Slab Building

Part of Pottery and Ceramics

Building pottery from flat clay slabs — the method for creating boxes, tiles, troughs, and angular forms impossible to make on a wheel.

Why This Matters

Not every vessel is round. A rebuilding community needs flat tiles for roofing and flooring, rectangular troughs for water channels, square boxes for storage, and large flat surfaces for drying food or evaporating salt. The potter’s wheel produces only round forms. Slab building produces everything else.

Slab construction is also the fastest way to make large, flat objects. Rolling out a tile takes two minutes. Hand-forming the same tile from coils takes twenty. When you need fifty roof tiles, a hundred floor tiles, or a dozen drainage channel sections, slab building is the only practical method.

The technique is conceptually simple — roll clay flat, cut it to shape, join the pieces — but the details of thickness, joining, and drying determine whether your piece survives firing or cracks apart. Done properly, slab-built pottery is as strong as wheel-thrown work and far more versatile in shape.

Rolling Slabs

Tools Needed

• Rolling pin: A smooth, straight cylinder of wood, 40-60 cm long, 5-8 cm diameter. A section of straight branch works. A piece of pipe works even better.
• Guide sticks: Two strips of wood, each the same thickness as the desired slab. Place one on each side of the clay and roll across them — the rolling pin rides on the guides, producing a uniform slab.
• Work surface: A canvas cloth, burlap, or smooth board. Clay sticks to bare tables. Fabric allows you to peel the slab off without distortion.
• Cutting tools: A sharp knife, wire, or taut string for cutting straight lines. A needle tool for curves.

Rolling Process

1. Prepare the clay — Wedge thoroughly. Slab clay should be slightly stiffer than throwing clay. If it is too soft, slabs sag and distort when lifted.
2. Form a thick block — Pat the clay into a rough rectangle, approximately 3-4 cm thick.
3. Place guide sticks parallel on the work surface, spaced wider than the block.
4. Roll out — Set the rolling pin on the guide sticks and roll across the clay. Roll in one direction first, then rotate the slab 90 degrees and roll again. This prevents the clay from stretching unevenly in one direction.
5. Flip the slab halfway through rolling. Peel it off the surface, flip it, lay it down, and continue rolling. This ensures both sides are compressed equally.
6. Target thickness:

Application	Thickness	Notes
Tiles (floor/roof)	10-15 mm	Must be even to prevent warping
Small boxes and vessels	6-10 mm	Thinner = lighter but more fragile
Large troughs/channels	12-20 mm	Needs structural strength
Decorative panels	8-12 mm	Balance between weight and durability

Consistent Thickness

Uneven slabs warp during drying and firing because thin areas shrink faster than thick areas. Always use guide sticks. Check thickness by pressing a needle tool through the slab at multiple points and measuring the depth.

Cutting and Shaping

Templates

For repeatable shapes (tiles, box panels), make templates from thin wood, bark, or stiff leather. Lay the template on the slab and cut around it with a knife. This ensures identical pieces that fit together properly.

Cutting Techniques

• Straight cuts: Use a straightedge (ruler, flat stick, taut string) as a guide. Cut with a sharp knife held vertically — angled cuts create joints that do not meet flush.
• Curved cuts: Freehand with a needle tool or knife. Score the line first, then cut deeper on a second pass.
• Bevel cuts: For mitered corners (like a picture frame joint), hold the knife at a 45-degree angle while cutting. This creates a larger glue surface and stronger joints.

Letting Slabs Stiffen

Freshly rolled slabs are too floppy to stand upright. For vertical walls and boxes, let slabs stiffen to the soft leather-hard stage:

• Lay slabs on a flat surface, cover loosely with cloth
• Wait 1-4 hours depending on humidity and thickness
• Test: the slab should hold its shape when lifted by one edge but still be flexible enough to bend without cracking
• If it cracks when bent, it is too dry for joining

Joining Slabs

This is where most slab-building failures happen. Poorly joined slabs crack apart during drying or blow apart during firing. The technique is called score and slip.

