Why Mitred Boxes Are Worth the Effort

Slab building is the most direct way to make a box in clay. You roll out a sheet, cut your panels, and join them. The trouble is that the obvious approach — butting panels edge to edge and smoothing a coil over the seam — tends to look lumpy, and those fat external seams can crack as the piece dries. Mitring solves both problems at once. By cutting each panel with angled edges at the corners, you create a clean 45-degree joint that sits flush and neat, like the corner of a picture frame. The result is a box that looks deliberate and handmade in the best sense, rather than improvised.

It does demand accuracy, but accuracy in slab building is mostly about patience and a couple of simple tools rather than rare talent. Give yourself a full session for your first attempt and you will finish with something worth keeping.

Getting Your Slabs Flat, Even and Leather-Hard

Everything depends on the slabs. Roll them out between two battens of equal thickness so the sheet comes out uniform — 6 to 8mm is a good working thickness for a box of, say, 15cm across. Thinner than 5mm and the walls will warp in the firing; thicker than 10mm and the box becomes heavy and slow to dry.

Roll on a porous surface such as plaster board or a fabric-covered board, and flip the slab once or twice as you work so it does not stick. Then leave it alone. You want the clay at leather-hard: firm enough to stand up when lifted, still flexible enough to take a sharp cut without crumbling. Press a fingertip in gently — it should leave an indentation but no wet shine.

• Roll in two directions, turning the slab a quarter turn between passes, to avoid a grain that pulls the panel out of square.
• Cover slabs with plastic while you work on the next one, so the first does not stiffen past the useful stage.
• If the slab starts to dry unevenly, wrap it in damp newspaper under plastic for an hour and try again.

Marking Out and Cutting Accurate Mitres

Decide on the outer dimensions of the box first, then work out your panel sizes. For a box with sides of thickness t, the two opposite walls should be cut to the full outer length, and the other two cut shorter by twice the thickness plus a little allowance for the joint. Sketch it on paper before you touch the clay; a few minutes with a pencil saves a lot of swearing.

For the mitres, a mitre box or an adjustable set square is ideal. Mark the 45-degree line with a needle tool against a steel rule, then cut with a taut wire or a sharp knife held vertically — do not lean the blade, or the angle will be wrong down the depth of the cut. Cut each panel with the same tool and the same setup so the angles match.

Check your work by dry-fitting the four walls on the base. The corners should meet with a hairline seam, not a gap and not an overlap. If a panel is a millimetre out, trim it now rather than filling the error later.

Joining with Slip and a Firm Hand

Score both faces of every mitred edge with a needle tool or fork, then paint on slip — clay mixed with water to the consistency of double cream, ideally made from the same body you are using. Let the slip sit for a moment until it goes tacky, then press the joint together.

Hold the corner closed and run a finger down the inside seam to push a little clay into the joint. On the outside, do not smear the slip around; wipe the seam with a damp sponge and a light touch so the sharp corner survives. Work around the box, joining all four walls to the base and to each other, and check the diagonals with a tape measure — equal diagonals mean the box is square.

Stand the box on a board and let it firm up before you handle it much. The joints are at their most fragile in the first hour.

Reinforcing the Internal Corners

This is the step that separates a box that survives the kiln from one that splits at a corner three weeks later. Once the walls are firm enough to hold their shape, add a coil of soft clay down each internal vertical corner, pressed in with a wooden modelling tool. Blend it into the wall and base without making the seam bulky.

• Roll coils thin — about 5mm — and press rather than smear, so the clay compacts against the join.
• Add a small triangular fillet where the base meets the walls, especially on larger boxes.
• Keep the inside corners dense and smooth; trapped air pockets there will cause trouble in the firing.

If the box will hold weight or be handled often, a thin coil around the inside of the base rim is also worth the effort.

Drying, Finishing and Firing

Dry the box slowly and evenly. Cover it loosely with plastic for a few days, then uncover one side at a time. Rushing this stage is the most common cause of cracked corners, particularly in a warm studio or near a radiator.

Once leather-hard, tidy the outside with a rib or a damp sponge, refine the rim, and consider a foot ring or a simple chamfer at the base. Bisque fire slowly to around 1000°C, then glaze and fire to the temperature your clay body requires — most stoneware bodies for domestic use will want 1220 to 1260°C.

Mitred boxes reward a slow, careful approach, but the technique becomes quick with practice. Make three or four in a row, and by the last one your corners will be crisp, your joins strong, and your boxes genuinely useful around the house and garden.