What is Clay?
Clay is a naturally occurring mixture of minerals and traces of metal oxides and organic matter. These clay particles have a crystalline structure of extremely fine grains. When combined with water, this enables the flat particles to arrange themselves and create a thick, pliable, mouldable substance ideal for crafting pottery.
Why are Clay Minerals Important
Clay minerals play a vital role in pottery due to their fine, flat particles and distinct structures that offer important qualities such as plasticity (the ability to be moulded with water), cohesiveness, water retention, and specific firing attributes (including colour, strength, and durability). These factors convert pliable clay into robust ceramic items.
The main clay minerals we find are Kaolinite, Illite and Montmorillonite, which we know as bentonite. These clay minerals influence the clay’s properties and final appearance. These minerals enable potters to create particular textures, colours, and strengths tailored for diverse kinds of pottery.
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Key Traits of Clay Essential for Pottery
Plasticity and Malleability
When wet, it’s soft and plastic, that is, malleable and mouldable and, at the same time, slippery.
Drying and Shrinkage
When dry, its clay particles get closer, causing it to shrink and very often crack during drying and kiln firing. Understanding what occurs to clay is critical for avoiding clay and fire defects, such as cracking, breaking, exploding, and glaze defects.
Vitrification and Durability
Fired clay becomes dense, solidifying into a ceramic material. It becomes hard, strong, and water-resistant, making it ideal for functional uses.
The Process of Making Clay for Pottery
Specific Use
The starting materials for making clay for pottery are not just plain clay; instead, they consist of a blend of materials known as the clay body. This mixture includes various types of clay along with additional materials that are tailored for specific uses.
For instance, some kinds of clay bodies are more suitable for extrusion than for throwing, or for slip casting rather than slab work. Ideally, it’s important to formulate a clay body tailored to the specific application you intend to use it for.
Firing Temperature
Clay bodies can also be formulated for a particular firing temperature. For instance, clays created for Cone 6 mid-fire, or earthenware clays, for softer temperatures up to 1100°C.
Firing Methods
Different clay bodies suit different firing methods (such as wood firing, electric firing, gas firing) by aligning their properties, like porosity, vitrification, thermal shock resistance, to the required firing temperatures and techniques. For instance, earthenware is designed for low-fire applications where vibrant colours are desired; stoneware produces sturdy, adaptable mid-to-high-fire functional items; whereas porcelain focuses on crafting high-fired pieces that are translucent and robust. Additionally, specialised clays such as Raku clay can endure intense thermal shock during quick cooling processes, while bodies intended for woodfire/salt glazing are created to achieve specific atmospheric effects.
Specific Handling
Clay bodies can be customised for specific clay handling. Earthenware is formulated for hand-building and sculpting purposes. Stoneware produces sturdy pieces crafted on the potter’s wheel, fired at mid- to high-temperature ranges. Porcelain is ideal for creating fine, smooth, and translucent items, whereas Ball Clay enhances plasticity and Fire Clay offers heat resistance.
Specific Final Fired Properties
Clay bodies can be developed for specific final-fired properties, such as a particular colour or texture.
Types of Clay Used by Potters
The three general types of clay that potters use are: Earthenware (including red earthenware and white earthenware), Stoneware and Porcelain. There are also some specialised clays, such as Raku, Paper Clay, and Bone China.
General Characteristics of the Main Types of Clay
Earthenware
Earthenware clay is characterised by its softness, high plasticity (which allows for easy shaping), and the need for low-temperature firing. It is commonly used for decorative objects. However, because it is porous, glazing is required to make it suitable for holding liquids; this makes it less sturdy than stoneware but surprisingly capable of withstanding thermal shocks.
Key Attributes:
Low Firing Temperature: Fired within the range of 982°C to 1150°C (1800°F to 2100°F).
Porosity: Remains absorbent unless glazed; ideal for plant containers, but requires glazing for functional items like mugs.
Colour Variations: Typically shows reddish-brown hues (terra cotta) but can also appear white, buff, or yellow depending on mineral composition and firing conditions.
