Topic: Articles

Skilled Centering

 

A well-wedged mound is the first step toward efficient centering and skilled throwing. Skilled centering facilitates throwing by maintaining strong clay with compressed and aligned particles and an even moisture content throughout the centered mound. The biggest problem during centering is not just controlling the off-center high points and finding the center, but also maintaining the strength and homogeneous consistency of the clay established during wedging. Overworking and the resulting over saturation of the clay during centering erodes the strength of the clay, ultimately compromising the thrown form.

Note: Directions in this article are given for a counter-clockwise spinning wheel. For ease of communication the points around the wheel head will refer to the face of a clock with 6:00 at the potter’s belly button.

1. Wedging

Wedging prepares the mound for centering.

A. Create a high point at the center bottom to dispel air when attaching the mound of clay to the bat or wheel head.

B. Make fairly straight vertical sides to simplify centering (1).

C. Create a subtle high point at the top of the mound to allow the clay to move up the core of the mound during centering.

D. For vertical forms prepare a vertical mound. Open forms start with a lower, wider mound.

2. The Hit

The hit, done well, results in a fairly centered vertical mound, minimizing the work of final centering. The hit also expels air and attaches the mound to the wheel. Trapped air under the mound may cause it to release from the wheel head during centering or throwing. Removing the splash pan facilitates centering.

A. Cup your hands around the clay with your small finger under the change of direction at the base of the hit (2).

B. Stabilize your head with your nose over the center of the wheel (3).

C. Practice moving the wedged mound straight up and down over the center point of the wheel to get a feel for the weight of the clay, the vertical alignment, and the center point of the wheel.

D. Keep your elbows out—elbows next to your body during the hit cause an angled rather than vertical mound.

E. As your hands move down in the hit, continue the movement in a follow through from the mound out across the wheel head.

1 Shape the wedged mound with straight sides and curved ends.

2 Cup the mound, look down on the wheel, and hit the clay against the wheel head.

3 Continue the movement in a follow-through across the wheel head.

3. Reposition

Repositioning the mound moves the clay closer to center and straightens the mound if necessary. If the mound is off center or at an angle after the hit and the suction is strong, reposition the mound instead of rewedging and repeating the hit (4).

A. If the mound is at an angle, touch the high point of the base where it’s the most off center. At the same time, touch the high point on the opposite side at the top where it is most off center and move these points toward the center to straighten the mound (5).

B. Rotate the wheel head so that the point that is the greatest distance off center is toward your body. Embrace the clay, press down, and move the mound toward the center (6 and 7).

4 Straighten and reposition the mound after the hit.

5 Touch the high points of the base and the top and move toward the center. 6 and 7 Rotate the wheel so the off-center point is toward your body. Push down and move toward the center.

4. Slap Centering

Slap centering begins the centering of the clay with dry hands and quick directed slaps to dispel high points and minimize the work of final centering. Don’t overwork the clay at this step. At the end of slap centering, the mound should have straight sides with a soft curve and subtle high point at the top.

A. Center your head above the wheel and look for off-center high points.

B. With the wheel moving very slowly, use loose arms and cupped hands to slap and release, dispelling many high points at one time (8). Touch quickly and release immediately, releasing the top of your hands first and move in the direction of the turn of the wheel to avoid torquing the mound. Your hands should not contact the clay on the same horizontal plane (9).

5. Seal the Mound and Center the Top

Sealing the mound creates a small buttress at the base of the mound preventing the water from eroding the suction (10).

A. Throw a bead of clay at the base of the mound by rolling your index finger in, down, and out creating a tiny buttress (11).

Centering the top of the mound creates a dynamic high point to assist in wedging during centering.

B. Moving against the spin, throw in a tight spiral from the edge to the center top of the mound and back to the edge, leaving a high point at the center top of the mound (12).

8 Slap centering with dry hands to remove any high points and create straight sides.

9 Slap and release quickly, dispelling many high points as the wheel slowly turns.

10 Seal the mound to the wheel head by creating a small buttress of clay.

11 A tiny buttress minimizes the buildup of water that erodes clay at the wheel head.

Lubrication

Lubrication minimizes friction. During final centering, lubricate carefully to minimize the breakdown of the structural integrity of the clay caused by too much water or a torque in the mound caused by too little lubrication.

Lubricate by creating slip from the mound using a loosely held wet sponge or your wet hand. Move quickly and remove the sponge or hand from the surface of the clay before the slip is eroded. Replace the lubrication as it’s eroded during centering. A sponge in your right fingers allows replacement of lubrication during final centering.

Wedging During Centering

Wedging while centering consists of directing the movement of the clay down, across the wheel head, and up the center of the mound, mixing and blending the clay to maintain the compression and alignment of clay particles and the homogeneous consistency of the clay (13). Overworking, and thus over lubricating, the top 13 of the mound leads to a weakening of the structural strength of the top 13 of the pot, often resulting in thin walls and weak rims.

Conversely the bottom 13 of the mound is often underworked and difficult to move, contributing to thickness in the base of the walls and the floor of the pot. Pay attention to your angle of approach to efficiently direct the movement of the clay toward the core of the mound and dispel unwanted high points as you move against the spin of the wheel (14). With adequate preparation, the need for coning, which may erode the strength of a well-wedged mound, is minimized.

