Clay Q&A: Our Experts Answer Your Questions
Choosing the right clay starts with understanding the properties that define each clay body. From plasticity and firing temperature to material composition, knowing how clay behaves during forming and firing helps both beginners and experienced potters select materials suited to their projects. Insights into this knowledge ensure your pottery performs as intended, whether you’re creating functional tableware, sculptural forms, or experimental designs.
Choosing the right clay can feel overwhelming, especially with so many terms used to describe different clay bodies. Whether you are a beginner buying your first bag or an experienced potter looking to expand into new techniques, understanding these terms is the key to making smart choices. Below, our experts answer some of the most frequently asked questions about clay so you can confidently select the right one for your project.
What defines “pottery clay”?
Pottery clay is any clay body specifically formulated for use in ceramics. Unlike natural clay that might be dug from a riverbank or construction site, pottery clay is refined, processed, and blended to ensure consistent performance in shaping, drying, and firing.
Raw clay in its natural state often contains impurities such as rocks, roots, or excess sand that make it unsuitable for reliable pottery. Commercial pottery clay, by contrast, has been processed to remove contaminants, carefully balanced for plasticity and strength, and classified according to firing temperature and performance characteristics.
What does “plasticity” mean in pottery clay?
Plasticity refers to the clay’s ability to bend, stretch, and hold shape without cracking. A highly plastic clay is smooth and flexible, making it ideal for wheel throwing. A less plastic clay may chalky, and crack when pushed beyond its working limits.
Plasticity is a product of how the surface chemistry and the electrolytic charge of the particles express themselves, the particle shape, the mineralogy of the particles and the presence of impurity non-plastic particles in the matrix.. An ideal clay body has particles of many sizes so they can pack together and fill all the microscopic voids. Fine-grained clays such as porcelain tend to have high plasticity, while coarse clays can be less forgiving and more prone to cracking while working. Potters should consider the balance: too much plasticity can make a clay prone to slumping or collapsing under it’s own weight, while too little can make it difficult to shape.
What do the terms “low-fire,” “mid-fire,” and “high-fire” mean?
These terms describe the firing temperature range the clay is designed for. Each range affects both the durability of the finished piece and the type of glaze options available.
- Low-fire (cone 06–04, ~1,800–1,950°F): Often chosen for beginners and for decorative work. Colors stay bright, but pieces are more porous and less durable.
- Mid-fire (cone 4–6, ~2,100–2,225°F): A balance between strength and functional durability. Stoneware is common in this category, offering functional and artistic versatility.
- High-fire (cone 8–10, ~2,300–2,400°F): Produces dense, durable ware. Porcelain and certain stonewares belong here, making them ideal for functional kitchenware.
Choosing the right temperature range is essential, as glazes and kilns must be compatible with the clay’s rating.
What is the difference between earthenware, stoneware, and porcelain?
This is one of the most common questions about pottery clay, and the distinctions are worth knowing.
- Earthenware: A low-fire clay that remains somewhat porous. It is not as durable for functional ware and mostly for decorative work.
- Stoneware: A mid- to high-fire clay that becomes vitrified (non-porous) at maturity. It balances strength and workability, making it a favorite for everyday functional pottery.
- Porcelain: A high-fire clay made from very pure kaolin. It is smooth, ultra white, and often translucent when thin. Porcelain is strong, vitreous and almost glass-like when fired but more demanding to work with.
Each clay type brings unique strengths. Earthenware offers color, stoneware offers durability, and porcelain offers refinement.
Why are grog and sand added to some clays?
Grog and sand are non-plastic materials that modify the behavior of clay. Grog is pre-fired clay ground into particles, while sand is a natural mineral addition. Usually raw, pulverized quartz is the type of sand added to clay, unlike play sand or construction sand which can contain impurities that are detrimental to clay bodies. Both reduce shrinkage, improve drying, and increase resistance to cracking.
Potters working on large sculptures or pieces with thick walls often rely on heavily-grogged clays. The texture may feel rougher in the hands, but the reduced shrinkage and increased porosity help prevent defects and explosions in the firing process. Wheel throwers often prefer smoother clays with little to no grog, as the coarse particles can interfere with delicate forming and are abrasive on the hands.
What does “vitrification” mean, and why is it important?
