What Are The Fibers In Clay?

Clay is a naturally occurring material composed primarily of fine-grained minerals, such as illite, kaolinite, and montmorillonite. It is formed over long periods as a result of the weathering and erosion of rocks. Clay contains several crystalline silicate minerals micronized over time to particle sizes less than 4 micrometers in diameter. The composition and mineral content of clays can vary widely, resulting in different physical properties and uses.

Clays exhibit plasticity when mixed with water in certain proportions. This allows clay to be shaped into various forms for making pottery, ceramics, bricks, and other objects. When fired at high temperatures, the clay hardens permanently into a ceramic material. Clay bodies can be modified by adding non-plastic materials such as grog or sand to improve workability and fired properties.

Common Clay Fibers

Some of the most frequently used fibers in clay are derived from plants. These natural fibers come from readily available agricultural byproducts and have been used for centuries to reinforce and strengthen clay.

Straw is one of the most common plant fibers used in clay bodies. After crops like wheat, rice, oats and barley are harvested, the dried straw stalks remain. These stalks are typically 1/2 to 2 inches long and can be chopped and added to clay. The tubular shape of straw fibers improves airflow and drying in the clay. Straw fibers burn away during firing, creating small voids and a more porous fired clay body.

Wood fibers like sawdust, wood shavings and finely ground wood powder can also be incorporated into clay. Soft woods generally work better than hard woods. As the wood burns out during firing, it leaves behind carbon that gives the clay a dark, richer color. The carbon also helps clay vitrify at lower temperatures.

Manmade Fibers

Two common manmade fibers used in clay are nylon and polyester.

Nylon fibers are strong, flexible synthetic polymers that resist stretching and shrinking. They are abrasion resistant and unaffected by moisture. When blended into clay bodies at up to 3% by volume, nylon fibers reduce cracking and increase impact resistance during drying and firing. However, nylon can create small air pockets in the clay that lead to pitting.

Polyester fibers are made from polyethylene terephthalate polymers. They have high tensile strength and are resistant to stretching. Polyester is chemically stable, maintaining strength and shape through drying and firing cycles. Like nylon, polyester is added to clay bodies at around 3% by volume to reduce cracking and add durability. The fiber length and aspect ratio of polyester can be optimized for increased fracture toughness.

Natural Fibers

Natural fibers found in clay are derived from both animal and mineral sources. Some of the most common natural fibers used in clay bodies include:

Animal Hair

Animal hair such as horsehair is sometimes added to clay to produce lightweight and insulating ceramic pieces. The hair burns out during firing, leaving behind tiny air pockets and channels that make the material porous. This reduces the weight and increases thermal insulation properties compared to non-porous ceramics.

Mineral Fibers

Asbestos is a natural silicate mineral fiber that was historically used in some clay bodies. However, asbestos is now known to be hazardous to human health and no longer commonly used. Other natural mineral fibers like wollastonite may still be incorporated in small amounts as reinforcing agents. The mineral fibers can improve mechanical strength and fracture resistance.

Fiber Lengths

The length of fibers added to clay has an important effect on the final properties of the clay body. Fibers are typically classified into two main categories based on length:

Short Fibers

Short fibers are generally defined as being less than 1/16th of an inch (1.5mm) long. Common examples of short fibers used in clay are wood fibers and cotton linters. The short length means these fibers are not oriented in the clay body and act more like non-fibrous fillers. Short fibers help prevent cracks from propagating in drying clay. They also improve the green strength of unfired clay. However, short fibers do not significantly improve the strength of fired clay. They also have little impact on clay shrinkage.

Long Fibers

Long fibers are longer than 1/16th of an inch (1.5mm). Examples are plant fibers like hemp or synthetic fibers like glass or carbon fiber. Due to their length, long fibers become oriented in the direction of clay flow. This orientation means long fibers are very effective at reducing drying shrinkage. Long oriented fibers also dramatically increase the strength and toughness of fired clay bodies. However, long fibers are harder to mix evenly into clay and can cause issues with extrusion. Overly long fibers may also lead to surface defects if they become exposed.

Fiber Orientation

The orientation of fibers within a clay body can significantly impact the properties and workability of the clay. Fibers are typically either randomly oriented or aligned in a particular direction.

Random fiber orientation occurs when the fibers are distributed in all directions throughout the clay. This is the default orientation when fibers are simply mixed into a clay body. Random orientation provides reinforcement in all directions, improving overall strength, toughness and crack resistance.

