# Where Does Clay Come From? From Granite to the Ground Under You URL: https://myclay.online/blog/where-does-clay-come-from Published: 2026-08-01 Category: Materials Tags: Materials, Wild clay, Pottery basics Reading time: 5 min > Clay forms when feldspar in rocks like granite weathers chemically over thousands of years. Some stays where it formed as primary clay; most is carried by water and deposited elsewhere as secondary clay. Clay is what rock becomes when water works on it for long enough. > **From the studio:** The first time a supplier told me a clay body had been reformulated because a seam had run out, the geology stopped being abstract. The bag looked identical and threw slightly differently, and I had to relearn a body I thought I knew. ## It starts with feldspar Granite and similar igneous rocks are made largely of three minerals: quartz, mica and **feldspar**. Feldspar is the vulnerable one. Rainwater is slightly acidic, because it dissolves carbon dioxide from the air as it falls. Over thousands of years, that weak acid attacks feldspar crystals and strips out the soluble elements — potassium, sodium, calcium — leaving behind aluminium and silicon bound together with water. What remains is a clay mineral: a flat, microscopically small plate of hydrous aluminium silicate. This is chemical weathering, and it is slow. Frost, roots, temperature swings and grinding by ice and water all help break the rock apart physically, exposing more surface for the chemistry to work on. ## Two kinds of deposit **Primary clay** stays where it formed, sitting directly on the parent rock. It is chemically pure, with coarser particles and low iron content, so it fires white. Kaolin — china clay — is the classic example, and it is what porcelain is built from. Primary clay is not very plastic, because the particles have not been ground fine by transport. **Secondary clay** has been picked up by water and carried away. Rivers move the fine particles downstream and drop them wherever the current slows: in floodplains, lake beds, estuaries and, over geological time, in beds that later become dry land. The journey changes the material. It grinds the particles finer, which makes secondary clay far more plastic. It also sorts and mixes: iron oxides, organic matter, calcium and other minerals get folded in, which is why secondary clay is usually grey, brown or red rather than white and why it fires at lower temperatures. Almost all clay you will encounter — in a riverbank, in a garden, in a bag from a supplier — is secondary. ## Why it ends up in bands Deposition sorts material by particle size. In slowing water, gravel drops first, then sand, then silt, and clay last, because the particles are so fine they stay suspended almost indefinitely. Repeat that over thousands of flood cycles and you get the horizontal [banding](/blog/where-to-find-natural-clay) visible in any road cutting or eroding riverbank: coarse layers where the water ran fast, fine clay layers where it stood still. That banding is what makes clay findable — you are looking for the layer that was laid down in calm water. ## Where it is mined Commercial extraction happens where beds are thick, consistent and shallow enough for open-pit quarrying. The best-known kaolin deposits include Cornwall and Devon in England, where china clay has been worked since the 1740s, and a wide belt across Georgia in the United States. Ball clays come from Devon and Dorset, Kentucky and Tennessee. Large deposits are worked in Brazil, Ukraine, India and China — the last of which gave porcelain its English nickname. Mining is straightforward: strip the overburden, extract the clay, then wash, sieve, settle, filter-press and dry it. That refining is most of the value. Raw clay in the ground is worth almost nothing; processed, blended, consistent clay is worth buying. ## Is clay renewable? Technically it is still forming, everywhere, all the time. Practically, it is classified as a **non-renewable resource**, because the rate of formation is measured in thousands of years and the rate of extraction is measured in tonnes per hour. The saving grace is abundance. Clay minerals make up a substantial fraction of all sedimentary rock on earth, and there is no realistic prospect of running out of clay in general. What can and does run out is a *particular deposit* with a particular whiteness, plasticity and firing range — which is why potters occasionally find that a favourite clay body has been reformulated. The mine ran out, and the supplier had to blend something else to match. ## The one-way trip Firing ends the cycle. Above about 500 °C the chemically bound water is driven permanently out of the clay minerals, and the structure that made the material plastic is destroyed for good. Unfired clay can be recycled endlessly — slaked in water, dried back, wedged and used again. Fired ceramic cannot. It can be crushed into [grog](/blog/what-is-grog-in-clay) and added back to a clay body as filler, but it will never again be clay. That irreversibility is exactly why pottery survives in the ground for thousands of years, and why broken pots are the most abundant artefact in archaeology. ## Why it matters to a potter Geology sounds academic until you notice how much of studio practice it explains. **Why clay bodies get reformulated.** A seam runs out, and the supplier blends something else to approximate it. Your familiar clay throws slightly differently, and nothing on the bag has changed. **Why porcelain costs more.** It needs primary kaolin of a specific whiteness and consistency, which occurs in far fewer places than the mixed sedimentary clays that make stoneware. **Why wild clay is usually low-fire.** Secondary clay picks up iron and other fluxes on its journey, and those fluxes make it melt at a lower temperature. **Why grey clay fires orange.** The iron in a waterlogged deposit is chemically reduced and turns grey; oxidise it in a kiln and it goes red-brown. **Why clay bodies are recipes, not dug material.** No single natural deposit has the right plasticity, firing range, colour and shrinkage all at once, so bodies are blended from several clays plus feldspar, silica and grog. None of this changes what you do on a Tuesday evening. It does make a surprising number of otherwise arbitrary facts fit together. ## Where to go next For the mineral chemistry, see [what is clay made of](/blog/what-is-clay-made-of). To find some yourself, [where to find natural clay](/blog/where-to-find-natural-clay) explains what to look for. And [is clay renewable or non-renewable](/blog/is-clay-renewable-or-nonrenewable) answers the question this always raises. ## Frequently asked questions **How long does it take for clay to form?** Thousands to millions of years. Chemical weathering of feldspar is extremely slow, which is why clay is classified as a non-renewable resource on any human timescale even though it is abundant. **Is clay found everywhere?** Nearly. Clay minerals occur on every continent and make up a large share of sedimentary rock and soil. What varies is quality — clay pure and plastic enough for pottery is much less common than clay in general. **Where is clay mined commercially?** From open-pit quarries wherever thick, consistent beds occur — Cornwall and Devon in England, Georgia in the United States, and large deposits in Brazil, Ukraine and China among many others. --- Source: https://myclay.online/blog/where-does-clay-come-from — https://myclay.online/