Bowl Making: The Inversion Technique Ivor Lewis

Appears in the May/Jun 2018 issue of Pottery Making Illustrated.

As potters, our products should exhibit economy and spontaneity in form and decoration through intelligent use of mind, eye, and hand. Bowls for holding should nestle in the hand. For mixing, they should be robust. For serving, they should be light and easy to carry.

Old Lessons

I was taught to make bowls by centering a ball of clay, opening it, then spreading it to create a concave inner contour from the base outward, squeezed from inside and out using both hands, and stretched as a rising wall upward to the rim. Several pulls were needed to achieve both the desired diameter and height. It was common for the wall to slump, leaving a humped ring at the foot of the wall inside near the bottom. For a beginner, correcting this defect was a struggle, it was a difficult to make a matching set, and quality was inconsistent with variations of height, diameter, and contour.

Studying the Problem

All bowls are basically a hemisphere, and the strength of a bowl during production is dependent on our approach to, or departure from, that ideal hemispherical form. When the foot ring is narrow with an overhanging base wall that is almost horizontal, so that there is a marked change in profile to achieve a near vertical wall, there is potential for the form to slump over the foot and cause a hump around the interior at the bottom of the pot.

To correct this defect, model the rising contour earlier, closer to the center of rotation, and reduce the inflection between the base and the wall, or alternatively, increase the diameter of the foot ring. However, if these changes are taken to either extreme, that desirable volume of a hemispherical shape is lost. The wall and the floor may merge to give a cone or the base spreads to create a dish. This effectively reduces the working volume of the vessel.

Understanding the Materials

To become successful at throwing, it’s necessary to understand the strengths and weaknesses of plastic clay. Throwing clay should contain enough moisture to provide adequate lubrication. It’s preferable to use throwing slip rather than water, since slip is a better lubricant. The clay can then remain slippery without absorbing unwanted moisture.

Thick clay is stronger than thin clay and can resist more stress without deforming. When a potter starts working to shape a bowl, the first effect is to make the clay thin and therefore weaker at the base of the wall, where it will bear the most stress. Stresses arise from the torque, from friction, and from gravity. At the throwing point, gravity, acting on the heavier mass extending and overhanging the foot, will cause the clay particles to shear and slide past each other, and slumping occurs.

A Different Approach

The key to avoiding slumping in bowls: expanding the profile to create volume should be the last action, not the first. My inversion technique, which takes a lot longer to describe than it does to perform, is a versatile method that can be used for small or large, deep or shallow, wide or narrow bowls. Once the method is understood and practiced consistently, uniform bowls can always be achieved. You’ll soon find you can change the profile to give broader interiors and steeper walls by varying the direction of your hand movement at the inversion stage. You’ll also find your production rate goes up, and you’ll get more pots from your clay. Less trimming is needed to achieve lightness, and bigger volumes are developed from smaller weights of clay. Furthermore, height, which enhances aesthetic elegance, is maintained.

The Inversion Technique

To begin, your clay should be plastic, smooth, uniform in texture, consistent, and until the technique is understood, it should be soft rather than firm—firm clay is less plastic and is difficult to center. Begin with 1–1½ pounds of clay. Work on a bat if possible and lightly moisten the clay to lubricate it.

Your immediate aim when learning isn’t to make very light, thin bowls, but to master the process. Your skill will develop as your confidence grows, and eventually you’ll make larger bowls with thinner walls. Note: In the following description, the wheel is revolving counterclockwise and throwing occurs on the right side of the pot. In the diagrams, a circle on the arrow shows fingertip movements.

Center and Open

Center 1 pound of clay on a bat, then form it into a flat-topped cylinder, slightly higher than it is wide (1). Try to make the diameter at the rim measure twice the height, with the foot about ⅓ the diameter of the rim. Open the cylinder by pressing into the center of the revolving clay (2A) to leave the floor about ¾-inch thick (2B). I prefer to sink my middle and ring fingers, held tightly together, into the clay. These glide over the notch between thumb and forefinger of my left hand, which gives rigid but supple support against the force of the spinning clay. This technique has two advantages—your fingers can reach farther into the clay than your thumb can, so it’s equally useful with large or small masses of clay; and there’s no need to change hand positions for the next stage.

Complete this step by pressing your left thumb (2C) against the base outside your rotating pot. This brings the foot ring back to the diameter you’ve chosen, and it compresses clay inside the base, helping to prevent future S-crack problems.

