3.1: Types of Clay and Their Properties

Understanding Different Clay Bodies

Raw Materials in detail

The formula of clay varies depending on its composition, as clay minerals can differ in their chemical makeup. However, a generalized formula for clay minerals, such as kaolinite, is:

Al2Si2O5(OH)4

This formula represents the basic structural unit of kaolinite, which consists of one aluminum (Al) atom surrounded by four oxygen (O) atoms in a tetrahedral arrangement, and two silicon (Si) atoms surrounded by four oxygen (O) atoms and one hydroxyl (OH) group in an octahedral arrangement. This basic unit repeats to form the crystal lattice structure of the clay mineral.

It's important to note that clay can also contain other elements and impurities, such as iron, magnesium, calcium, and potassium, which can influence its properties and behavior.

Clay, stoneware, and porcelain are all types of ceramic materials, but they have distinct compositions and properties.

1. Clay: Clay is a natural material composed of finely grained minerals and organic matter. It is formed through the weathering and decomposition of rocks rich in aluminum silicates. Clay is plastic when wet, allowing it to be molded and shaped, and it hardens when fired at high temperatures.

2. Stoneware: Stoneware is a type of clay-based ceramic that is fired at a high temperature, typically between 2,100°F and 2,300°F (1,150°C and 1,260°C). It is known for its durability, strength, and non-porous nature. Stoneware clay contains a higher percentage of flux materials, such as feldspar or talc, which vitrify during firing, resulting in a dense, waterproof surface.

3. Porcelain: Porcelain is a type of ceramic made from a specific type of clay called kaolin, along with other materials such as feldspar and silica. It is fired at a very high temperature, typically above 2,300°F (1,260°C), which causes the clay particles to vitrify and fuse together, resulting in a translucent, glass-like appearance. Porcelain is prized for its strength, whiteness, and ability to hold intricate details.

While clay, stoneware, and porcelain are all ceramic materials, they are not the same. They vary in composition, firing temperature, and properties. Clay is a broad term that encompasses various types of natural materials, while stoneware and porcelain refer to specific types of ceramic made from different types of clay and fired at different temperatures.

Clay is a naturally occurring material composed primarily of fine-grained minerals, organic materials, and water. Its scientific makeup varies depending on its composition, but generally, clay consists of hydrated aluminum silicates, such as kaolinite, illite, and montmorillonite. These minerals are formed through the weathering and decomposition of rocks, particularly feldspar-rich rocks like granite, and are typically found in sedimentary deposits.

The key difference between dirt and clay lies in their composition and physical properties. While dirt is a more general term that encompasses various types of soil and organic matter, clay specifically refers to a type of soil characterized by its small particle size and plasticity when moistened. Clay particles are much finer than those found in dirt, giving clay its unique properties, such as the ability to be molded and shaped when wet and to retain its form when dry.

Clay can be found naturally in a variety of environments, including riverbeds, lakebeds, coastal regions, and areas with volcanic activity. It is often associated with sedimentary deposits and can be mined or extracted from the earth's surface or subsurface. Additionally, clay deposits may be found in areas where geological processes have concentrated clay minerals over time, such as in areas with high levels of erosion or sedimentation.

Additional Reading Recommendations:

Wild Clay by Hitomi Shibata, Matt Levy, and Takuro Shibata

Clay and Glazes by Daniel Rhodes

Primary clays and secondary clays are two classifications of clay based on their geological formation and composition:

1. Primary Clays:

- Primary clays are clays that form directly from the decomposition and weathering of rocks in their original location.

- They are typically composed of pure clay minerals and have undergone minimal transportation or alteration from their original source.

- Primary clays tend to have high plasticity and are often used in ceramic production due to their favorable properties for shaping and firing.

2. Secondary Clays:

- Secondary clays are clays that have been transported from their original source by natural processes like water or wind and deposited elsewhere.

- During transportation and deposition, secondary clays may undergo physical and chemical changes, such as mixing with other materials and the loss of some clay minerals.

- As a result, secondary clays often contain impurities, such as sand, silt, and organic matter, which can affect their plasticity and firing characteristics.

- Secondary clays are generally less desirable for ceramics compared to primary clays but can still be used in certain applications, especially when mixed with other materials or processed to remove impurities.

In summary, primary clays form directly from the weathering of rocks and are typically pure in composition, while secondary clays are transported and deposited elsewhere, often containing impurities due to their geological history.

Quick Understanding

- Earthenware:

This low-fire clay is soft and porous, making it ideal for beginners. It’s often used for functional pottery like flowerpots and decorative tiles. Earthenware is available in red or white varieties, depending on its iron content. Glazes applied to earthenware require a clear topcoat to make them waterproof.

- Stoneware:

A versatile, high-fire clay known for its strength and durability. Stoneware is suitable for making functional items such as mugs, plates, and bowls. It can range in color from white to gray to brown, depending on the clay’s mineral content. Fully vitrified stoneware becomes non-porous and ideal for everyday use.

- Porcelain:

Porcelain is a fine, white clay that produces smooth, translucent results after firing. It’s challenging to work with because of its low plasticity (it can tear or collapse easily). Porcelain is often used for detailed sculpture and delicate functional items like teacups. It requires high firing temperatures, often exceeding 2,300°F, to achieve its glass-like finish.

Plasticity and Workability

- Plasticity Defined:

Plasticity refers to how easily clay can be shaped and molded. Highly plastic clays, like porcelain, allow for intricate detail but can be prone to cracking. Less plastic clays, like heavily grogged stoneware, are more stable but harder to manipulate. To increase plasticity, add bentonite or ball clay to your clay body.

- Tips for Testing Plasticity:

Roll out a thin coil of clay and bend it into a U-shape. If it cracks or breaks, the clay lacks plasticity and may need adjustment. Use this test to evaluate reclaimed clay or new clay bodies before committing to large projects. Practice wedging thoroughly to improve the clay’s workability.