Throwing an 18 cm bowl with a 6 mm wall and no S-crack

Bowls fail for a different reason than cylinders do. A bowl is wider than it is tall, so the rim carries the load and the floor carries the shrinkage. The two things that ruin bowls are a rim left thinner than the wall and a floor left thicker than the wall. This is how to get an 18 cm serving bowl to 6 mm at the rim and 8 mm at the floor in one session.

Open wider, pull less

The mistake is to throw a bowl like a short cylinder: open narrow and then pull the wall out. That produces an uneven wall and a rim that tapers. Instead open to 8 cm wide before pulling anything, then take the wall outward in three passes with the wheel at 130 rpm, moving the inside hand horizontally rather than vertically.

Horizontal pulling moves more clay per pass and leaves the wall more even, because you are pushing material sideways rather than dragging it upward against gravity. The rim then thickens slightly as you go, which is exactly what you want.

The deliberate rim

Leave the rim 1 mm thicker than the wall on purpose. Sponge and chamois work erodes the rim; by the time you finish shaping, a rim thrown exactly level with the wall is 0.7 mm thinner. A deliberate 1 mm surplus ends up level.

Finish the rim by pinching it between a wet thumb outside and a dry index finger inside, rolling it inward slightly rather than flaring it. Bowls with a slightly inward-rounded rim stack better, chip less, and hold heat longer for serving.

Floor and foot

Throwing an 8 mm floor on an 18 cm bowl means judging depth by feel: with the wheel at 120 rpm, press two fingers into the centre and open to 1.5 cm from the head, then widen outward while keeping the floor level. Use a rib from the centre out, then sight across the floor from the side — an 18 cm floor shows a 1 mm dip very clearly.

Trim the foot ring after two hours at 6 mm wide and 4 mm tall. A narrow foot ring on a wide bowl is unstable, so keep the ring diameter at 40 percent of the bowl diameter: 7 cm for an 18 cm bowl.

Drying a wide form

Wide forms dry from the rim inward, and that gradient is what makes rims crack. Dry the bowl upside down on a board covered in newspaper, with another sheet of newspaper laid loosely over the top so moisture leaves the whole surface at a similar rate. Turn it right way up after 24 hours and check the rim with a fingernail — it should be leather-hard all the way round at once.

Never dry a bowl on a bat with the rim in contact with a hard surface. The rim shrinks 6 percent while the base is still pinned, and the rim is what cracks.

Steps

1. Centre 1.2 kg of stoneware at 190 rpm and open to 1.5 cm from the wheel head.
2. Widen the opening to 8 cm before pulling any wall.
3. Compress the floor with a rib from the centre outward, 8 to 10 overlapping passes.
4. Pull the wall outward horizontally at 130 rpm in three passes, moving the inside hand sideways rather than upward.
5. Leave the rim 1 mm thicker than the wall and finish by rolling it inward between a wet thumb and a dry finger.
6. Stop, sponge the rim, and cover the bowl loosely for two hours before trimming.
7. Cut a 6 mm by 4 mm foot ring at 40 percent of the bowl diameter, then dry the bowl upside down on newspaper for 24 hours.

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.

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