Most attachments fail at the drying stage, not in the kiln, and they fail because the two pieces were at different moisture states when they met. Leather-hard joining is the discipline of putting two pieces together when both are stiff enough to hold their shape and damp enough to fuse. This is the test to apply, the slip to use, and the four steps that make an attachment permanent.
H 11 · Ø16 / Ø17.5 / Ø9 cm
The moisture test that decides everything
Press a fingernail into the pot and into the piece to be attached. Both should give a mark that holds its shape with no tackiness and no clay adhering to the nail. If the attached piece is stiffer, wrap it in a damp cloth for 15 minutes. If the pot is stiffer, wrap the attachment in plastic and wait.
The tolerance is about two hours of drying time. Beyond that, one piece shrinks while the other is still plastic and the join tears. This is why the productive habit is to make attachments in the same session as the form, or to store both wrapped until they match.
Slip that actually bonds
Use slip made from the same body, mixed to the thickness of single cream, with a pinch of vinegar per 100 ml to flocculate it. Vinegar makes the slip cling to the scored surface instead of running off, and it slows the rate at which the slip dries, which extends the working time on a join to about a minute.
Do not use water as a substitute for slip, and do not use commercial glaze-binder slip. Water weakens the join; commercial slip is formulated for a different shrinkage than your body and will show as a ring under the glaze.
Four steps: score, slip, press, weld
Score both surfaces with a needle tool in a crosshatch about 8 mm across and 1 mm deep. Apply slip to both scored faces with a small brush and let it sit 15 seconds until it goes from glossy to matte — that is the moment the slip has drawn the surface moisture up and will actually stick. Press the pieces together with a slight rotation, then hold them for ten seconds.
The fourth step is the one people skip: weld. Take a small coil of the same clay and press it along the inside of the join with a wooden tool, exactly as with a slab corner. Then blend the outside with a rib. An attachment that is pressed but not welded has roughly half the strength of one that is.
Supporting while it sets
Every attachment is under stress for the first hour, when the added piece is still heavier than its join can comfortably bear. Support spouts from below with a rolled newspaper prop, lug handles from the side with a foam offcut. Remove the props once the join has set — when you can press the attachment with a finger and feel no movement at the seam.
If an attachment does crack off during drying, do not glue it back with slip while both are stiff. Rewet the broken faces, let them return to plastic together, and re-join from scratch. A re-joined attachment that was patched while stiff will fail in the bisque.
Steps
- Test both pieces with a fingernail and adjust moisture with a damp cloth or plastic wrap until the nail marks match.
- Mix slip from the same body to single-cream thickness and add a pinch of vinegar per 100 ml.
- Score both mating surfaces in an 8 mm crosshatch, about 1 mm deep.
- Brush slip onto both scored faces and wait 15 seconds until the gloss goes matte.
- Press the pieces together with a slight rotation and hold for ten seconds.
- Weld a small coil of the same clay along the inside of the join with a wooden tool, then blend the outside with a rib.
- Prop the attachment with newspaper or foam and remove the prop once the seam stops moving under finger pressure.
Where it goes wrong
- Using water instead of slip. Water wets the surface and lets the two pieces separate as they dry.
- Skipping the weld. A pressed join is half as strong as a welded one and fails at the drying stage.
- Attaching to a pot that has already gone past leather-hard. The stiff piece cannot compress, so the join never closes.
- Leaving the props on too long. They hold moisture against the join and leave a damp patch that shrinks differently.
| Symptom | Cause |
|---|---|
| Spout cracked off during drying | moisture mismatch between spout and body, usually the spout being drier. |
| Attachment popped off in the bisque with a clean break | the join was pressed but not welded. |
| Dark ring visible around the attachment under the glaze | slip from a different body, or too much slip left in the join. |
| Dent left in the wall next to the attachment | the piece was pressed on with too much force for the wall thickness. |
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.