Aligned fiber orientation is when the majority of fibers are arranged in the same direction. This can be achieved by extruding or rolling the clay to induce fiber alignment. Aligned fibers provide maximum reinforcement in the direction of orientation, which can produce high strength and stiffness along that axis. However, properties perpendicular to fiber alignment are typically weaker.

The desired fiber orientation depends on the final application. For example, aligned fibers are advantageous for handles and spouts that need strength and stiffness in one direction. Random orientation is better for overall structural integrity in sculptural and architectural ceramics.

Effects on Clay

Adding fibers to clay bodies can significantly impact the properties and behavior of the clay. Some key effects that fibers have on clay include:

Strength

One of the main reasons for adding fibers to clay is to increase the clay’s strength. Fibers act to reinforce the clay matrix, improving green strength as well as fired strength. The long fibers help hold the clay particles together, increasing resistance to cracking and breakage.

Plasticity

Fibers tend to decrease the plasticity of clay. Plasticity refers to how easily the clay can be deformed when wet. Highly plastic clay is very stretchy and malleable. The addition of fibers makes the clay stiffer and less plastic.

Drying

Fibers also affect the drying behavior of clay. In particular, fibers help reduce shrinkage and cracking during drying. As the clay loses moisture and starts to shrink, the fibers provide internal support to resist deformation. This results in less warping and cracking as the clay dries.

Fiber Volume

The amount of fibers added to clay is usually measured as a percentage of the total volume. Most clay bodies contain 0.5-5% fiber content by volume. More fibers provide increased strength, but too much can negatively impact workability and drying.

A typical stoneware body may have 1-3% fiber content. Higher than 5% is not common, as the clay becomes too “hairy” and hard to work with. However, some specialized clays go up to 8-10% fibers for maximum durability.

Lower percentages around 0.5-1% are more common for porcelains and some sculpture clays where some added strength is desired but workability is prioritized. Going above 3% for porcelains is rare.

The optimal percentage depends on the clay body, the fibers used, and the desired characteristics. Finding the right balance is important to gain strength benefits without sacrificing too much workability.

Clay Bodies With Fibers

Clay can be combined with fibers to create clay bodies with unique properties. Some examples of clay used with fibers include:

Stoneware clay – Stoneware clays are often mixed with fibers like nylon or polypropylene to increase strength and durability. The fibers help the clay resist cracking and crazing during drying and firing. Up to 3% fiber content may be added.

Porcelain – Porcelain can be reinforced with fiber to help minimize warping and add strength. Fibers like silica fiber or organic fibers are commonly used in amounts up to 2%. The fiber increases porcelain’s resistance to thermal shock.

Earthenware – Natural fibers like straw and hair are sometimes incorporated into earthenware clay bodies in small amounts. This can improve dry strength and fire out at low temperatures leaving small voids. Up to 5-10% fiber content may be used.

Paper clay – Paper fibers comprise a major component of paper clay bodies, typically around 25-75% of the total volume. Cellulose fiber from paper allows the clay to be lightweight and strong in both wet and dry states.

Refractory fibers – Refractory ceramic fibers made of alumina-silica or alumina-boria-silica are used to reinforce refractory clay bodies designed for very high temperature applications.

Uses of Fiber Clay

Fiber clay has several important uses, especially in sculpture, pottery, and construction.

For sculpture, fiber clay enables artists to create large-scale and freestanding sculptural works that would not be possible with regular clay. The addition of fibers provides tensile strength, allowing sculptures to stand freely without support even when very tall or with extended parts. The fibers also reduce cracking and breakage when sculpting complex shapes. Many modern sculptors utilize fiber clays like paperclay to create monumental yet intricate sculptures.

In pottery, fiber clays allow the creation of lighter, thinner, and larger vessels. Fibers give strength and structure to thin-walled pots, enabling very large pieces to be lightweight and functional. The added durability also reduces cracking and warping during drying and firing. Many ceramic artists incorporate paper pulp or other fibers when throwing large pots and platters.

For construction applications, fiber clay is used to create bricks, tiles, and other building materials that are lighter and more resistant to cracking than standard clay. Adding plant fibers creates construction materials like adobe that have improved durability and insulating properties. Fiber clay construction materials are often used in sustainable building projects.