The Park Royal clay firing story, batch by batch.

This clay did not arrive as clay. It came out of the ground at a construction site in Park Royal, west London, the same dig that is making way for the new HS2 station. To find out whether it could be fired into anything usable, it first had to be turned back into a workable body and then taken up through temperature, one batch at a time, to see where it held and where it failed.

From spoil to test bars

The processing followed the method set out in Matt Levy’s book Wild Clay. The raw lumps were broken up, dried completely, then slaked in water until they fell apart. The slurry was mixed, sieved through a 100 mesh to take out grit and roots, and left to firm up on a plaster batt until it could be cut into test bars. Each bar started at 10 cm long so that shrinkage could be read directly off it after firing.

The bars were split into six batches, A to F, and fired to rising peak temperatures from 700 to 1150 C. The schedule was the same for all of them: 100 C per hour up to 600 C, then a full ramp to the batch maximum.

The record, batch by batch

These are the per-batch averages, measured off the fired bars. Linear shrinkage is against the 10 cm starting length.

• Batch A, 700 C: 9.2 percent shrinkage, 17.6 percent water absorption.
• Batch B, 900 C: 11.1 percent shrinkage, 10.7 percent water absorption.
• Batch C, 1000 C: 15.1 percent shrinkage, 5.1 percent water absorption.
• Batch D, 1050 C: 15.8 percent shrinkage, 0.7 percent water absorption.
• Batch E, 1100 C: 9.6 percent shrinkage, 24.8 percent water absorption.
• Batch F, 1150 C: 11.9 percent shrinkage, 38.3 percent water absorption.

What the numbers seem to say

Read in order, the bars tell a fairly clean story. As the firing gets hotter the clay tightens: water absorption falls from 17.6 percent at 700 C to its lowest point, 0.7 percent, at 1050 C, and shrinkage rises to its peak of 15.8 percent at the same point. That is the clay reaching its densest, most mature state in these tests.

Above 1050 C it turns over sharply. At 1100 C absorption jumps to 24.8 percent and at 1150 C to 38.3 percent, while shrinkage drops back. The bars at those temperatures had begun to bloat, which fits the numbers: as the clay over-fires it puffs and opens up rather than densifying further, so it both absorbs more water and measures less net shrinkage. So far this suggests the clay matures close to 1050 C and should not be pushed much hotter, at least not on this schedule and at this bar size.

It is worth being careful about what one set of test bars can show. These are averages from a single processing run and a single firing schedule, in one kiln. They map the firing range. They do not prove how a thrown pot or a cast piece of the same body would behave at scale, where wall thickness, atmosphere, and soak time all come into it.

Open questions

• How sharp is the edge at the top? The jump between 1050 and 1100 C is large, so the safe working ceiling could sit anywhere in that gap and is worth narrowing with batches in between.
• Does a soak at temperature, rather than a straight ramp, move where the clay matures or how forgiving the over-fire is?
• The casting work uses the same clay with 2.5 percent barium carbonate added to make it slip-castable. Does that addition shift the firing range shown here, and by how much?

The full per-batch data, the photographs of every bar, and the raw lab notes are on the findings page: Park Royal Clay findings, which also links out to the complete report.