The mechanism
How a pottery wheel actually works.
Not the romantic version. What the machine does, what your hands do, and why the wall gets thinner as it gets taller.
The wheel makes one thing: rotational symmetry. Everything else is you.
The short version. A pottery wheel spins clay on a flat metal head. You brace your hands against something fixed, usually your own body, and let the rotation carry the clay past a stationary point of pressure. Because every part of the clay passes that point once per revolution, the result is symmetrical whether or not you are any good.
Water reduces friction so your hands can stay in contact without tearing the surface. The wall thins because clay squeezed between two points has nowhere to go but up.
The sequence
Five things happen, in this order.
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1
Wedging
Before any of it, the clay is kneaded to drive out air and line up the particles. Trapped air is the usual cause of a piece blowing apart in the kiln, and unaligned clay fights you on the wheel.
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2
Centering
The clay is forced into a shape whose axis matches the wheel’s axis. You do it by bracing and applying steady pressure while the wheel does the work. It is the hardest step by a wide margin, and it is a gate: nothing downstream works until it is done.
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3
Opening
Thumbs or fingers go down through the middle to a set depth, leaving the floor. Get the floor wrong here and no amount of skill later saves the piece, because you cannot add clay back to a bottom that is too thin.
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4
Pulling the wall
A knuckle inside and fingers outside, opposite each other, squeeze the wall between them and travel upward. The clay between them is displaced, and the only direction available is up. That is the whole trick.
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5
Shaping and finishing
The cylinder becomes a bowl, a mug or a vase by pushing from inside or collaring from outside. Speed comes down as the piece gets taller, because the same rotation moves the rim much faster than the base.
Why it behaves like that
Four bits of physics worth knowing.
Symmetry
The product is the rotation
You are not sculpting. You are holding a shape and letting the clay pass through it. That is why a beginner’s pot is still round, and why an unsteady hand produces a spiral rather than a lump.
Plasticity
Clay deforms and stays deformed
Wet clay is plastic: past a yield point it flows and keeps the new shape. Push it too far past that point and it loses structure entirely, which is what a collapse actually is.
Water
Lubricant, and the thing that ruins you
Water lets your hands slide instead of tearing the surface. It also softens the wall. Too little and you drag, too much and the wall loses the strength to hold itself up. Most early collapses are a water problem, not a strength problem.
Leverage
Brace against something fixed
A hand floating in the air cannot hold a line. Elbows locked into your body, forearms braced on the wheel edge, and the fixed point is what the clay is measured against. Fighting the clay with muscle is the mistake almost everybody makes first.
The numbers behind it
Where the tolerances actually bite.
This is the part that tends to interest people with technical jobs, because it is the part where pottery stops being a craft story and starts being a dimensional problem.
| Clay body | Shrinkage from wet to fired |
|---|---|
| 52 Buff | 10% |
| B-Mix with grog | 11% |
| B-Mix | 12% |
| B-Mix with specks | 12.47% |
| Speckled Buff | 12% |
| Half and Half | 12% |
| Berlin Gray | about 12% |
| #16 Porcelain | 13.9% |
Which means the thing you make is never the size of the thing you get. A mug thrown to fit a particular lid, or a planter thrown to drop into a particular pot, has to be thrown oversized by the right percentage for that specific clay.
Then it goes through two firings. Bisque at cone 05, which is free here, and a glaze firing at cone 5½. Glaze firing is charged at 5 cents a cubic inch of the bounding box, with a $4.80 minimum and a cap of $600 on any single piece.
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