Slipping and scoring is how you join two pieces of clay together. It works by creating a mechanical bond between the surfaces — the scored grooves give the slip something to grip, and as both pieces dry together the join becomes as strong as the surrounding clay. Get the technique right and joins hold through drying, bisque, and glaze firing. Get it wrong and they crack at the seam.
When to Use It
Use slip and score whenever you’re joining two separate pieces of clay — attaching a handle, building walls onto a slab base, adding a coil, joining slab panels, or repairing a crack in greenware. Both pieces should be at the same stage of dryness, ideally soft to firm leather-hard. Joining pieces at very different moisture levels causes stress as one piece shrinks faster than the other, which leads to cracks at the seam.
What You Need
- Scoring tool — a serrated rib, fork, needle tool, or dedicated scoring tool
- Joining slip — made from the same clay body you’re working with. See: How to Make Joining Slip
- A brush or your finger to apply the slip
Step-by-Step
1. Score Both Surfaces
Use your scoring tool to scratch both surfaces in a crosshatch pattern — lines in one direction, then perpendicular lines crossing them. Score an area slightly larger than the actual contact patch. The crosshatch creates more surface area and mechanical grip than parallel lines alone.
- Tip: Score firmly enough to roughen the surface, but not so deep that you’re cutting through thin walls. The goal is texture, not gouges.
2. Apply Slip to Both Surfaces
Apply a thin, even layer of joining slip to both scored surfaces using a brush or your finger. The slip should be the consistency of thick cream — not watery, not paste. Too much slip creates a thick wet layer that shrinks unevenly as it dries and can actually weaken the join.
- Tip: Always use slip made from the same clay body as the pieces you’re joining. Different clay bodies shrink at different rates. A mismatched slip will pull the join apart as it dries.
3. Press and Wiggle Together
Press the two pieces firmly together. A small amount of slip should squeeze out around the edges — that’s good. Give the pieces a slight wiggle as you press to work the slip into the scored grooves and eliminate air pockets at the join.
4. Blend and Smooth the Seam
Use your finger, a wooden tool, or a rubber rib to smooth the excess slip into the seam. Blend the clay from both pieces into each other across the join. This isn’t just cosmetic — blending integrates the two pieces and eliminates the sharp edge where joins are most likely to crack.
- Tip: On slab joins and coil work, always blend the inside of the seam first. The inside is structural. The outside can be cleaned up after.
5. Dry Slowly
Cover the piece loosely with plastic and let it dry slowly. Joins are vulnerable while the clay is still moving. Even drying gives the join time to stabilize before the clay becomes rigid.
Alternative Joining Liquids
Plain water slip works for most joins. For pieces at different moisture levels or tricky joins, stronger options help:
- Vinegar slip: A small amount of vinegar added to the slip acts as a mild deflocculant and increases tack. Useful for leather-hard to leather-hard joins.
- Magic water: A solution of sodium silicate and soda ash in water. Stronger bonding agent for difficult joins or pieces at very different stages of dryness.
Why Joins Fail
- One or both surfaces not scored: No mechanical grip. The slip sits on top rather than bonding into the clay.
- Pieces at different moisture levels: The drier piece pulls moisture from the wetter one unevenly, causing stress at the join as it dries.
- Too much slip: A thick layer dries and shrinks differently from the surrounding clay.
- Not blending the seam: A sharp edge at the join is a crack waiting to happen.
- Drying too fast: Cover and slow it down.
