Forty Bowls, Ten Days, and a Fan
A school craft club has an order for 40 earthenware soup bowls for a local lodge. They have 80 kg of local clay containing about 15% grog, a small electric kiln that fires to 1100°C, and ten working days. The bowls are 18 cm across with 8 mm walls. In the first batch of 12, six developed S-shaped cracks running down from the rim, and three handles attached with thick slip have separated at the join. The glaze batch is noticeably thicker than the club's usual mix and has crawled on two test bowls, leaving bare patches. The club dries work in an open shed at 32°C with a fan running all day. The lodge will reject any bowl with a crack or a loose handle. The club leader asks you to diagnose the failures and propose a realistic plan that still meets the order.
1. Explain the most likely causes of the rim cracks and handle separation, and why adding water to the clay would make the problem worse.
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The rim cracks are most likely drying cracks. The rims are thin, the shed is 32°C and a fan runs all day, so the rim loses water faster than the base; the clay shrinks at different rates and tears in an S-shape. The handle failures point in the same direction: if the handles were attached with thick slip, or attached when the bowl was already too dry, the join cannot shrink with the body. The slip may also have been too thick to key into the clay. Adding water would not solve this. Extra water increases total shrinkage and makes the body and handles shrink differently again; it can also weaken the clay. The better correction is to slow the drying, cover the rims, turn the fan off or move it away, attach handles at leather-hard with slurry of the same clay, score both surfaces and wrap the joins while they equalise.
2. Produce a ten-day production plan that can deliver 40 sound bowls, including a contingency for losses. State the key control point at each stage.
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Days 1–3: wedge the clay and throw or press 46 bowls, not 40, to allow for losses; cover them loosely with plastic and dry slowly away from the fan. Day 4: trim and attach handles at leather-hard, using scored joins and slurry; wrap handles for a day. Days 5–6: dry under plastic with the rims covered, then turn the fan off and allow gradual air drying. Day 7: bisque-fire the sound bowls after checking for cracks. Day 8: glaze and load. Day 9: glaze-fire at 1100°C. Day 10: inspect, reject cracked or loose-handled bowls, and pack the best 40. The control points are: no draught during first drying; handle joins checked before bisque; no bowl loaded if a tap sounds dull or a crack is visible; glaze thickness checked on a test tile. Keeping six spares is the contingency, because the first batch lost half.
3. Recommend how to correct the glaze batch and adjust application and firing to avoid crawling.
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The glaze is too thick, and crawling usually means the layer is heavy and has poor adhesion. Sieve the batch to remove lumps, then adjust with water until it coats a test tile evenly; a simple dip test is to check that the layer is about as thick as a matchstick on the bisque. Clean dust from each bowl with a damp sponge before glazing, stir the glaze often, and dip quickly rather than pouring heavily. Wipe the foot ring clean. Fire at 1100°C with a normal soak, and place test tiles in the kiln so the result can be checked before the whole load is judged. Do not try to rescue the two crawled test bowls by adding more glaze; the crawling edge will not heal reliably. If the adjusted glaze still crawls, thin it further and apply two thin coats rather than one thick coat.