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Wheel Throwing in Cross Section
Posted By Vince Pitelka On December 12, 2012 @ 10:51 am In Daily,Features,Wheel Throwing Techniques | 1 Comment
When I was taking my first wheel throwing class, I remember staying in the studio late one night centering clay over and over again. Once I got centering mastered, I remember getting super frustrated because I kept throwing the clay off center when I tried to open it. Then, of course, there was the struggle to pull nice even tall walls. Sound familiar?
Since these are not uncommon challenges, I thought I would post this helpful excerpt from Vince Pitelka’s Clay: A Studio Handbook (which is back in print!!). The cross section photos should be a helpful guide for beginners out there and those who teach them. – Jennifer Harnetty, editor.
Penetrating the Lump
After the clay mass is well centered and wheel-wedged, you are ready to begin forming a vessel. With the wheel still running at high speed, lubricate the spinning lump, wrap your hands around either side for stability, and with the tip of one thumb create a dimple in the center of the top. When working with a small lump of clay, squeeze some water into that dimple, and simply continue pressing the thumb down into the lump, creating a narrow V-shaped opening in the center of the lump. Stop when the tip of your thumb is about 1/2-inch from the bat or wheelhead.
When working with a larger lump, after squeezing water into the dimple slowly press one finger of your left hand (the second finger with the index finger twisted around behind it to back it up works very well) down into the center of the lump, holding the finger at a slight angle, and keeping the fingertip right on the center axis as you press down, again producing a V-shaped hole. Keep your right hand wrapped around the right side of the lump for stability, and use the left side of your right thumb as a steady rest and guide, sliding the fingers of your right hand against it as they penetrate the lump. Stop pressing down into the lump when your fingertip is approximately 1/2-inch from the wheelhead.
Measuring the Thickness of the Bottom
The decision of how thick to leave the bottom of a pot depends on whether or not you plan to trim away any clay from the bottom. Until you develop an accurate sensitivity to bottom thickness, it is worthwhile to check it at this point. Hold your needle tool in one hand with the index finger against the base of the needle. Poke the needle tip through the inside bottom of the “vessel” until it touches the wheelhead or bat surface, and slide your index fingertip down along the shaft of the needle tool until it touches the bottom of the “vessel” Lift finger and needle tool out together. The distance between your fingertip and the end of the needle tool gauges the thickness of the bottom. You can use this system whenever you wish during the throwing process to measure the thickness of the bottom of a pot. If you plan to trim the bottom, it should be at least 3/8-1/2-inch thick. If you are not going to trim the bottom, 1/4-inch thickness is adequate for small and medium-size vessels.
The Claw, Widening the Bottom
Slow the wheel down a bit for this operation. As your skill develops, you may wish to do the penetrating and claw (widening) steps in one continuous movement with the wheel at high speed. Squeeze water from your sponge over the spinning lump so that it flows down over the inside and outside walls. With your right hand wrapped around the right side of the lump for stability, place the second finger of your left hand into the hole in the lump, with the fingertip touching the bottom of the hole and with your index and ring fingers “backing up” the second finger, providing more strength and rigidity. Keep the edge of your left index finger resting against the side of your right thumb for stability (with right hand still wrapped around lump), and move your three left fingers (together as described) towards you in a claw motion, so that you widen the inside and bottom of the “vessel” without widening the opening at the top. As you widen the bottom try to keep it as level as possible. If you end up with a raised bump or “pedestal” in the center of the bottom it is because you are pushing down as you widen the bottom. If you end up with a concave cross section across the bottom it is because you are lifting up your fingers as you widen the bottom. Avoid both these circumstances, trying instead for a flat uniform bottom. There are circumstances where you will want a curved bottom, but for the sake of skill development it is good to work on creating uniform flat bottoms.
During the latter part of the “claw” movement many potters will squeeze with the left hand and apply slight pressure against the outside of the “doughnut” with the right hand. This is called “recentering” and some people find it helpful in ensuring a uniform “doughnut” before proceeding with lifting the walls. In my experience, if initial centering, wheel wedging, penetrating, and widening are done correctly, recentering usually is not necessary.