Plasticity: Very manageable and easy to shape while maintaining form effectively.
Durability: Generally softer than stoneware and not as durable, yet it handles sudden temperature shifts remarkably well.
Applications: Frequently used for garden pottery, tiles, ornamental pieces, and artistic pottery where vivid colours and ease of manipulation are valued.
Stoneware
Stoneware clay is a resilient and adaptable ceramic recognised for its density, non-porous nature when fired, and sturdy composition, making it well-suited for functional pottery such as dinner sets and cooking utensils. Its notable features include a high firing temperature of 1200°C to 1300°C (approximately 2200°F to 2500°F), the capacity to transform into a firm, stone-like substance through vitrification, earthy hues (including buff, grey, and brown), and exceptional plasticity that aids shaping.
Stoneware often incorporates impurities, such as grog or sand, to enhance texture. Compared to earthenware, it has lower absorbency and can be finished either unglazed or glazed, as preferred.
Key Attributes:
Durability: After firing, it becomes extremely hard and durable, resisting scratches.
Low Porosity: When fired, stoneware is dense and water-resistant—ideal for storing food or liquids.
High Firing Temperature: It matures at mid-range to high temperatures (Cone 4-10), leading to effective vitrification.
Colour Range: In its unfired state, it’s typically grey; once fired, it shows colours from buff to brown depending on the impurities used during the process.
Plasticity: Offers excellent working characteristics due largely to the inclusion of ball clay along with grog for improved handling abilities and textural finish.
Vitrification: At elevated temperatures, this process results in a glass-like non-porous condition that enhances overall strength.
Porcelain
Porcelain clay has an exceptionally fine, white, smooth texture, along with high strength and the ability to be translucent when fired at elevated temperatures, approximately 1200°C to 1400°C (2380°F to 2460°C). It is delicate and requires good control, low plasticity—which can be enhanced with the addition of bentonite—high shrinkage during firing, and a distinctive ringing sound when tapped. This type of clay is perfect for creating elegant, robust, and intricate ceramics; however, it poses handling challenges due to its susceptibility to distortion.
Key Attributes:Â
Colour: Initially appears white or grey but transforms into a pure, brilliant white after firing; can be translucent if kept thin.Â
Texture: Composed of extremely fine particles that are dense and smooth.Â
Workability (Wet): Soft and pliable yet highly sensitive to water; easily loses shape or dries out rapidly.Â
Firing: Requires high temperatures for vitrification (over 2300°F/1260°C), resulting in glass-like qualities.Â
Durability: Remarkably strong and hard while being non-porous as well as resistant to chemical exposure and thermal shock post-firing.Â
Shrinkage: Exhibits a considerable shrinkage rate of up to 16%.Â
Sound: Emits a bell-like tone when tapped.
How to Formulate a Clay Body
Clay bodies are made up of three types of clay components: clays, fluxes and silica.
Clay
Clay is the main component, offering the essential structure and plasticity. Kaolin is frequently used in porcelain formulations due to its high purity (characterised by its white hue and minimal impurities). Occasionally, ball clay is incorporated to enhance plasticity.
Flux
Feldspar is the most commonly used flux. It helps to lower the melting temperature of silica and other minerals in the clay body. It promotes vitrification (the process of converting clay into a more glassy, non-porous composition) at the desired fire temperature.
Silica
Silica is a glass-forming material. It is what eventually fuses during fire, resulting in a tight, strong ceramic matrix.
In addition to these fundamental elements, various materials can be incorporated to refine clay formulations. Grog, which consists of pre-fired crushed clay and sand, help decrease shrinkage while enhancing texture. Fireclay provides strength at elevated temperatures, whereas plasticisers such as bentonite enhance the material’s workability.
Talc is useful for managing thermal expansion, especially in low-fire clays. Furthermore, stains can be introduced to achieve vibrant colours. These additives allow ceramic artists to modify their clay’s handling properties and firing behaviours according to their particular requirements.
