Adding 2.5% barium carbonate made Park Royal clay cast.

The clay dug from the Park Royal construction site fires well. Across batches A to F it reaches its tightest, most mature state near 1050 C, at about 0.7% water absorption. But firing was never the problem. The problem was getting it into a mould in the first place.

As it came, this clay would not slip-cast. Slip casting needs a clay that has been deflocculated, a watery slip that pours and releases cleanly rather than sitting as a thick, sticky paste. This clay refused to do that.

What happened

The usual route is a sodium deflocculant, sodium silicate or soda ash, added in tiny amounts to thin the slip without adding water. We tried the sodium deflocculants we normally reach for. Every one of them either did nothing at all or made things worse, gelling the slip into something stiffer than where it started.

Adding 2.5% barium carbonate changed that. With it in the body, the slip would deflocculate and the clay became fully suitable for slip casting. Barium carbonate is not a deflocculant itself; what it seems to have done is clear something that was stopping the sodium additions from working. The same body that had resisted every sodium addition now poured and cast.

What this means, tentatively

In these tests, 2.5% barium carbonate took a dug clay that could not be cast and made it castable. That is a useful, repeatable result for this clay. It is not a general rule. It is one body, one set of tests, and a single addition level that worked; it does not prove that 2.5% is the right amount, or that barium carbonate is the only fix.

There is a plausible reason it worked. Barium carbonate is also used to lock up soluble sulphates in clay bodies, where it reacts with the sulphates so they no longer interfere. Soluble salts are a known cause of deflocculation trouble: they can leave a slip impossible to thin with sodium because the salts already in the clay are working against you. If this Park Royal clay carries soluble sulphates, the barium carbonate clearing them would explain why the sodium deflocculants failed and why the barium addition unlocked the slip. We are treating that as a working explanation, not a settled one. We have not measured the sulphate content here, so the mechanism is a reasonable read of the result rather than something these tests confirm.

Open questions

• Is 2.5% the right amount, or would less do the same work? We have not yet tested lower additions.
• Does the slip cast cleanly at scale, over many pours, or only in small tests?
• Are soluble sulphates actually present in this clay, and at what level? Measuring that would test the working explanation directly.
• Does barium carbonate change the fired body, its colour, its maturing range, or only the wet behaviour?

The firing data, the full batch table, and the pre-processing steps for this clay are written up on the Park Royal clay findings page.