drain casting

Think of coating the inside of a chocolate mould: you pour in melted chocolate, swirl it so a shell sets against the cold wall, then tip out the extra to leave a hollow chocolate egg. Drain casting is the ceramic version. It is a way of running slip casting so you end up with a hollow, thin-walled part instead of a solid one, and it is how most hollow slip-cast ware — vases, cups, pitchers, some crucibles — is actually made.

The steps are simple. Fill the porous plaster mould completely with slip. The plaster begins wicking water out, and a cast wall starts to grow inward from the mould surface, just as in ordinary slip casting. But you do not wait for the whole cavity to fill in solid. Once the wall has thickened to the value you want — timed by experience, since thickness follows the square root of time — you invert the mould and pour the still-liquid slip in the centre back out. What remains is a shell that hugs the mould's inner contour, open where you drained it. The outer surface is crisp and shaped by the mould; the inner surface is whatever the drained slip left, so it is a little rougher.

Drain casting is set against its sibling, solid casting, where the mould cavity is thin enough (or fed from both faces) that the growing walls meet in the middle and the whole piece fills solid — used for plates and handles where you want a fully dense cross-section. Choosing between them is choosing whether the finished part should be hollow or solid. Drain casting saves material and weight and gives graceful hollow forms, but wall thickness is set by timing alone, so consistency depends on a disciplined, repeatable schedule.