Hand-built work fails at the seams, and the seams fail because the slab was rolled in one direction. A slab carries the memory of the roller: it shrinks more along the direction of rolling than across it, so a box built from four slabs cut the same way will tear itself apart at the corners. This is how to roll an even 8 mm slab, rotate it 90 degrees, and join it so the corner is stronger than the wall.

Cross-section of a 9 cm form: rim 22 cm, belly 22 cm, foot 12 cm. — **Vessel cross-section** — Section drawn to the rim, belly, foot and height recorded for this entry.
H 9 · Ø22 / Ø22 / Ø12 cm

Even thickness is a fixture problem, not a skill problem

Two battens of 8 mm plywood laid either side of the clay and a rolling pin whose ends ride on the battens gives an even slab every time. Rolling freehand gives a slab that varies from 6 mm to 11 mm, and the thin parts shrink faster than the thick parts, which is what warps a flat tile into a potato chip.

Roll in one direction, lift the slab, rotate it 90 degrees, and roll again. Two passes at 90 degrees to each other equalise the particle orientation in both axes. For a slab that will be cut into a base or a wall larger than 20 cm, add a third pass at 45 degrees.

Resting the slab

A freshly rolled slab is not ready to use. Rolling compresses the middle and leaves the two faces at slightly different moisture, and if you join it immediately the slab will move after the join and open the seam. Lay it between two sheets of plastic on a board and leave it 45 minutes for a 8 mm slab, 90 minutes for a 12 mm slab.

The test is simple: press a thumb into the surface. It should leave a clear print with no tackiness and no cracking at the edge of the print. If the print edges crack, the slab has gone past leather-hard and you should wrap it and wait, or spritz the surface and re-wrap.

Joining: bevel, score, slip, press

Cut both edges at 45 degrees rather than square. A square butt joint has only the width of the slab holding it; a bevel doubles the contact area and puts the seam in compression when the box shrinks. Cut the bevels with a knife held at 45 degrees against a straight edge — do not eyeball them, because a 5 degree error over 20 cm opens a 1.7 mm gap.

Score both bevelled faces with a needle tool in a crosshatch, paint on slip of single-cream thickness, press them together with a slight sliding motion, then run a coil of the same clay along the inside of the seam and press it in with a wooden tool. The inner coil is what actually holds the corner: it converts a butt joint into a fillet, and the fillet is stronger than either slab.

Working the corners

Do not cut all four walls and assemble them in one go. Build up in pairs: join two walls and let the right angle set for 30 minutes with a support bat outside the corner, then add the third and fourth. A box assembled wet in one session slumps at the corners while you are still working on the second one.

Support the outside of every corner with a small triangular offcut of foam or a rolled newspaper bat until the box is leather-hard. When you can pick up the box by one wall without the shape changing, it is ready for the base to be joined and the whole thing wrapped for slow drying.

Steps

Lay two 8 mm plywood battens on a canvas-covered board, spaced to the slab width you need.
Roll the clay between the battens in one direction until the roller rides entirely on the battens.
Lift the slab, rotate it 90 degrees, roll again. Add a third pass at 45 degrees for slabs over 20 cm.
Rest the slab between two sheets of plastic for 45 minutes for 8 mm, 90 minutes for 12 mm.
Cut the edges at 45 degrees against a straight edge, not by eye.
Crosshatch-score both bevels, apply single-cream slip, and press together with a slight sliding motion.
Roll a coil of the same clay along the inside of the seam and press it in with a wooden tool to form an inner fillet.
Support the outside of each corner with a foam offcut, build up in pairs, and rest 30 minutes between pairs.

Where it goes wrong

Rolling the slab in both directions without rotating, which just makes an oval slab. The rotation is what equalises shrink.
Using slip that is too thin. Milk-thin slip has no clay body in it and dries to a dusty layer that fails at the seam.
Joining slabs cut from different batches. Two bodies with different shrinkage rates will separate at the seam no matter how good the join is.
Handling a slab by its middle. A slab picked up in the centre bends and stretches; always lift with two hands or on a bat.

Symptom and cause
Symptom	Cause
Corner cracked open during drying	the slabs were joined wet, or the slip was too thin; try the inner fillet coil.
Slab warped into a curve after firing	the slab was rolled in one direction only or not rested before joining.
Seam visible under the glaze as a light line	the slip came from a different body, or the join was not compressed enough.
Box collapsed at one corner while the next was being joined	right angles were not supported while they set.

Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.

Same cone — Cone 6

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Rolling slabs to an even 8 mm and joining them so the seam outlives the pot

Tools