Score and Slip Method

1. Score both surfaces to be joined. Use a fork, comb, serrated tool, or knife to scratch a crosshatch pattern into the clay where the joint will be. Score lines should be 2-3 mm deep.
2. Apply slip to both scored surfaces. Slip is liquid clay — the consistency of thick cream. Make it by dissolving scraps of the same clay body in water.
3. Press together firmly — Push the surfaces into each other so the slip squishes out the sides. The scored grooves interlock with slip, creating a mechanical and adhesive bond.
4. Reinforce with a coil — Roll a thin coil of soft clay (5-8 mm diameter) and press it into the interior corner of the joint. Smooth it into both surfaces with a finger or wooden tool. This fills any gaps and dramatically strengthens the joint.
5. Smooth the exterior joint with a rib or finger for a clean appearance.

Same Clay, Same Moisture

All pieces being joined must be from the same clay body and at the same moisture level. Joining wet clay to dry clay, or stoneware to earthenware, creates differential shrinkage that cracks the joint every time. If pieces have dried unevenly, lightly mist the drier piece with water and wait 30 minutes before joining.

Building Common Forms

Rectangular Box

1. Cut 4 wall panels and 1 base panel to exact dimensions. Account for clay thickness — if walls are 8 mm thick, the base should be narrower by 16 mm (two wall thicknesses) so the walls sit on top of it.
2. Score and slip the base edges and the bottom edges of two opposing walls. Attach them.
3. Score and slip the remaining two walls and attach them to the base and to the already-attached walls.
4. Reinforce all interior corners with coils.
5. Add a rim coil to the top edge for strength.

Tiles

1. Roll slabs to uniform thickness (10-15 mm).
2. Cut to size using templates.
3. Place on a flat, porous surface (plaster, dry wood, sand).
4. Flip every 6-12 hours during drying. Tiles that dry from one side only warp into curves.
5. Once fully dry, stack with spacers for kiln loading.

Tile Warping Prevention

Place a flat board and weight on top of drying tiles. The weight resists curling. Use a kiln shelf or flat stone during firing to keep tiles flat. Sand the bottom surface of leather-hard tiles with coarse grit to ensure they sit flat.

Trough or Channel

1. Roll a base slab and two wall slabs.
2. Score and slip the long edges of the base.
3. Attach walls at the desired angle (vertical for a box channel, flared for a trough).
4. Reinforce interior corners with coils.
5. For waterproofing, apply slip or glaze to the interior before firing.

Cylinder from a Slab

1. Roll a slab long enough to wrap around a cylindrical form (a log, pipe, or rolled newspaper works as a form).
2. Wrap the slab around the form.
3. Score and slip the overlapping edges where they meet.
4. Press and smooth the seam.
5. Cut a circular base from another slab, score, slip, and attach.
6. Remove the interior form once the clay stiffens enough to hold its shape.

Drying Slab-Built Pieces

Slab-built work is more prone to cracking during drying than wheel-thrown work because of the joints and flat surfaces.

Rules for Safe Drying

1. Dry slowly — Cover with plastic or damp cloth for the first 24-48 hours. Then gradually open ventilation.
2. Dry evenly — Rotate pieces daily. Cover thin sections (rims, handles) with small damp cloths if they are drying faster than thick sections.
3. Support flat surfaces — Tiles and flat bases should dry on porous, flat surfaces. Hang-drying causes sagging.
4. Avoid drafts — Moving air dries exposed edges faster than protected interiors, causing warping and cracking.
5. Total drying time: 5-14 days depending on thickness, humidity, and clay type. The piece must be bone dry (no cool-to-the-touch spots) before firing.

Troubleshooting

Problem	Cause	Prevention
Cracks at joints	Insufficient scoring/slip, or moisture mismatch	Score deeper, use more slip, match moisture levels
Warped tiles	Uneven drying	Flip regularly, weight during drying, dry on porous surface
Walls collapsing inward	Slabs too soft when assembled	Let slabs stiffen more before joining
Blowout at joints during firing	Trapped air or incomplete bonding	Press joints firmly, reinforce with coils, dry completely
Surface cracking (alligator pattern)	Rolled too thin in spots, or dried too fast	Use guide sticks, dry slowly under cover
Uneven thickness	Rolling without guide sticks	Always use guide sticks; flip and rotate during rolling

Slab building expands your ceramic repertoire far beyond what a wheel can produce. Combined with wheel-throwing, it gives a rebuilding community the ability to manufacture virtually any clay form needed — from dinner plates to drainage systems.