Opening and flooring: how thick the base actually needs to be

The floor of a pot is the part that fails first and the part makers think about least. Open too deep and you cut through into the wheel head; open too shallow and the pot cracks off the bat. Here are the numbers — 1.5 cm to the head for a 1 kg ball, 8 to 10 mm finished floor, 6 mm at the edge of a 15 cm pot — plus the rib work that stops S-cracks before they start.

Choosing an opening depth

Open with two fingers of the right hand pressed down through the centre of the spinning mound and the left hand containing the outside. Stop when the tip of your middle finger is 1.5 cm above the wheel head for a 1 kg ball. For a 3 kg ball throwing a 25 cm platter, stop at 2 cm, because the platter will be trimmed down to a 10 mm floor and you need material to trim.

You can feel the head through the clay once the floor gets thin — the sound of the fingertip changes and there is a hard resistance under the clay. Learn that sensation early. It is the difference between a pot and a hole.

Floor thickness by pot size

The rule that survives firing is 8 to 10 mm for a drinking vessel up to 15 cm across, 10 to 12 mm for a serving bowl, and 12 to 15 mm for anything over 30 cm. The reason is not strength in the hand, it is differential shrinkage: a floor that is twice as thick as the wall shrinks at a different rate and pulls the wall inward, cracking around the foot.

So the actual discipline is to make the floor only slightly thicker than the wall, then trim it to the final dimension at the leather-hard stage. A 6 mm wall with an 8 mm floor is a well-balanced cylinder; a 6 mm wall with a 15 mm floor is a pot that will dunt on cooling.

Compressing the floor

Once the opening is at depth, take a wooden or hard-rubber rib and press the floor outward from the centre in overlapping strokes, eight to ten passes, moving from the middle toward the wall. You are not moving clay so much as aligning particles at the surface and closing the shear planes the fingers opened.

This step is the single cheapest insurance against S-cracks. An uncompressed floor opens cracks at the base in maybe one pot in ten; a compressed floor makes it rare. It costs 15 seconds.

Levelling and checking true

With the wheel at 150 rpm, hold a needle tool flat against the floor and let it score a shallow circle to mark the intended inner base of the wall. Then sight across the floor from the side: a properly levelled floor shows a flat, even surface with no dip in the middle. A dip means you opened too deep and pressed harder in the centre than at the edge.

If the floor is dished, do not try to fix it by adding clay. Push the floor back up from underneath — put a hand under the bat if you can reach — or accept it and design the pot as a shallow bowl, which is what a dished floor actually is.

Steps

1. Centre the mound and reduce the wheel to 150 rpm before opening.
2. Press two fingers of the right hand into the centre of the spinning mound, keeping the left hand on the outside to contain the shape.
3. Open to 1.5 cm from the wheel head for a 1 kg ball, 2 cm for a 3 kg ball.
4. Widen the opening by pushing outward with the fingers while the left hand supports the outside wall, stopping about 2 cm short of the intended base width.
5. Compress the floor with a rib, working from the centre outward in overlapping strokes, eight to ten passes.
6. Score a shallow reference circle with a needle tool to mark where the wall will sit.
7. Sight across the floor from the side, correct any dish by lifting the floor from underneath, then sponge out the slip before pulling the first wall.

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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