Before widening an opening, check how much clay remains underneath it. That floor must survive cut-off and, if planned, trimming. Measuring now is easier than discovering a hole after the walls are raised.
Allow for the foot
Leave material beneath the interior floor if you plan to trim a foot. A flat-bottomed practice cylinder needs a different allowance. Choose the underside design before pressing downward so that the base is neither accidentally thin nor a large block left for later carving.
The interior also sets the opening shape. A cylinder needs a relatively flat floor and a controlled transition into the wall. A bowl needs a curve continuing upward from the base.
Make the central depression
Begin with clay that is centered through its full height. Make a small central depression and deepen it under controlled pressure. If the opening travels around the wheel, stop expanding it and reassess. Widening an eccentric hole generally makes unequal wall thickness more obvious.
Support the working hand with the other hand where comfortable. Lower the fingers steadily as resistance changes. Keep the contact lubricated, and sponge out pooled water so you can see the floor.
Check floor depth
With the wheel stopped, a needle tool can help check how much clay remains between the interior floor and the bat. Insert it carefully and use your finger as a reference for the clay depth, keeping the sharp point controlled. Then close and smooth the small test opening. If you are unsure how to read the measurement, practice on a piece you plan to cut apart.
Base thickness depends on the project, clay and planned foot. For a practice cylinder, compare the depth reading with a cut section afterward. That shows whether you read the tool correctly and how much your later movements changed the floor.
Widen to the intended profile
After establishing depth, widen the interior while preserving that floor. Repeated downward pressure at the floor-to-wall junction can leave a thin ring around a thick center. Guide the movement into the profile you planned.
Smooth the base and remove standing water before pulling. If you have opened through to the bat, reclaim the practice piece and begin again with a shallower opening. Plugging a saturated hole is an unreliable beginner repair for a vessel you intend to keep.
Questions about this topic
Should every pot have a thick base for trimming?
Leave only the allowance appropriate to the planned foot and form. Excessively thick bases create more trimming work and may dry differently from the walls.
Why does one side of my opening look thicker?
The opening may be off center, or the original mound may not have been centered throughout. Examine that cause before pulling a tall wall.