Compacting and Leveling the Bottom
When you widen the bottom you are pulling clay away from what remains as the bottom of the pot, whereas during subsequent lifting you will be compressing the walls of the vessel from both sides. If the bottom remains uncompressed, it will shrink more than the walls, and S-shaped cracks may form during drying or firing. To avoid this, apply mild fingertip pressure against the spinning bottom, moving from the center to the right edge and back again several times. This is also the ideal time to level any irregularities in the bottom.
Back by popular demand!
Lifting the Walls
If all has been done correctly up to this point, you are at what is called the doughnut stage, ready to begin lifting the walls. Hold both hands as you would to shake hands with someone. Bring them together, and cross and lock your thumbs together, creating what we call the caliper position. Bend your fingertips slightly inward. If you bend them too much you will rake clay off the surface, and if you bend them too little you will be working with the flats of your fingers and will have little control. With your hands held in this caliper position, you have formed both a lifting tool and a measuring device, and during the lifting process it is important to always think of your hands as not only moving and thinning the clay, but also constantly gauging the thickness of the walls.
Bend your hands/caliper downwards at the wrist. It will feel awkward initially, but you will get used to it quickly. While undertaking the lifting process, keep your elbows resting on your thighs or tucked in against your torso for stability.
When you have experimented with this position, bring the wheel to medium speed and squeeze your sponge above the rim of the spinning vessel so that water flows down both inside and outside. Lower your hand/caliper over the right-hand wall of the vessel, with your left hand inside and your right hand outside. From this point on, always work on the right-hand side of the vessel, where the clay is moving away from you. With the wheel at medium speed, close your hand/caliper so that your slightly-bent fingertips press against the lower extreme of the walls inside and outside, and slowly lift your hands. During the very first lift, the wall at the base of the vessel will be considerably thinner than the doughnut above it, so do not apply significant pressure until you come up against the doughnut, but at that point increase the pressure and continue lifting. In the first lift, you should be able to raise the doughnut, producing a uniform wall approximately 1/2-inch thick. The most common fault at this point is to apply too much pressure beneath the doughnut, so that the lower walls are thinned too much and no longer have the strength to withstand the torque necessary to thin the walls above.
As you lift, your fingertips should leave very gradual spiral marks up the side of the vessel. Stop just short of the lip of the vessel. If the size and shape hasn‚Äôt changed at all in the first lift, you are not pressing hard enough. Always lift in one continuous pass from the bottom to the top, and between each lift always squeeze more water over the rim (with the wheel spinning). Stop each lift just below the rim, and do not allow your fingers to slip off the rim, as this will distort it badly. Repeat the lifting movement as many times as is necessary to bring the walls to the desired height and thickness (1/4-inch thick right below the rim). In each lift after the first one be sure you apply pressure right from the bottom, in order to maintain even wall thickness, but try to avoid making the walls too thin anywhere.
To measure the thickness of the walls, hold your needle tool in your right hand as you did for measuring the thickness of the bottom. Hold a finger of your left hand against the inside wall where you want to measure thickness. At the corresponding spot on the outside, poke the needle tool through the wall until it barely touches your finger on the inside. Complete the measurement just as you did in measuring the thickness of the bottom.
If you find that you are ending up with wide bowl-like cylinders, then you need to concentrate on keeping them narrow. If you are starting with a fist-size ball of clay, then the inside bottom shouldn’t be more than 3-4 inches wide. Centrifugal force tends to direct the clay outward from the center, and you must counteract this. When you are lifting, think of your hands in the caliper position as a single tool. As you lift the walls, you must purposefully direct that tool inward towards the center axis of the pot, resulting in a tall tapered cylinder.
As you are lifting the walls, if you encounter small hard bumps in the clay, they are either foreign objects (like a small piece of sponge or fired clay) or air bubbles. Poke your needle tool into the bump. If the needle goes right through, it is probably an air bubble. When you do your next lift the air will squeeze out through the hole left by the needle tool. If the bump is a foreign object, then you must decide whether to leave it or remove it. If it is very small you may choose to ignore it, but if it is large you can remove it and press a small piece of clay into the hole.
For more interesting wheel throwing techniques, download your free copy of Five Great Pottery Wheel Throwing Techniques: Tips on Throwing Complex Pottery Forms Using Basic Throwing Skills.
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