What is Clay?

What is clay? Until recently, I had not thought about it in much detail. As an ecologist, I understand soil, where clay content plays an important role. As a potter, I know how commercial clays vary in type and quality. What I had not fully considered was what clay is made from, or how clay dug straight from the ground differs from the material I throw on the wheel. So I did a little research.

Natural clay forms over millions of years as rocks such as granite weather into tiny mineral particles. It is made primarily of silica (SiO₂), alumina (Al₂O₃) and chemically bound water (H₂O), but it can also contain minerals such as magnesium, iron, potassium, sodium and calcium. This structure makes clay soft, flexible and easy to shape when wet. When left in the sun, it dries out and becomes firm. People have used this property for at least 20,000 years, first to make figurines and later to create practical vessels such as pots and jars. As agriculture developed, the need to store grain and water helped drive pottery-making for practical use, eventually leading to the stronger, more reliable commercial clay bodies we use today.

Commercial clay is made by combining raw ingredients. A basic clay recipe includes natural clay, silica, fluxes and additives such as grog. These ingredients are blended in different proportions depending on the required clay body and its desired properties; for example, terracotta uses different ingredient ratios from stoneware.

The first ingredient is natural clay, which forms the base of the recipe. It is essentially the same material our ancestors dug from the ground thousands of years ago to make figurines and bowls. However, we now have access to a variety of natural clays, each with distinct properties that can be blended to suit different requirements. Common types include:

• Ball clay: added for plasticity. It makes the clay smooth, stretchy and easier to throw on the wheel;

• Kaolin/China clay: used for purity and high heat resistance;

• Fire clay: raises the melting point of the clay body and helps prevent warping or slumping during firing; and

• Bentonite: a clay formed from the alteration of volcanic ash that helps bind ingredients together and makes the clay easier to work with. 

The second ingredient is silica, usually added as flint or quartz sand. Pure natural clay shrinks quickly as it dries, which can cause cracking. Because silica does not absorb water, it does not shrink in the same way. When added to natural clay, it acts as a structural skeleton within the clay body and helps control thermal expansion, reducing shrinkage and cracking.

The third ingredient is flux, most commonly feldspar, nepheline syenite or talc. Silica has an extremely high melting point, so flux is added to lower the clay body’s firing temperature to a standard kiln range. This is important because it allows the ingredients to fuse, or vitrify, into a dense, hard ceramic. In simple terms, this is what turns porous, fragile clay into hardened, durable ceramic.

This is the basic clay body recipe used by commercial manufacturers. Additives can be included to suit the clay’s intended use, such as throwing, sculpture or raku firing. For example, grog (crushed ceramic particles that resemble sand) can be added to sculptural clay bodies to provide extra strength.

Here is a brief summary of the ingredients used in different types of clay:

Clay Type	Typical ingredients	Properties
Porcelain	50% kaolin, 25% silica, 25% feldspar	Pure white, translucent and smooth. When fired to the correct temperature, it becomes non-porous and does not need glaze to be waterproof. Fired at high temperatures.
Stoneware	25% kaolin, 25% ball clay, 25% silica, 25% feldspar	Durable and dense; suitable for functional dinnerware. When fired to the correct temperature, it becomes non-porous and does not need glaze to be waterproof. Can be fired at mid-range or high-fire temperatures.
White earthenware	50% ball clay, 40% talc, 10% kaolin	Fired at low temperatures; highly porous and needs glaze to be waterproof.

Clay also comes in different colours. Wet raw clay straight out of the bag often appears grey because it contains trapped organic matter, which burns away during firing. This is why grey clay can become white ceramic after it has been fired. Terracotta gets its classic red colour from high levels of red iron oxide. Porcelain is normally pure white but can also be coloured: cobalt oxide produces blues, while chrome and copper oxides produce greens. There is a whole science behind oxides and how colours are produced through chemical reactions and kiln conditions. It deserves a separate discussion, so I will leave that for another time.

That covers the basics of what clay is made from. But where do these ingredients come from?

Let’s start with China clay, also known as kaolin. China, unsurprisingly, remains a major source, with large China clay deposits found in several provinces. China clay is also quarried in the USA, Brazil, Uzbekistan, Ukraine, France, Germany, Spain and the Czech Republic. The closest producer to the UK is Cornwall and Devon, and the UK is the largest China clay producer in Europe.

In addition, ball clay is also produced in the UK, with the southwest England having some of the world’s rarest and highest-quality deposits. It has been mined here since the 16th century. Ball clay is also mined in the USA, as well as in China and India.

Feldspar is the most abundant mineral group on Earth, making up about 60% of the planet’s crust. Ceramic production, however, requires high-purity deposits. These are found mainly in India, as well as Turkey, Iran, China, Thailand and the USA. For a more local source to the UK, the main producers of ceramic-grade feldspar are France, Germany, Portugal and Spain.

Like feldspar, quartz is a common mineral, but commercial pottery requires ceramic-grade quartz, which limits who can produce it. Ceramic-grade quartz must have very low iron content because even small traces can spoil porcelain or clear glazes. This type of quartz is mined in India, China and the USA, with France and Sweden being the closest European producers.

Porcelain needs especially pure ingredients, so suitable sources are more limited. This is one reason true porcelain costs more to produce. The higher the required purity, the fewer sourcing options are available.

As this shows, clay minerals come from around the world. Their environmental impact is significant and complex. Many are abundant and can be sourced from several regions, often within the same continent or country. For hobby potters and ceramics suppliers, however, tracing the environmental impact of raw clay ingredients can be difficult. Again, the subject of environmental impacts cannot be covered here and deserves its own blog post.

So, that is what clay is: no longer just mud dug straight from the ground, but a complex blend of ingredients shaped by science and sourced from around the world.