Hairline Cracks, Nine Days and One Kiln Load
Ndapewa is slab-building a rectangular vessel for the Three-Dimensional Design component: 28 cm tall, with incised geometric bands based on pattern work in her sketchbook. The kiln is loaded in nine days. She has one bag of earthenware left and cannot obtain more of the same clay body. She joined the four walls with slip but did not score the seams. The base is 14 mm thick, the walls 6 mm, the rim 4 mm. She has kept the piece sealed inside a plastic bag between sessions. On Monday two hairline cracks run up from the base corners and one corner has opened about 2 mm. There is condensation on the inside of the bag. Her teacher is away on a course; the technician glances at it and says, 'Just fill the cracks with slip and let it dry.'
1. Diagnose the cracks. Why is the technician's advice likely to make the situation worse rather than better?
Read the worked answer
The cracks are a symptom, not the fault. Four causes are stacking up:
- Thickness differential. Clay shrinks linearly by roughly 5–7% as it dries. A 14 mm base holds water far longer than a 6 mm wall, so the walls stiffen and stop shrinking while the base is still moving. The stress has to go somewhere, and it concentrates at the corners - the sharpest change of direction in the form. That is exactly where the cracks have appeared.
- Unscored seams. Slip alone is a glue line. It shrinks at a different rate from the body and shears under the stress above.
- Condensation in the bag. A sealed bag traps moisture unevenly: the thin rim may dry while the thick base stays wet, or the reverse. Either way the piece dries from the outside in, which is the wrong direction.
- No internal fillet. A right-angled interior corner is a stress raiser. A fillet of clay spreads the load.
Why 'fill it with slip' fails: wet slip added to a part-dry wall swells that patch locally, then shrinks again - so the crack reopens, usually wider. It also seals moisture inside the wall. In the kiln that water turns to steam at 100 °C and a sealed earthenware wall can spall or blow apart, taking neighbouring work with it. Filling treats the crack; it does not remove the stress that made it.
2. Nine days, one bag of clay, one kiln. Decide between repair and remake, and justify the decision with a drying schedule.
Read the worked answer
Decision: repair - but the first act of the repair is drying, not filling.
Why not a remake: four slabs plus assembly is roughly a day and a half of contact time, and a 28 cm vessel with a thick base then needs four to five days to reach bone dry before a safe bisque. Nine days minus building minus drying leaves almost no margin, and the new piece would enter the kiln damper than the repaired one. Remaking also risks a second, unproven set of faults. One bag of clay makes the gamble expensive.
The repair, in order:
- Equalise the drying (days 1–4). Unwrap entirely. Stand the vessel on newspaper or a plaster bat so moisture is drawn out through the base - the thickest part. Tent it loosely with plastic, never sealed; keep it out of draughts and direct sun; rotate it daily.
- Correct the thickness while leather-hard. Invert and scrape the base down toward 8 mm from the inside if reachable, or from the outside of the foot. Equal walls shrink equally.
- Then weld the seams. Widen each crack slightly, score both faces, paint slip, press a coil of the same body into the joint from the inside, compress with a wooden tool, and fillet the interior corners.
- Support. Pack crumpled newspaper inside so the walls cannot slump as they stiffen.
- Test before committing. If a seam reopens within 24 hours, the stress is still active: cut a clean chamfer along the crack and make the repair a deliberate incision, if and only if the brief allows surface variation.
- Bone dry check (day 7–8). Warm to the inside of the wrist - no chill means no water. Load only then.
3. What should go into the portfolio and sketchbook about this episode, and why does that material earn marks?
Read the worked answer
Photograph and annotate the failure as evidence, not as apology.
The assessment rewards problem-solving and realisation, not only a finished object. A crack that is documented, diagnosed and resolved is stronger evidence of design thinking than a vessel that simply happened to work.
What to include:
- A dated photograph of the cracks with the two millimetre measurement, taken before repair. Scale matters; a ruler in the frame makes the record usable.
- A short diagnostic note (30–50 words): 'Cracks at the base corners. Base 14 mm, walls 6 mm - uneven shrinkage. Seams were not scored. Corrected by equalising thickness and welding the joints with coils.'
- A second photograph of the repair in progress, showing the interior coil and fillet.
- A drying log: three dated lines recording weight or feel and whether the base was still cool.
- A design change sheet, if the cracks altered the surface pattern, showing the original and revised drawings side by side with one sentence explaining the decision.
Why it earns marks: it evidences the design cycle - identify, test, decide, evaluate - which is exactly what the criteria name. It also protects her in moderation: if the seam is visible in the fired piece, the record shows it was a controlled decision rather than an accident.
What to leave out: the technician's advice, the plastic bag full of condensation, and any narrative of blame. The portfolio records judgement, not misfortune.