Vitrification is the stage at which a clay body becomes glass-like and impermeable to water. A vitrified clay is dense, strong, and safe for functional ware such as dinner plates and mugs. A clay body that is not fully vitrified will remain porous, even if glazed, which can lead to problems with durability and the intrusion of water into the fired clay body. If water gets into a finished ware it can lead to bacterial/ fungal growth, which is why most low-fire clays are not considered dinnerware safe.
Manufacturers formulate clay bodies to mature at specific cones. Overfiring can cause warping, bloating or melting, while underfiring leaves the clay weak and absorbent. Choosing clay with the right cone rating ensures that vitrification happens your intended firing temperature.
How do clay manufacturers create different clay bodies?
Creating a clay body involves blending different natural clays and minerals to achieve desired properties. A typical recipe might combine:
- A primary clay (like kaolin) for whiteness and purity
- A secondary clay (like ball clay) for plasticity
- Feldspar or other fluxes to lower the melting point
- Silica for strength and thermal stability
The ratio of these ingredients determines whether the clay is best suited for handbuilding, wheel throwing, or sculptural work. Expert blenders test batches by shaping, drying, and firing to make sure the clay performs consistently.
What are some unusual or unique clay bodies?
While earthenware, stoneware, and porcelain dominate the market, specialized clay bodies also exist.
- Paper clay: Regular clay mixed with paper fibers, which makes it lighter and more flexible during construction. It can be joined at drier stages more easily than traditional clay.
- Colored clays: Pigments or stains are added to the clay itself, producing vivid colors that remain after firing. These clays allow surface decoration to be integrated directly into the body.
- Wild clays: Some potters experiment with locally dug clays, sometimes blending them with commercial bodies to create unique textures and colors or to change the firing properties. These clays may require significant testing before they are reliable.
Such clays provide creative possibilities beyond the standard options, although they often require more technical skill and labor to use successfully.
How should beginners choose their first clay body?
For those just starting out, it helps to focus on simplicity and ease of use. Consider the following steps:
- Choose a mid-fire stoneware. These clays are forgiving, strong, and work with a wide range of glazes.
- Select a smooth body. Avoid heavy grog at the beginning, as it can be difficult on the hands during wheel throwing.
- Start with small quantities. Purchasing 25 pounds allows you to test the clay without committing to a large batch.
- Ask for advice. Suppliers often provide guidance on matching clay to projects and kiln types.
Starting with a versatile clay body ensures beginners can practice techniques while minimizing frustration.
What should experienced potters consider when changing clay bodies?
Experienced potters may want to experiment with new clays for creative or functional reasons. However, switching requires attention to compatibility. Glazes behave differently on different clay bodies, and firing schedules may need adjustment. Potters should always test small batches before committing to a full production run.
Why does shrinkage matter?
All clays shrink as they dry and fire, usually between 10–15 percent. Shrinkage mostly affects the final size of the pieceand knowing how big to make something so it will shrink to the intended size.. Understanding shrinkage is especially important when fitting lids, adding slips that may have different shrinkage rates, and combining multiple clay bodies in one piece.
How does atmosphere affect clay in the kiln?
The kiln’s atmosphere can dramatically change the outcome of a clay body and glaze color. In oxidation (abundant oxygen in kiln atmosphere), clays fire true to their formulated color. In reduction (limited to no oxygen), clays may darken or develop richer hues due to changes in the chemistry of iron and other minerals. Potters working with gas kilns or wood kilns often experiment with these “reduction” effects, because the burning of fuels steals all the oxygen from the kiln atmosphere and in turn the clay body/ glaze. Those with electric kilns fire in oxidation as there is “fresh air” in the kiln chamber and nothing is burning and consuming oxygen.
How do I match a clay to my project?
Choosing a clay body comes down to balancing project needs with the properties of the clay body. Functional ware requires strength and vitrification. Sculptural pieces may benefit from grog for stability, the porous bodies also allow for moisture to escape and large thick works can survive a firing without exploding. Beginners do best with forgiving mid-fire stoneware, while advanced potters may explore porcelain or experimental blends.
Clay is the foundation of every ceramic project. By understanding the terminology and characteristics of clay bodies, potters at every level can make informed choices and bring their creative visions to life.
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Our team is always happy to answer your questions, and can guide you to the perfect match for your goals.
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