What Makes The Clay?

Clay is a naturally occurring material composed primarily of fine-grained minerals, which exhibit plasticity and harden when fired or dried. The most common clays include kaolinite, montmorillonite-smectite, and illite. Clays are distinguished from other fine-grained soils by differences in size and mineralogy.

Some key properties that make clay a versatile material include its plasticity, shrinkage on drying, cohesion, and ability to absorb water. These attributes allow clay to be molded into a variety of shapes and forms while maintaining its structural integrity after firing. The firing process permanently hardens the clay body through sintering.

Due to its abundance, versatility, and durability, clay has served a wide range of purposes for thousands of years. It is one of the oldest building materials, and has many uses including pottery, bricks, tiles, and porcelain. Clay is also a vital raw material in the manufacturing of cement and detergents. In modern times, specialized clay types are used in diverse applications ranging from cosmetics to drilling fluids.

Clay Composition

The primary component of most clays is clay minerals. These are hydrous aluminum phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. The most common clay minerals include kaolinite, montmorillonite-smectite, illite, and chlorite.

Clay minerals are formed over long periods of time through the chemical weathering of rocks. They are fine-grained and plate-shaped. The particle size of clays is referred to as clay-size, defined as less than 2 micrometers. This contributes to clays’ high surface area, which gives them unique properties of plasticity, shrinkage, swelling, and adsorption.

Plasticity refers to the ability of clays to be molded when wet, then maintain their shape when dried. This makes clay ideal for pottery and ceramics. Shrinkage and swelling refers to clays expanding when wet and shrinking when dried. Adsorption is the ability of clays to adhere to molecules on its surfaces.

The four main types of clays are kaolinite, montmorillonite-smectite, illite, and chlorite. Kaolinite is the softest and least plastic. Montmorillonite is the most plastic and has the highest adsorption. Illite and chlorite have properties in between kaolinite and montmorillonite.

Clay Formation

Clay is formed from the weathering and decomposition of rocks over long periods of time. The weathering process breaks down rocks into finer and finer particles. There are two main types of weathering that create clay:

Chemical weathering – This involves chemical changes in minerals that make up rocks. Water interacts with minerals, causing chemical reactions that transform rocks into clays. For example, feldspar, a common mineral in granite, chemically weathers into kaolinite, a key component of clay.

Physical weathering – This involves physical breakdown of rocks from temperature changes, freezing/thawing cycles, plant roots growing, etc. The physical forces break rocks into smaller and smaller pieces.

After rocks weather and break down into fine particles, they are transported by wind, water, gravity and ice to depositional environments like river deltas, lakes, oceans, and floodplains. Here the clay particles settle out and accumulate over time. The weight of overlying sediments compresses the deposits, causing the clay particles to compact together to form clay.

The rate of weathering and type of source rocks influence the kinds of clay minerals that form. Higher rainfall and warmer temperatures accelerate chemical weathering. Granite and feldspar-rich rocks weather into kaolinite, while volcanic ash and basalt weather into montmorillonite and illite. The geologic processes that create clays take place over millions of years.

Properties of Clay

Clay has several unique properties that make it an ideal material for pottery, sculpture, and construction. The key properties of clay are:


Plasticity refers to the ability of clay to be molded, shaped, and deformed without cracking or breaking apart. When clay contains water, its plate-like mineral particles can slide over one another, allowing the clay to be worked into different forms. Plasticity allows clay to hold its shape once formed.


Cohesion refers to the ability of clay particles to stick together even when wet. This allows clay to be molded into solid objects that retain their shape without crumbling. The cohesive properties of clay are what enable potters to shape clay vessels and craftsmen to form bricks and tiles out of clay.


Clay can readily absorb significant amounts of water, allowing it to become pliable and workable. The absorption is made possible by the small size and electrically charged surface of clay particles. When clay absorbs water, its volume increases. As it loses moisture, the volume decreases and shrinkage occurs. Controlling water absorption and evaporation allows firing of clay into ceramic objects.


