What is clay made of? At its core, clay is a mass of microscopic mineral plates, mostly kaolinite, illite and smectite, mixed with water and small amounts of quartz, iron oxide, mica and organic matter. Those plates come from rock, usually feldspar, that has broken down over a very long time. Pottery clay from a supplier goes one step further: it is a recipe of several clays and minerals blended for a firing temperature. This guide is research-based, drawn from USGS, mineral references and maker and dealer data rather than our own lab work. Facts checked October 2026.
What is clay made out of?
Geologists define clay partly by size. USGS describes clay minerals as particles smaller than 2 micrometers, and the ISO 14688 standard uses the same cutoff. That is far finer than silt or sand, and it is the main reason clay behaves so differently from ordinary soil.
The particles are a family of minerals called clay minerals. They are layered silicates built from silica and alumina with water bound into the structure. The main groups are:
- Kaolinite: the mineral in kaolin and china clay. Its formula is Al2Si2O5(OH)4. It fires white and is the base of porcelain.
- Illite: a common clay mineral that tends to form when rock weathers under more alkaline conditions.
- Smectite (including montmorillonite): very fine and swelling. Bentonite is a smectite clay, and potters add small amounts to boost plasticity.
- Chlorite and others: sometimes grouped with clay minerals, usually as minor parts of a natural clay.
Pure clay minerals are white or pale. The red, brown, tan and gray colors of most natural clay come from impurities, mainly iron oxide, plus organic matter that burns away in the kiln. That is why a red earthenware turns terracotta orange when fired and a white porcelain stays white.
Where does clay come from?
Clay comes from the ground, formed from weathered rock and found in two kinds of deposits. Which kind a clay comes from decides how pure it is and how it handles.
Primary (residual) clay
Primary clay stays where it formed, on top of or next to its parent rock. Because it has not traveled, it picks up few impurities. Kaolin is the classic example: white, quite pure and not very plastic, because its particles are relatively coarse for a clay.
Secondary (sedimentary) clay
Secondary clay was carried away by rivers, streams or glaciers and settled in calm water such as lakes, floodplains and old seabeds. Along the way the particles get ground finer and collect iron, other minerals and plant matter. The result is more plastic and usually darker. Ball clays, fire clays, stoneware clays and red earthenware clays are secondary clays.
Where does pottery clay come from in the US?
Clay-King names Georgia, Tennessee, North Carolina and Missouri among the major US sources of pottery clay. A few well-known raw clays show the spread:
- OM-4 ball clay is mined near Mayfield, Kentucky, according to Digitalfire.
- EPK (Edgar Plastic Kaolin) is a water-washed kaolin from Florida. Seattle Pottery Supply reported in December 2025 that its source mine had stopped again, so EPK is hard to buy in 2026.
- Cedar Heights Goldart is a stoneware clay mined in southern Ohio.
- Georgia kaolins such as Wilco UPF are being used as EPK substitutes.
Most of the moist clay sold in boxes is mixed by clay companies from these dry raw clays rather than dug and boxed straight from one pit.
How is clay formed?
Clay forms mainly by chemical weathering. Rainwater picks up carbon dioxide and becomes a weak carbonic acid. Over very long periods, that acid attacks silicate minerals in rock, especially feldspar, one of the most common minerals in the Earth's crust. The feldspar breaks down, soluble parts such as potassium and sodium wash away, and the leftover alumina and silica rebuild into clay minerals.
USGS lists three main routes:
- Weathering: rock breaks up physically and decomposes chemically at the surface. Rock type, water, temperature and time decide which clay mineral forms. Warm, wet, acidic conditions tend to make kaolinite; more alkaline conditions favor illite.
- Diagenesis: buried sediments slowly change into more stable clay minerals under pressure and heat.
- Hydrothermal action: hot water in geothermal areas alters rock and volcanic ash into clay.
Clay-King describes pottery clay deposits as forming over millions of years. Clay is a natural resource that is not replaced on any human timescale, which is one reason reclaiming scraps is good studio practice.
What makes clay plastic?
What makes clay different from sand or silt is plasticity: wet clay can be pushed into a new shape and it stays there. The reason is the shape of the particles. Clay minerals are thin, flat plates. When wet, a film of water sits between the plates, so they can slide past each other without letting go. Once you stop pushing, the water films hold them in place.
Finer particles and more plate surface mean more plasticity. That is why ball clay and bentonite are very plastic and kaolin is less so. It also explains shrinkage: as the water between the plates leaves, the plates pull closer together and the piece gets smaller, first in drying and again in firing.
