a fine-grained natural rock or soil material that is plastic when moist but hard when fired, making it ideal for shaping into bricks. Bricks are rectangular blocks that are formed from clay and hardened by firing at high temperatures in a kiln. The brickmaking process transforms clay into a durable, heat-resistant building material.

This process involves several steps, starting with extracting clay from the earth, refining it, molding it into the desired brick shape, drying the bricks, and finally firing them at high heat. During firing, chemical changes occur that permanently harden the clay into a ceramic material. After cooling, the finished bricks are ready for use in construction and masonry projects.

Clay Composition

Clay is composed of a variety of naturally occurring minerals and materials. The main clay minerals are kaolinite, montmorillonite‐smectite, illite, and chlorite. Kaolinite is the most abundant mineral in clay deposits and gives clay its plasticity. Montmorillonite‐smectite provides clay with its high absorption and swelling capacity. Illite contributes strength and chlorite provides greenish colors. In addition to the clay minerals, clay also contains quartz, feldspar, calcite and organic matter as natural impurities.

The specific composition of a clay deposit depends on how it was formed geologically. The minerals present influence the properties and quality of the clay. High kaolinite clays are very plastic and are sought after for making porcelain and whiteware. Montmorillonite-smectite rich clays swell considerably more. Illite and chlorite contribute fluxing elements that lower the vitrification temperature of clays. The non-clay mineral impurities also affect clays. Higher quartz content reduces shrinkage and density.

Clay Extraction

The process of turning clay into bricks begins with the extraction of clay from the earth. There are two main methods for extracting clay – digging and mining from clay deposits.

Digging involves excavating clay directly from the ground. The topsoil is removed to reach deposits of clay beneath. Backhoes, bulldozers, and other heavy equipment are used to dig and gather clay. The clay is then stockpiled on-site.

Mining clay involves extracting clay from deeper deposits underground. Underground shafts are created and mining equipment is used to excavate and haul clay to the surface. Blasting with explosives may be required to break up clay deposits. Mined clay is brought up from underground through vertical shafts.

Both digging and mining are effective techniques for accessing and extracting large volumes of natural clay for brick manufacturing and other industrial uses. The choice between digging or mining depends on the depth and size of local clay deposits.

Clay Preparation

Once the clay has been extracted, it goes through a preparation process to improve its workability and properties for brick making. This involves crushing, grinding, screening, and blending the raw clay:

Crushing – Lumps of clay extracted from the earth contain rocks and other impurities that need to be removed. The clay is crushed into smaller pieces to separate these particles.

Grinding – After crushing, the clay is fed into large grinders or tube mills which further pulverize it into a fine powder. This increases the surface area of the clay for improved mixing and bonding.

Screening – The ground clay powder is passed through vibrating screens to filter out remaining coarse contaminants based on particle size.

Blending – Different types of clay are blended together in specific proportions to achieve the desired plasticity, strength, appearance, and other properties in the final brick product. Water may also be added to reach an optimal moisture content.

The prepared clay resulting from these processes provides a homogeneous, workable material ready for molding into bricks.

Molding Bricks

After the clay has been prepared, the next step is molding it into the shape of bricks. There are a few different methods for molding bricks:

Brick Molds

Brick molds are rectangular boxes open on top and bottom, made of wood, plastic or metal. Clay is pressed into the mold either by hand or using an automated machine. This creates bricks with smooth sides and sharp corners. Brick molds allow efficient mass production of uniform bricks.

Extrusion

With extrusion, clay is forced through a die opening with the desired brick shape. As the continuous column of clay emerges, it is cut into individual bricks with wires. Extrusion is a fast, automated method ideal for mass production. The extruded bricks have slightly rounded edges compared to molded bricks.

Molding by Hand

In some cases bricks are molded by hand. The clay is rolled into balls or blocks, then pressed into wooden molds open on top and bottom. This is slower but allows unique handmade styles. The hand-molded bricks have rougher, more irregular surfaces with rounded edges.

Choosing the molding method depends on factors like production scale, consistency, and desired finish. But in each case, the clay is shaped into the standard brick rectangular form at this stage of the process.

Drying Bricks

After bricks have been molded, they must be dried before firing. There are two main methods for drying bricks:

Slow Air Drying

Air drying is the traditional method for drying bricks. The wet, molded bricks are loaded onto drying racks and left to sit in open air for 1-2 weeks. This allows moisture in the bricks to slowly evaporate until the bricks are ready for firing.