As clay loses moisture, it shrinks in volume. This shrinkage occurs because water exits the spaces between clay platelets as it dries. Shrinkage continues during kiln firing of clay. The amount and rate of shrinkage depends on the clay composition and must be carefully controlled during ceramics production. Too much shrinkage can lead to cracking and breakage.

Types of Clay

There are several different types of clay distinguished by their mineral composition. Some of the most common and important types of clay include:


Kaolinite is a mineral clay composed of hydrated aluminum silicate. It is formed from the chemical weathering of rocks like granite. Kaolinite clays are often white or light-colored and have low plasticity. They are valued for their refractory properties and are a key ingredient in porcelain.


Montmorillonite is composed of hydrated sodium, calcium, magnesium, or aluminum layered silicate. It forms from the decomposition of volcanic ash. Montmorillonite has very high plasticity and is able to absorb large amounts of water. It is commonly used in industrial applications including drilling fluids.


Illite is a non-expanding clay composed of hydrated aluminum, potassium, magnesium, iron silicate. Its structure is similar to mica. Illite forms from the weathering of silicate minerals like feldspar. It is an important component in common clays used for ceramics and construction.


Bentonite is a type of montmorillonite clay consisting mostly of the mineral sodium. It forms from the weathering of volcanic ash in marine environments. Bentonite can absorb large amounts of water and expand significantly. It is used for drilling, clarifying wines and oils, and as a binding agent.

Ball Clay

Ball clays are kaolinite clays with high plasticity and high fired strength. They consist of hydrated aluminum silicate with minor amounts of other minerals. Ball clays are often used in clay bodies and ceramic glazes.

Fire Clay

Fire clays are composed primarily of kaolinite with some illite and montmorillonite. They have high refractoriness, meaning they can withstand very high temperatures without deforming. Fire clays are essential for making kiln linings, fire bricks, and ceramics requiring high firing temperatures.

Clay Uses

Clay has been used for thousands of years to make pottery, bricks, ceramics, paper, cement, cat litter and many other products that are part of our everyday lives. Here are some of the most common uses of clay:

Pottery: One of the oldest uses of clay is to shape it into pottery for holding liquids, cooking food, and decorative purposes. Clay is an ideal material for pottery because it can be molded when wet, maintains its shape when dried, and becomes hard and durable when fired in a kiln. Pottery artifacts dating back over 20,000 years have been discovered, showing just how long clay pottery has been an important part of human culture and civilization.

Bricks: Clay bricks have been used as a durable building material for thousands of years. Bricks are manufactured by shaping clay in molds, then firing them in kilns to harden them. The durability, affordability, and availability of clay has made fired clay bricks an essential building block for construction around the world.

Ceramics: Porcelain and other ceramics are made by shaping and firing special varieties of clay at high temperatures. The finished ceramic products have a hard, durable, glass-like surface and are ideal for making tableware, tiles, figurines, sanitaryware, and electrical insulators.

Paper: High quality paper can be produced using clay as a component. Adding clay fillers to paper pulp results in paper that is smoother, brighter, more opaque, and better for printing. Clay-coated paper is also more resistant to moisture.

Cement: Clay is an important ingredient in Portland cement, a binding agent used in concrete and mortars. The alumino-silicate materials in clay react with calcium during the cement-making process to form compounds that give strength and durability to concrete structures.

Cat Litter: Clumping clay is an absorbent material perfect for cat litter. Its moisture-absorbing properties keep odors under control. Clay cat litter is environmentally friendly and biodegradable too.

Clay Extraction

Clay is extracted from the earth through mining operations. The two main types of clay mining are open pit mining and subsurface mining.

Open Pit Mining

Open pit mining involves removing clay from the ground surface. It is used when clay deposits are located close to the surface. In open pit mining, the topsoil is removed first. Then the overburden, which are the rock layers and soil above the clay seam, are blasted away with explosives. This exposes the clay deposit, which can then be extracted.

Subsurface Mining

When clay deposits are too deep below the surface for open pit mining, subsurface mining methods are used. Common types of subsurface clay mining include shaft mining, drift mining, slope mining, and room and pillar mining. These techniques involve digging tunnels, shafts, or rooms underground to access and extract the clay.


