Ceramic clay is made primarily of decomposed rock, specifically minerals rich in aluminum and silica that form when granite and other rocks break down over millions of years. The most common clay mineral is kaolinite, which contains aluminum, silicon, oxygen, and hydrogen in roughly equal parts aluminum to silica. But the clay you’d actually shape on a wheel or press into a mold contains more than just this base mineral. Potters and manufacturers blend several ingredients to control how the clay handles, how it shrinks, and how dense it becomes after firing.
How Clay Forms in Nature
All ceramic clay starts as rock. Rain, wind, ice, and chemical weathering slowly break down feldspar-rich rocks like granite into fine particles. These particles eventually become clay minerals, with kaolinite being the most important for ceramics.
Where those particles end up determines the type of clay. Primary (or residual) clays stay at or near the site of the parent rock. Kaolin is the classic example: relatively pure, white, and coarse-grained because it hasn’t traveled far. Secondary (or sedimentary) clays have been carried by water, wind, or glaciers and deposited far from their origin. Along the way they pick up organic matter, iron compounds, and other minerals that change their color, plasticity, and firing behavior. Ball clay, a fine-grained secondary clay prized for its workability, is a good example.
The Core Ingredients in a Clay Body
A “clay body” is the specific recipe a potter or factory uses. Most clay bodies blend four categories of ingredients, each with a distinct job.
- Clay minerals (kaolin and ball clay): These provide plasticity, the quality that lets you shape clay without it crumbling. Kaolin is less plastic but fires white and contributes structural strength. Ball clay is far more plastic and easier to work with but contains more impurities. Most stoneware recipes use both.
- Flux (usually feldspar): A flux lowers the temperature at which the clay body melts and fuses together. Feldspar contains potassium, sodium, and calcium ions that promote melting and help the other ingredients bond into a dense, glassy structure during firing. Nepheline syenite serves a similar role.
- Silica (quartz or flint): Added silica gives the fired piece hardness and resistance to chemical attack. It also helps control thermal expansion, making the finished ceramic less likely to crack when it heats and cools.
- Additives: Depending on the purpose, a recipe might include grog (crushed fired ceramic), sand, or bentonite. Grog in particular reduces shrinkage and prevents cracking during drying.
How Recipes Change by Clay Type
Earthenware
Earthenware is the oldest and lowest-fired type of ceramic. Its defining characteristic is a high iron content, which gives it that familiar terracotta red or warm buff color. Red earthenware clays can contain 6.5% to 8.5% iron oxide by weight, along with other minerals like quartz, biotite, and hornblende that were mixed in during glacial transport. Yellow ochre, a form of iron bound with water, is another common natural colorant in earthenware. When fired, it loses its water and converts to red iron oxide, deepening the color.
Because earthenware fires at lower temperatures, the clay body never fully vitrifies (fuses into a glassy, non-porous state). That’s why earthenware pots need a glaze to hold water.
Stoneware
Stoneware fires hotter and produces a denser, more durable result. A common stoneware formula uses about 80% total clay (a mix of kaolin and ball clay), 10% silica, and 10% feldspar for high-temperature firing. At a lower firing temperature, the feldspar proportion increases to around 15% while total clay drops to about 75%. A simplified starting recipe that many potters learn is equal parts: 25% kaolin, 25% ball clay, 25% silica, and 25% feldspar.
The balance between kaolin and ball clay matters for workability. Kaolin contributes about 37% alumina and is nearly non-plastic on its own. Coarser ball clays contain 27% to 31% alumina with moderate plasticity, while ultra-fine ball clays run 24% to 27% alumina and are much easier to shape. Bentonite, sometimes added in small amounts to boost plasticity, has only about 20% alumina. The general rule: as alumina drops, plasticity goes up.
Porcelain
Porcelain uses a high proportion of kaolin, which is what gives it a white, translucent quality when fired thin. The trade-off is that kaolin’s low plasticity makes porcelain notoriously difficult to throw on a wheel. Porcelain recipes compensate with carefully chosen ball clays and sometimes a small percentage of bentonite, plus a generous amount of feldspar to promote full vitrification at high temperatures.
Bone China
Bone china is a special case. It replaces a large share of the clay with bone ash, which is essentially tricalcium phosphate derived from animal bones. A typical bone china body contains 35% to 45% bone ash, 25% to 30% china clay (kaolin), and 25% to 30% feldspar. Some formulations push bone ash close to 50%. The bone ash gives bone china its characteristic translucency, whiteness, and surprising strength despite how thin it can be made.
What Grog and Other Fillers Do
One of the biggest practical challenges in ceramics is shrinkage. As clay dries and then fires, it loses water and contracts. If it shrinks unevenly, it cracks. Grog, which is simply ground-up fired ceramic, addresses this directly. Adding grog reduces both drying and fired shrinkage because the grog particles have already gone through their shrinkage during a previous firing. They also act as tiny crack-stoppers: when a micro-crack starts to form, it runs into a grog particle and stops before it can spread.
The difference can be significant. A clay body that normally shrinks 5.5% during firing can drop to just 2.5% shrinkage with an addition of 15% coarse grog and 15% fine silica sand, while maintaining the same density and strength. Larger grog particles are better at stopping cracks, while finer particles are more effective at reducing overall shrinkage. Sculptors and tile makers tend to use grittier, grog-heavy clay bodies because their pieces are thick and prone to cracking. Potters throwing on a wheel generally prefer smoother bodies with less grog, since coarse particles can tear the clay under their hands.
Why the Same Mineral Behaves Differently
What makes ceramic clay fascinating is that small changes in the recipe produce dramatically different results. The same core minerals, aluminum silicates plus quartz and feldspar, can yield a porous terracotta flowerpot, a waterproof stoneware mug, or a translucent porcelain teacup. The differences come down to particle size, impurity levels (especially iron), the ratio of flux to clay, and how hot the kiln gets. A red earthenware clay packed with iron fires at around 1,000°C and stays porous. A porcelain body with almost no iron fires above 1,300°C and becomes glass-like.
Even within a single clay type, swapping one ingredient shifts the outcome. Replacing some feldspar with nepheline syenite lowers the required firing temperature. Adding more ball clay improves workability but increases shrinkage. Every clay body is a balancing act between what the maker needs at the wheel and what they want out of the kiln.