Clay from London construction sites: a working guide
London ground is full of clay, and building sites dig huge amounts of it. This is a plain guide to what that clay is, where it goes now, whether it is any good for ceramics, and what we found firing one batch of it.
London sits on clay. Dig almost anywhere in the city and you reach it, and building sites reach it constantly, because foundations, basements, and tunnels all go down through it. The amounts are large. On most sites that clay is treated as something to move on, not something to keep.
This guide is a starting point. It covers what construction-site clay actually is, where it goes now, whether it is any use for ceramics, what we found when we fired one batch of it, and what it would involve if a site wanted to send some our way. Each section is short on purpose and points to a longer note where there is more to say.
What construction-site clay actually is and why it is dug
Most of what comes out of a London foundation or tunnel is not the topsoil from the surface. It is the subsoil and the clay strata below, the ground that has to be removed so something can be built in its place. In much of London that means London Clay, a thick, fairly even layer of clay that was laid down under the sea long ago, along with other local clays depending on where the site sits.
It is dug because it is in the way. A basement, a foundation, a deep trench for pipes and cables, or a station all need the ground taken out before the structure goes in. The clay is a by-product of making room, which is why it tends to be thought of as spoil rather than as a material.
Where it goes now
On most sites the clay is dug, classified, loaded onto a lorry, and driven away. The lorry haulage, and the fee the landfill charges to take it, are real costs. Landfill tax is added on top, and the rate depends on what the material is. Clean, naturally occurring subsoil and clay usually count as inert, and from 1 April 2026 inert material is taxed at the lower landfill rate of GBP 8.65 per tonne, not the standard GBP 130.75 that contaminated or mixed waste attracts.
A lot of clean excavated material never reaches landfill at all, because it can be reused under the development industry’s code of practice for excavated soil. The developer stays the waste producer and the duty-of-care holder throughout. For the detail of the tax split, the reuse route, and what handing material to a studio involves, see the note on what could happen to clay dug out of a London site.
This is general information, not legal or regulatory advice. Take advice from your own environmental consultant, a CL:AIRE Qualified Person, or the Environment Agency before classifying or moving anything.
Is it any good for ceramics
Sometimes. A clay that came out of the ground as spoil is not a studio clay body, and it has to be processed and tested before you know what you have. Some London clays fire well, some are awkward, and some are not worth the effort. The only honest answer for any given clay is to test it. For what the question really means, and the numbers behind it, see the note on whether London clay is any good for pottery.
What we found firing one
Our first project worked through one batch, dug from a Park Royal construction site, the same dig making way for the new HS2 station. Fired across a range from 700 to 1150 C, it reached its tightest, least porous state near 1050 C, at about 0.7% water absorption. Above that it over-fired and bloated, and absorption climbed steeply, so hotter was not better. As it came, the clay would not cast in a mould at all, until 2.5% barium carbonate was added and it became castable.
That is the short version. The measured numbers, the table, and the charts are on the Park Royal clay findings page, and there is more on the Park Royal clay project. For the batch by batch account see the Park Royal clay firing story; for the casting result see how adding barium carbonate made the clay slip-cast; and for why there is a best firing temperature at all, see the explainer on how a clay matures. For a fuller account of what comes out of the ground on a London site, see what happens to clay dug from a London building site.
If your site produces it
If you have clay coming out of the ground and are curious what a small studio would do with it, the working relationship is simpler than it sounds. We take clean, naturally occurring London clay in small documented quantities, we keep what we do legal under the framework the development industry already uses, and we publish what we learn. The soft version of the ask is the honest one: we would rather hear about your material than not. The full version, with how the rules fit and what to send, is on the work with us page for materials.
How early this is
This is one studio in Park Royal, west London, working through a small number of materials a year, by hand. We are not claiming to solve construction waste or to take volume off anyone’s hands. What we can offer is genuine interest in a material, careful testing, and results published in the open, so that whatever we learn is useful beyond this one studio.