After extraction, raw clay goes through various processing to remove impurities and prepare it for use. Processing may include crushing, grinding, screening, blunging, degritting, refining, aging, and more. These processes improve the clay’s quality and consistency for manufacturing.

Clay Production

The production of clay involves several key steps including purification, mixing clays, forming, drying, and firing. Purification removes impurities and helps improve the plasticity and workability of the clay. This involves processes like blunging which mixes the clay in water to dissolve impurities. The purified clay is then mixed with other clays and materials to create desired properties and colors.

Once purified and mixed, the clay goes through forming which shapes it into products. Forming methods include throwing on a potter’s wheel, slipcasting into plaster molds, extruding through dies, and handbuilding techniques like coil building. The shaped clay pieces are then dried slowly and evenly to remove the moisture content. Drying too quickly can cause cracks and breakage.

The final step is firing the clay in a kiln at high temperatures. Firing hardens the clay through sintering and transforms the raw clay into a finished ceramic material. Different clays require firing at different temperatures based on their composition. The final fired ceramics have increased strength and durability for use in products.

Clay Art and Craft

Clay has been used for artistic expression and crafts for thousands of years across many cultures. Some of the most common uses of clay in art include pottery, sculpture, and jewelry.


Clay pottery is one of the oldest and most widespread art forms made from clay. Clay is an ideal material for making pottery because it can be easily shaped while wet, maintains its form when dried, and becomes hard and durable when fired in a kiln. There are many techniques used in pottery including coiling, slab construction, throwing on a wheel, and slipcasting. Pottery can be left natural colored or decorated using glazes, paints, or other materials. Major types of pottery include earthenware, stoneware, and porcelain.


Clay sculpture utilizes the malleable properties of clay to create three-dimensional works of art. Clay is a popular medium for sculpture because it can be molded into detailed shapes and textures. It also maintains strength and stability when fired. Clay sculptures span a wide range of styles from realistic portraiture to abstract modern designs. Like pottery, sculpture can be painted, glazed, or fired in its natural state. Some sculptors also produce limited edition bronze castings from their clay designs.

Polymer Clay

Polymer clay is a type of modeling clay that cures when baked in a regular oven. It comes in a variety of colors that can be blended and mixed together. Polymer clay is popular for making jewelry, miniature food items, and small decorative objects. It is appreciated for its bright colors and ability to achieve fine detail. A major benefit of polymer clay is that it does not require a kiln for firing.

Clay Jewelry

Jewelry has been crafted from clay for thousands of years. Clay jewelry ranges from traditional beads, pendants, and ear rings to more contemporary designs. Polymer clay in particular has opened new possibilities for intricate and lightweight jewelry. Precious metal clay is also growing in popularity which contains fine silver or gold particles that are revealed when fired. Clay gives jewelry designers tremendous flexibility to shape, texture, and color their pieces.


In summary, clay is an important natural material that has many uses and applications in our world today. Clay is formed from the chemical weathering and decomposition of rocks over long periods of time. The four main types of clay are kaolinite, montmorillonite-smectite, illite, and chlorite. Each type has unique properties that make them suitable for different uses. Clay is a versatile material used for pottery, bricks, ceramics, cements, and as a key ingredient in many industrial and cleaning products.

The most common use of clay is in the production of pottery and ceramics. Clay’s plasticity when wet, and hard durability when fired, make it an ideal material for sculpting and shaping into various forms and objects. For thousands of years, clay has been essential for crafting dishes, pottery, tiles, and other decorative items across the world’s cultures.

On an industrial scale, clay is an essential raw material for manufacturing Portland cement, which is a vital construction material for roads, buildings, and infrastructure. Clay is also used in drilling muds for oil and gas extraction due to its viscosity and ability to lubricate the drilling process.

The unique absorptive properties of clay are harnessed for cat litter, animal feed supplements, wastewater treatment, and purifying oils and fats. Some specialized clays have applications as pharmaceutical ingredients as well.

As an abundant natural resource, clay will continue to be prized for its versatility, usefulness, and sculptural beauty for many centuries to come.

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