Can Clay Be Used To Make Bricks?

Bricks are small building units typically made from clay and fired in a kiln. Clay bricks are one of the longest-used and most common building materials in the world. People have been making bricks from clay for thousands of years.

Clay bricks are manufactured by molding clay into brick shapes through extrusion or pressing, then firing the bricks at high temperatures in a kiln. This process hardens the clay through vitrification, making the bricks durable and resistant to weathering.

Clay bricks have been a popular building material across many cultures due to the widespread availability of clay as a raw material. They have been used to construct buildings, pavements, archways and other structures. Their modular size, durability, aesthetics and insulating capabilities have made clay bricks integral to architecture throughout history.

Clay Composition

Clay is a naturally occurring material composed primarily of fine-grained minerals. The types of clay used to make bricks include:

• Kaolin – Also known as china clay, kaolin has a high content of kaolinite. It is usually white in color and has a fine, soft texture.
• Ball clay – This type of clay has high plasticity and high strength when fired. It is composed of kaolinite and other minerals such as quartz.
• Fire clay – Fire clays are refractory clays that can resist high temperatures. They typically contain a mix of kaolinite, quartz, feldspar and other minerals.
• Common clay – As the name suggests, common clays are widely available. However, they need additives to improve their brick-making properties.

The key properties that make these clays suitable for brick manufacturing are plasticity and shrinkage capacity. Plasticity refers to the ability of clay to be molded without cracking or losing its shape. Shrinkage capacity allows the bricks to harden and become denser and stronger during firing without distortion.

Clay Preparation

Before clay can be used to make bricks, it must go through an extensive preparation process. This process transforms raw clay into a material with the right consistency and properties for brick making.

The first step is digging the clay from quarries or pits. The clay is extracted from deposits near the earth’s surface, usually from open mines or quarries. The quality and composition of clay can vary significantly depending on the source.

After digging, the clay goes through a weathering process. This involves exposing the clay to the air and elements, allowing it to disintegrate and break down further. Weathering improves the plasticity and workability of the clay. The clay is left exposed to the weather for weeks or months depending on the initial quality.

After adequate weathering, the clay is mixed with water to achieve an even consistency. Water is added and mixed thoroughly to disperse lumps and dissolve soluble salts. The clay is then left to soak for sufficient hydration. Enough water is added to make the clay moldable and workable.

The next step is tempering, which improves the strength and plasticity of the clay. The clay mixture is wedged and kneaded to achieve a uniform consistency. This aligns the clay particles and removes air pockets. Tempering helps remove excess water and improves the workability of the clay before molding.

Molding Bricks

After the clay is prepared, the next step is to mold the bricks. There are several types of molds that can be used:

• Wooden molds – These are handmade wooden frames without bottoms. The interior dimensions match the desired size of the brick.
• Metal molds – Made of cast iron or steel, these durable molds allow for efficient, consistent brick sizes.
• Plastic molds – Cheaper but less durable than metal, plastic molds are a lightweight option suited for small-scale production.

The molding process involves the following steps:

1. The mold is lightly coated in sand or ash to prevent sticking.
2. Clay is pressed into the mold cavity using a wooden tamper or mallet.
3. A strike-off bar is used to level off excess clay.
4. The mold is overturned onto a pallet or board, releasing the molded brick.
5. The process is repeated until the desired number of bricks are made.

Proper compression and filling of the mold ensures the bricks hold their shape in the subsequent drying and firing process.

Drying

After bricks are molded, they must undergo a drying process to remove moisture before being fired. There are several methods for drying bricks:

Airing

One of the oldest and simplest drying methods is to air-dry bricks. The molded bricks are stacked in the open with space between them to allow air circulation. This method relies on natural airflow and sun exposure to slowly evaporate the moisture over several weeks.

Solar Drying

Solar drying involves drying bricks in transparent roofed structures that trap heat from the sun. This passive solar heating speeds up evaporation. Vents allow moisture to escape while trapping the heat. Solar dryers can dry bricks in approximately one week.

Artificial Drying

Artificial drying uses kilns or tunnels with heaters to actively dry bricks. The temperature is controlled to drive off moisture gently without cracking the bricks. With artificial drying, bricks can be dried in just 1-2 days. This speeds up production but requires more energy input.