Now it’s time to change the clay’s shape to an open cylinder that’s slightly higher than the intended bowl, with enough thickness in the base from which a foot ring can be turned. Continue using your left hand ring- and middle-fingertips (3A) to stretch the clay at the base of the cavity outward until the inside opening (3B) is about four times the thickness of the wall (3C). Raise the wall. Have your left hand inside the pot and your right hand outside so that the fingertips of each are directly opposed. Keep this distance (3C) constant as you move your hands upward to the rim. As you thin out the clay by lifting and applying pressure, the wall will get taller (3D). Because the thin clay is concentric with the thick clay, pressure from the latter doesn’t cause slumping through the cantilever effect. This will give a circular wall of even thickness from bottom to top. Compress the rim, which should be level, to consolidate the clay and ensure that it runs true and concentric with the spin of the wheel (3E). If the rim waves up and down, don’t try to trim it. Remember this irregularity and make corrections to your centering technique. Consolidate clay in the base (3F) to prevent S cracks.

The measurement between the rim and the bat (3G) should be slightly higher than the intended height of the bowl. If your pot is shallow, draw the wall higher from the bottom, but keep its wall thickness uniform. If it gets too wide, compress and collar from the outside, working the clay upward from the bat. Your cylinder should be slightly higher than it is wide.

The Trumpet Bell Shape

The next step is to form a trumpet bell shape. Supporting the outside with your right hand (4A), use your left hand (4B) to push the revolving clay outward. As the clay deforms under this force, both hands are lifted so they move upward and outward together to the rim in response to the changing shape of the clay. Your right hand will resist this force, but should support the clay rather than work it. Note: You’re not squeezing the clay, you’re stretching it. Don’t try to increase the height of the wall. As your fingertips continue shaping the clay, both hands move in an arc (4C). This stretches your clay horizontally, causing the wall to thin without being lifted higher. Your movement leaves clay relatively thick, firm, and strong at the base of the wall so that it supports thinner, weaker clay near the rim. This reinforcement prevents slumping. Two or more passes may be needed to pull the clay out to the desired rim diameter. At this stage, the preshaped clay should resemble the funnel form of an inverted trumpet bell as shown in figure 5. With practice, you’ll be able to achieve this shape in one pull. You have achieved a mechanically strong structure, which resists slumping. Strength is enhanced because a minimum amount of moisture has been absorbed by your clay as you worked it. You are also making use of the mechanical advantage of a tapered cantilever beam.

Mop excess water from the interior, then use your fingers to apply pressure to the base inside the pot. This consolidates the clay and is effective in preventing S cracks after the foot ring is trimmed. Use a rounded rib (5A) to clean excess slurry from the exterior, then true and firm the lip of the rim with your fingers.

Trim the band of clay (5B) from the foot at the wheel head, and leave a slight undercut for wiring off, since it will be difficult later to gain access to the underside, especially on low-profile shapes.

Stretch Out, Lift, and Lift Up

Moisten the inner surface of the bowl so that it has a uniform coating of slip, but keep the outside free from excess lubricant. Your left hand will work the interior of the bowl while your right hand gives support on the exterior—support without pressure means less friction so less lubrication is needed. Place your right-hand fingertips outside the bowl against the clay near the wheel head (6A) and your left-hand fingers inside at the base of the bell (6B). Have the wheel turning at less than half speed.

Now, with the wheel turning at a moderate speed, push your left-hand (interior) fingers outward in a horizontal direction (7A). Use your exterior (right) hand for support, but don’t use it to thin your clay. Resist any impulse to press back with your right hand. This would squeeze the clay, which isn’t your intention. As your inner fingers stretch the clay, they will be slowly raised in a long arc (7B). The differential movement of stretching and lifting changes: the stretch out is fast, and the lift is slow, then as you reach the halfway point, the lift up becomes quicker than the stretch. Your final movements should be slow and should give a vertical wall with a thin rim. You have now successfully created a hemispherical basin.

The sequence of shapes that created this hemisphere avoided the obvious and direct route and interposed a stage that exploited the properties of plastic clay. As you gain skill through practice, you’ll find it becomes easier to achieve a smooth curve with one pass when making small bowls, and no more than two passes when making larger bowls.

Refining the Curve

With the wheel spinning slowly, use a round-profile rib (8A) to skim slurry from interior of the bowl. This also helps to refine and adjust the curve and remove irregularities. Work your rib from rim to base, but give only light supporting pressure from your right hand on the outside. If there’s an excess of slurry in the bottom, mop it out with a sponge, but finish with the rib laid over at an angle to make the clay smooth. With your chamois, finish the rim (8B) by molding a smooth, curved lip, then cut under the foot close to the bat.

Lifting your bowl may distort it, so leave it on the bat and take that from the wheel head. If you’re throwing without a bat, lift the bowl from the wheel with dry hands and make sure the wire gives a clean dry cut. When the clay is leather hard, excess clay can be trimmed and a foot ring defined (9A). Round the sharp edges (9B) to prevent tabletops from being scratched, and so that it feels good in the hand. Except for necessary decoration, allow your pot to dry before touching it again or taking it from the bat. It will then be ready for drying and firing.