Plasticity is the reason clay bodies are blends. A pure kaolin is too short to throw well, so makers add ball clay or a little bentonite to make it workable.
What is pottery clay made of?
The clay you buy in a 25 lb bag is a clay body: a recipe, not a single dug material. Most bodies combine three jobs:
| Ingredient | Job in the body | Examples |
|---|---|---|
| Clay | Plasticity and green strength; supplies alumina and silica | Kaolin, ball clay, fire clay, stoneware clay, red clay |
| Flux | Melts and glues particles together in firing | Feldspar, nepheline syenite, talc in some low fire bodies |
| Filler / glass former | Controls shrinkage and builds the fired structure | Silica (flint, quartz) |
| Opener | Adds tooth, speeds drying, cuts warping | Grog (fired, ground clay), sand |
| Colorant (optional) | Color or speckle | Iron oxide, manganese, stains |
A classic porcelain example published by Glazy is 50% kaolin, 25% feldspar and 25% silica. Stoneware bodies swap in more ball clay and fire clay for plasticity and a buff or gray color. Earthenware bodies lean on iron-rich red clays and stronger fluxes so they mature at low temperatures. Our types of pottery clay guide compares the three families, and the ball clay page covers the most common plasticizer.
How a body is balanced decides its cone. More flux and iron mean a lower firing range, which is why earthenware clay fires at cone 06 to 04 while stoneware and porcelain need cone 5 to 10. Adding grog makes a body better for big hand-built pieces but rougher to throw.
How is clay made for potters?
Commercial clay goes through a few steps between the pit and your wheel:
- Mining: raw clay is dug from deposits, from large open pits to small hand-dug seams.
- Slaking and sieving: dry clay is broken up and soaked into a slurry, then screened to remove stones, roots and debris.
- Drying and milling: many raw clays are dried and airfloated to a fine powder (Goldart and Redart are sold at 200 mesh) and bagged as dry materials.
- Blending: clay companies weigh the clays, feldspar, silica and grog for each body, often after small test batches.
- Mixing with water and de-airing: the dry batch is mixed with water, then pushed through a de-airing pug mill so it comes out dense and free of air pockets.
- Bagging: the pugged clay is cut into blocks, usually 25 lb, sealed in plastic and boxed in pairs.
What happens to clay in the kiln?
Firing changes clay into ceramic, and the change cannot be reversed. In order:
- Leftover water evaporates early in the firing. Pots that are still damp can crack or burst here.
- Kaolinite starts to lose its chemically bound water at about 1,022°F to 1,112°F (550°C to 600°C). After this, the piece can no longer slake back into clay.
- Quartz in the body changes form at 1,063°F (573°C) with a sudden size change, which is why makers advise slow heating and cooling through that range.
- At higher heat the flux melts, particles fuse, and in stoneware and porcelain new crystals such as mullite form. That is vitrification.
A bisque firing takes clay through the first changes; the glaze firing finishes the job at the body's rated cone.
Common mistakes and myths
- Calling clay "just dirt". Garden soil is mostly sand, silt and organic matter. Clay is a specific mineral size and type. Soil with a lot of clay can sometimes be refined, but most yard soil cannot be thrown or fired.
- Firing wild clay without testing. Dug clay can carry lime, sulfur and stones that spit, bloat or melt. Our making clay at home guide covers testing.
- Treating cosmetic clay as pottery clay. Kaolin and bentonite sold for skin care have no firing data and are not meant for pots.
- Assuming any clay fires at any cone. A low fire body can slump or melt in a cone 6 kiln. Always fire to the maker's stated range.
Safety: clay dust and raw materials
Clay and most clay body ingredients contain crystalline silica. Dry clay dust is the hazard, so clean with water and a damp mop or sponge, never dry sweep, and keep scraps damp. Wear a NIOSH-approved respirator when you mix dry clay or raw materials, and read the maker's SDS for every material. Our clay dust safety guide has the full routine.
What you need
You do not need to mix your own body to start. A boxed stoneware matched to your kiln's cone is the simplest choice, and our best clay for pottery guide compares the bodies that suit home potters, with cone, shrinkage and absorption from each maker.
Sources: USGS, Environmental Characteristics of Clays and Clay Mineral Deposits; Wikipedia articles on clay and kaolinite (particle size, mineral groups, kaolinite formula and dehydration range); Clay-King, How Pottery Clay Is Made and Processed; Glazy help, What Is Clay; Digitalfire material pages for OM-4 ball clay, EP kaolin and Goldart, and the quartz inversion glossary entry; Seattle Pottery Supply EPK update (December 2025); Katherine Fortnum Ceramics, Where Does Clay Come From. Facts checked October 2026.