Choosing a drying method involves tradeoffs between speed, energy use, and investment in equipment. Airing is the simplest approach but very slow. Solar drying accelerates natural drying at low energy cost. Artificial drying is the fastest but most energy intensive. Many modern brick plants use a combination of natural, solar and artificial drying.

Firing

The next step in brick making is firing, which hardens and stabilizes the clay bricks. Firing typically takes place in special ovens called kilns that can withstand the high heat required for firing clay. There are several types of kilns used for brick firing, but they generally rely on heating the bricks to temperatures between 900-1200°C.

The firing process causes chemical reactions that vitrify and harden the clay. As the temperature rises, the clay undergoes oxidation, decomposition of organic matter, dehydration, and the formation of new crystalline structures. The kiln’s atmosphere also affects the reactions. An oxidizing atmosphere will burn off carbon, causing brick colors to lighten. A reducing atmosphere limits oxygen, producing darker bricks.

Firing times depend on the type of clay and desired properties of the finished bricks. Fast-firing light-colored clays may require 6-8 hours at peak temperature. Denser clays used for facing bricks often require 24 hours or more at maximum heat. The entire firing process can take several days when accounting for steadily increasing and decreasing the kiln temperature.

Well-fired bricks will have an even color distribution and a hard, durable structure. Underfired bricks risk cracking, warping, or crumbling more easily. Careful control over the firing time, temperature, and kiln atmosphere is crucial to achieving strong fired clay bricks.

Properties of Fired Bricks

Bricks made from clay develop several important properties during the firing process that determine their quality and suitability for construction purposes.

Strength

Firing bricks at high temperatures results in strong ceramic bonds between clay particles, giving bricks high compressive strength. Well-fired bricks can typically withstand loads of 4,000 psi or more without crumbling. Strength is important for structural integrity.

Porosity

The level of porosity depends on the clay composition and firing temperature. More porous bricks absorb more water, which can decrease durability but can also help with insulation. Dense bricks with low porosity provide greater structural strength.

Hardness

Firing makes bricks incredibly hard and durable. Typical bricks achieve a Mohs hardness of 7-8, approximately the same hardness as granite. This allows them to withstand abrasion and breakage over time.

Advantages of Bricks Made from Clay

Clay bricks offer several advantages that make them a popular construction material. Some of the main benefits of clay bricks include:

Durability – Fired clay bricks are very durable and can last for centuries. The firing process permanently hardens the bricks, making them extremely weather and impact resistant. Properly made clay bricks do not decay or degrade over time.

Availability – Clay suitable for brickmaking is abundant and found worldwide. Unlike some other construction materials, the raw materials for clay bricks do not need to be imported or transported far distances. Clay bricks can be produced locally in most regions.

Cost – Clay is an inexpensive raw material, and the production process for clay bricks is not complex. This makes clay bricks one of the most economically feasible construction materials. The local availability of clay also helps keep costs down.

Disadvantages of Clay Bricks

Although clay bricks have been used successfully in construction for thousands of years, they also come with some drawbacks. Two of the main disadvantages of bricks made from clay are their weight and brittleness.

Clay bricks are much heavier than other common building materials such as wood, concrete, or steel. The average clay brick weighs about 5 pounds. This adds a lot of dead load weight to a structure built with bricks. More weight requires a stronger foundation and support structures. This increases the material costs of brick buildings.

Fired clay bricks are also quite brittle. They have low tensile strength and can crack under stresses. Brick masonry walls do not stand up well to lateral loads, such as earthquakes or high winds. They need extra bracing and reinforcements to provide ductility and prevent collapse. The brittle nature of bricks also leads to wear and damage over time. Bricks can chip, crack, or break if subjected to impact.

Conclusion

Clay has been used to make bricks for thousands of years and continues to be an important building material. The process involves extracting clay, preparing it, molding it into the desired shape, drying, and firing in a kiln. Fired clay bricks have many desirable properties like durability, fire resistance, and thermal mass. However, clay bricks also have some disadvantages like being damaged by moisture and the firing process producing greenhouse gases.

Looking to the future, clay bricks will likely continue being a useful material, especially in developing areas without access to modern building materials. Efforts can be made to improve the sustainability of clay brick production by using cleaner kilns and exploring renewable energy sources for firing. More research into improving clay brick properties is worthwhile, as there are still areas where traditional fired clay can be enhanced. Overall, the simplicity, availability, and versatility of clay means it will have an ongoing role in brickmaking.