How to Develop a Clay Body
Currently, most potters use commercially processed clay bodies. Suppliers combine and sift raw materials to manage characteristics such as plasticity, shrinkage, and colouration. For those starting out, using a ready-made clay body streamlines the process and minimises unexpected outcomes. As you become more skilled, you might consider experimenting with combinations of your own clay bodies or incorporating additives such as grog or sand to customise the clay to your preferences.
Important Properties to Consider When Developing a Clay Body
When creating your own clay body, it’s crucial to clearly understand and specify its intended use, the firing temperature, the firing methods you plan to use, and the unique attributes you desire for your finished product. Key factors such as plasticity, shrinkage, absorption (porosity), and vitrification play a significant role in how clay behaves.
Plasticity: Carefully evaluate your clay’s ability to be shaped without cracking and keep its shape. Consider the broad range of particle sizes to allow them to pack together and fill the space efficiently. For instance, the use of finer, plate-shaped particles (such as ball clay) will enhance plasticity, whereas coarser particles (like pure kaolin) will diminish it.
Shrinkage and Drying: All types of clay undergo shrinkage when transitioning from wet to dry as moisture evaporates, and again during the firing process as the particles bond. The overall shrinkage can typically range between approximately 10% to 15%, or even higher, depending on the specific clay used. Porcelain varieties may experience even greater shrinkage, potentially reaching up to 20%.
Porosity: Absorption indicates the extent to which the fired clay can retain water, revealing its degree of porosity or sealing. Earthenware may exhibit an absorption rate of 10% or more, making it relatively porous; in contrast, stoneware typically ranges from 0.5% to 5%. Porcelain often has an absorption rate below 0.5%, indicating it is effectively vitrified.
Vitrification: Vitrification is the process by which clay transforms into a glass-like, non-porous form when heated to elevated temperatures. Every type of clay has an optimal “maturing” temperature range, which is the specific temperature at which it achieves adequate vitrification without distortion or melting.
Stoneware and porcelain can reach full vitrification during firing, while earthenware typically attains only partial vitrification, resulting in increased porosity. The degree of vitrification influences the material’s strength, absorbency, and longevity. Excessive firing may lead to issues such as bloating or collapse, whereas insufficient firing can result in fragility and high absorbency.
After formulating the clay, test it with the Triaxial Test.
Triaxial Glaze Blending
A triaxial blend diagram is a triangular graph (or a road map of experimentation) utilised in ceramics to methodically explore every potential combination of three distinct materials or glazes, with each corner representing 100% of one material. Each point within the chart signifies a specific recipe where the proportion of the three ingredients totals 100%. John Britt from Ceramic Arts Network presents a visual guide of the process (for glaze mixing) on a YouTube video. Â
Source: Ceramic Arts Network
When you are developing completely new clay recipes, or glaze recipes, you need some kind of system for keeping track of your samples and organising them to cover all the different combinations of the materials you are working with. The Triaxial Diagram will allow you to visualise your tests.
A triaxial blend diagram for pottery clay uses the characteristics of mixtures that comprise three primary components (such as clay, feldspar, and silica/quartz) arranged on a triangular grid. Each corner represents 100% of one ingredient, demonstrating how properties like strength, colour, and melting point vary throughout the triangle.
This tool is beneficial for empirically developing clay bodies or glazes. It involves conducting tests along the edges (referred to as line blends) and then exploring combinations within the triangle’s interior, which can yield unexpected outcomes and help identify ideal compositions for materials with undetermined chemical compositions.
Conclusion
Understanding the nature of clay and selecting the appropriate type for your pottery project is an essential process that impacts the results of your work. By familiarising yourself with the characteristics and uses of various clays, you can make knowledgeable choices that reflect both your creative aspirations and practical requirements. Whether you choose the classic appeal of earthenware, the strength of stoneware, the refined elegance of porcelain, or some specialised clay bodies—or even venture to create your own clay body—each option presents distinct avenues for exploration.
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