Is London clay any good for pottery?

The honest first move is to break the question apart. “London clay” is not one material. It can mean the geological London Clay formation, or it can mean whatever clay happens to come out of a particular hole in the ground. Those are not the same thing, and clays dug a few miles apart, or a few metres down from each other, can behave very differently. So the useful question is narrower: which clay, and good for what.

We can only speak to one clay. We dug it from a Park Royal construction site in west London, the new HS2 station dig, and tested it as a single batch. What follows is about that clay, in these tests. It is not a claim about London Clay the formation, and it is not a claim about every clay you could dig in London.

What “any good” even means for a clay

“Good” is not one property. For pottery it usually breaks into a few separate things.

• Plasticity and workability. Can you actually shape it without it cracking or slumping. Does it hold an edge.
• A findable firing range. Is there a temperature where it tightens up into a usable body, and is that window wide enough to hit reliably.
• How dense it gets. Fired pottery that holds water needs low porosity, which we read off water absorption. The lower the number, the tighter the body.
• Whether you can form it the way you want. Throwing, hand-building and slip-casting each ask different things of a clay, and a clay can be fine for one and refuse another.

A clay can score well on some of these and badly on others. So the answer is rarely a flat yes or no.

What this London clay actually did

On firing, this clay did well. We fired the same processed clay at six temperatures, from 700 to 1150 C, and measured how much water the fired bars soaked up.

At 700 C it had barely started, with water absorption at 17.6%. As it fired hotter it tightened: 5.1% at 1000 C, and down to 0.7% at 1050 C. That 0.7% is close to fully sealed, the kind of dense, low-porosity body you would want for ware that has to hold water. For this clay, 1050 C is roughly where it matures.

Hotter was not better. At 1100 C absorption jumped back up to 24.8%, and at 1150 C to 38.3%, because past its maturing point the clay over-fires and bloats. So the usable window sits at the top of the range and is narrow. You can see the full set of numbers, the table and the charts on the Park Royal Clay findings page, and we have written up why hotter is not better in a separate note on how a clay matures.

The catch: forming it

Firing was the easy part. Forming it was not.

As it came, this clay would not cast in a mould. Casting means turning the clay into a thin, pourable liquid, a slip, and the trick is to thin it without simply adding water. That is done with a tiny amount of a chemical (a deflocculant). The usual ones we reach for, sodium silicate and soda ash, either did nothing or turned the slip stiffer than where it started. Adding 2.5% barium carbonate to the clay changed that, and it poured and cast cleanly. We treat the why as a working explanation rather than a settled fact: barium carbonate does not do the thinning itself, and the likeliest read is that it mopped up natural salts, called sulphates, in the clay that were stopping the usual thinners from working, though we have not measured how much sulphate is there. The full account is in our note on how 2.5% barium carbonate made this Park Royal clay cast.

So, is London clay any good for pottery

For this one clay, in these tests, the short answer is that it is promising for some uses. It fired to a dense, low-porosity body near 1050 C, which is a genuinely useful result, with the caveats that the firing window is narrow and that it would not slip-cast until we added barium carbonate. Throwing and hand-building ask less of a clay than casting does, so a clay like this may suit those better as it comes.

The caveats matter more than the result. This is one clay, one processing run, one firing schedule, one kiln. It is not all London clay. Iron-rich clays around London vary a lot from site to site, and the next dig could behave nothing like this one. What this tells us, tentatively, is that a wild clay pulled straight out of a London building site can fire to a serious, dense body, and that the harder questions tend to be about forming it, not firing it. If you want the wider picture of where clay like this comes from and what it takes to use it, see our working guide to clay from London construction sites.