6. Final Centering

Final centering moves all high points to a centered position to prepare for opening. Efficient final centering compresses and aligns the clay particles and fine tunes the shape of the mound—taller and narrower for vertical forms, lower and wider for open forms.

A. Hold a sponge in your right hand and lubricate in advance of your left hand.

B. Center the top 13: With a cupped hand, position your left thumb pad at the change of direction at the outer edge of the top of the mound at approximately 7:00. With a raised shoulder, tuck your left elbow into your body, direct the bones of your lower arm to the center core of the mound, use your body weight to apply pressure, and center the top 13 of the clay, releasing pressure at 9:00. Keep your right thumb perpendicular to your fingers and move it toward the center of the mound in order to find and control high points.

12 Center the top of the mound to remove high points and prepare for opening

13 Wedging during centering compresses and aligns particles to minimize coning.

C. Center the middle 13: Both the thumb and outside pads of the left hand control the middle 13 of the mound as the elbow moves down and the arm becomes parallel with the wheel head—do this without a splash pan in place. Begin at 7:00, keep your finger pads away from the clay, use the soft, flexible touch of your fingertips in order to feel the movement of the clay, and direct your body pressure at both pads of the left hand to the crest of the off-center high points. Move against the spin to dispel high points and release from the clay at 9:00.

D. Center the bottom 13: A tag team of the left and right hands accomplishes the final centering of the bottom 13 of the mound. As your left hand moves to center the bottom 13 keep an eye on the formation of a larger buttress at the base of the mound. As soon as you see the buttress getting larger, release the pressure of the left hand, protect the right fingers with a sponge, and starting at the top of the buttress, throw down against the spin (diagonally toward your body 3:00–5:30) (15). At the wheel head, rotate your right palm up to deposit the clay under the outer edge of the mound, create an undercut, and move clay across the wheel head and up the center core of the mound. This move creates a high point on the side of the mound about ½ inch above the wheel head. The wrist pad of the left hand moves to dispel the high point, centering the bottom ¼ inch of the mound without abrading the side of the hand. The right hand moves to the change of direction at the top of the mound to hold it on center, as the left hand dispels the high point at the bottom of the mound. Touch and dispel any remaining high points and the centering is complete.

Hand Ergonomics

Use of the hands warrants a brief discussion to prevent injuries and maximize efficiency. During centering, the job of the right hand is to center the top of the mound, lubricate in advance of the left hand, control the buttress at the base of the mound, and move the clay across the wheel head. Keep your hand cupped and pull or push with your arm and body. For a counter-clockwise spin, your left hand provides the power for centering. The job of the left hand is to control major high points to center the mound. Look at the palm of your left hand with your thumb extended and note both the muscle pad at the base of your thumb and the smaller pad over your wrist bone. With your hand cupped, these two muscle pads are located perpendicular to the end of your arm bones and are your high point controllers and direct the force from your body into the clay. If your hand is straight, there is no power and the pads at the base of your fingers contact the clay and cause significant friction. Keep the finger pads off the clay by cupping your hands. Fingertips of a cupped hand touch the clay and communicate movement of the clay and the location of off-center high points. Your extended thumb keeps the muscle pad strong and available for use in controlling high points. For centering, a move against the spin is critical. Your left hand moves away from your body between 7:00 and 9:00 and your right hand moves toward your body between 1:00 and 3:00.

Angle of Approach

Angle of approach is determined by the direction you need to move the clay. Your tucked elbow controls the angle of approach for your left arm and hand, and thus controls the movement of the clay. Posture and body position affect the angle of approach. Be aware of how natural body movements affect the needed angle of approach and how equipment may hinder the desired angle of approach.

The height of the potter’s chair in relation to the wheel head can prevent the use of body weight transfer.

Wedging during centering is hindered by the height of the splash pan because the left arm and elbow cannot move low enough to move clay in or up.

Check the angle of your fingers and arms when you are throwing or centering to make sure you are actually directing the clay where you want it to go (see figure 14).

14 Angle of approach is determined by the direction you need to move the clay.

15 Center the bottom of the mound. Use a damp sponge to add water as needed.

Wheel Speed

Speed is determined by type of wheel, types of clay, consistency of clay, amount of clay, and strength of the potter. The wheel speed must be fast enough to assist in the movement of the clay, but slow enough for the potter to control the movement of the clay. A fast wheel requires brute strength and lots of lubrication. If the wheel rotates too slowly, the clay doesn’t move easily. The spin of the wheel must assist in displacing the off center high points of clay from the surface of the mound to the core of the mound and back out to fill in the low points. Learn your clay consistency during preparation for centering and test your wheel speed as you throw to center the top of the mound. The bottom 13 may require an adjustment in the wheel speed due to the weight above.

Joyce Michaud is Professor of Art and Founding Director of the Graduate Ceramics Arts Program at Hood College in Frederick, Maryland. Her personal art work and teaching led her to research the physics involved in the art of throwing. She has researched and taught the fine art of throwing for 26 years. Her ceramic art is in the permanent collection of the Smithsonian’s Renwick Gallery. She has published the DVD Hand Throwing: East Asian Wedged Coil.

Photos: Timothy Jacobson

Comments

Comments are closed.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.

Larger version of the image

Send this to a friend