Air drying takes time but is a low-cost process that doesn’t require specialized equipment. The bricks must be protected from rain during the drying period. Air circulation around the bricks is important to enable uniform drying.

Accelerated Dryers

Many modern brick plants use industrial dryers to speed up the drying process. The most common type is a tunnel dryer, which blows heated air through a tunnel containing the bricks. This can dry the bricks in 1-2 days rather than weeks.

Other accelerated drying methods include shelf dryers and room dryers. These methods force air circulation around the bricks using fans and heating coils. Accelerated drying requires more specialized equipment and energy input, but greatly speeds up brick production.

The drying process is complete when the bricks have lost enough moisture to become strong enough for firing. Quickly dried bricks must be heated gradually in the kiln to prevent cracks from forming.

Firing Bricks

Firing is the most crucial step in transforming soft clay into hard, durable bricks. This process involves heating the molded bricks to extremely high temperatures in special ovens called kilns. The intense heat triggers chemical and physical changes in the clay that permanently harden it.

Kilns used for firing bricks are specially designed to withstand the ultra-high temperatures needed to properly fire clay, which can exceed 1000°C. There are several types of kilns used for brick firing, but they generally fall into two main categories: intermittent and continuous.

Intermittent kilns are batch-style kilns that fire a single load of bricks at a time. The bricks are loaded into the kiln, which is then sealed and heated gradually according to a schedule. Firing may take several days to complete before the kiln is cooled and unloaded. Tunnel kilns are the most modern type of intermittent kiln.

Continuous kilns allow for constant production. Bricks move slowly through the kiln on conveyors as their temperature is steadily increased. The hottest firing zone can reach over 1100°C. The bricks are then cooled gradually before exiting the kiln.

Regardless of kiln type, firing bricks involves carefully controlling temperature, heating rate, time at maximum temperature, and cooling rate in order to achieve the desired physical and chemical changes in the clay. When done properly, firing results in strong, dense, and durable bricks.

Cooling Bricks

After firing, the bricks must be cooled in a controlled way to ensure they do not crack or become damaged. This controlled cooling process is known as annealing.

The kilns are slowly cooled over a period of 24-48 hours down to about 200°C. This gradual cooling relieves internal stresses built up in the bricks during the high temperature firing process. If cooled too quickly, the bricks can crack, warp or become misshapen.

For annealing, the air supply to the kiln is reduced and the openings are partially bricked up to restrict airflow. This slow, controlled cooling allows the bricks to adjust to the temperature change without damage. Modern tunnel kilns often have automated temperature controls to precisely manage the cooling cycle.

Well-annealed bricks will have a more uniform structure and surface hardness. Proper annealing results in bricks that are dimensionally stable and able to withstand weathering conditions over many years.

Brick Finishing

After bricks have cooled down from the firing process, some additional steps may be taken to finish or enhance the bricks before they are ready for shipping and use. Common finishing steps include grinding, coating, and packaging.

Grinding involves using an abrasive material to smooth and even out the brick surfaces. This helps remove any small imperfections from the firing process. Grinding also helps standardize the size and shape of the bricks if there are any slight variations after cooling. The grinding process utilizes specialized grinding machines and abrasives like silicon carbide.

Coating bricks helps protect them and adds aesthetic qualities. Common coatings include glazes, paints, and hydrophobic treatments. Glazes provide a glass-like coating and can come in various colors. Paint coatings also add color variety. Hydrophobic treatments help reduce water absorption and prevent efflorescence (white salt deposits). Coatings are applied by automated spraying systems in most modern plants.

Packaging is the final step before shipping the finished bricks. Bricks are usually stacked and strapped onto wooden pallets, then covered with plastic sheeting or shrink wrap. This protects the bricks in transit and facilitates easy loading and unloading. Individual bricks may also be wrapped or boxed up. Proper packaging prevents damage and provides an orderly way to transport large quantities of bricks.

Conclusion

The process of making clay into bricks is a complex but fascinating one. It starts with the composition of clay itself, which contains minerals that help give bricks their strength and durability. Extraction and preparation refine the clay further, removing impurities and getting just the right consistency for molding. The molded bricks are then carefully dried and fired at high temperatures, undergoing chemical changes that harden them permanently. After cooling, the finished bricks are ready for any final finishing steps.

Brickmaking has come a long way over the centuries, with modern production methods allowing for mass production of consistent, high-quality bricks. At the same time, artisan brickmakers keep old traditions alive. As one of humankind’s oldest manufactured building materials, bricks will continue playing a vital role in construction long into the future. New brick varieties and production techniques point to exciting possibilities ahead for this stalwart building block.