Throwing a Form That Closes on Itself
Most thrown pottery is open at the top: a bowl, a mug, a vase. A closed form is one where the walls come back inward and meet at the top, creating a form with little or no opening. Think of a hollow sphere, a teapot body before the hole is cut, or a sculptural closed vessel.
These forms are sometimes called doughnut forms when they have a hole through the middle: like a literal torus shape, or closed forms or hollow spheres when the top is nearly sealed.
Why Closed Forms Are Challenging
- You cannot reach inside to support the walls from the interior once the opening becomes very narrow.
- All shaping must happen from the outside, requiring controlled outward pressure from inside using tools, or gentle pushing from outside using a rib.
- The form must be structurally sound enough to support itself: wet clay wants to slump outward.
Closed Forms in Practice
Closed forms are the foundation for:
- Teapot bodies (closed sphere with holes added for spout and lid gallery)
- Hollow sculptural pieces: decorative vessels, ikebana forms
- Lidded boxes when the lid and base are thrown as one and then cut apart at leather hard
- Doughnut or donut forms: fully enclosed toruses used in sculptural and functional ceramics
Critical Warning: Never Fully Seal a Closed Form
A completely airtight closed form will explode in the kiln. As the temperature rises, the air trapped inside expands. If there is no escape route, the pressure builds until the form cracks or bursts, damaging itself and everything nearby on the shelf.
Always leave a small hole, or pierce one with a needle tool once the form has stiffened slightly. Even a 3-4 mm hole is sufficient to allow trapped air and steam to escape safely.
Keep Exploring
Closed forms are the foundation of teapot making, one of the most technically demanding areas of studio pottery. The Japanese tradition of raku ware tea bowls and the broader mingei folk craft movement both celebrate the mastery of form that closed and spherical shapes require.
Common Mistakes and Quick Fixes
- Sealing forms airtight: Always leave or pierce a vent hole before firing.
- Trying advanced closure too early: Master simple hollow spheres before torus forms.
- Over-thinning walls during shaping: Keep enough wall mass for structural stability.
- No drying plan for enclosed forms: Dry slowly and evenly to reduce stress cracks.
Practice Exercise
Throw three closed forms with different vent-hole sizes and track drying and firing outcomes. You will quickly learn practical safe vent dimensions for your clay body and workflow.
