Coiling clay pots is a simple process of layering coils or other shapes, one at a time, then welding the layers together to create a solid form. It is an extremely versatile technique that is good for beginners but can be used for great craftsmanship and artistry. Even those who are new to clay should be able to make large and intriguing pottery using the coiling method.
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Choosing the Right Clay for Coiling
The clay used to build coil pots must be matched to the final form, meaning larger structures need more strength and thicker walls, so choose a clay body of appropriate strength and a low coefficient of expansion.
Clay bodies that have sand or grog in them tend to work best. Look for a shrinkage rate of 8 percent or lower. The clay should also be soft in consistency. Stiff clay won't weld together as well, which results in weaker joints that can crack apart.
Be sure that the clay is thoroughly wedged before you use it. This will homogenize it and remove air pockets that can cause blowouts in the kiln.
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Creating the Coil Pot Floor
The floor or bottom of a coiled pot is usually a slab or patty of clay and has the same thickness as the finished pot. When rolled out, the bottom slab should be substantially larger than the piece's diameter, since it will be trimmed down later.
Once you've made the slab, place it on a support surface that allows the piece to be moved safely. For a flat-bottomed pot, this support surface could be a bat or a disc of bisqued clay. For a pot with a curved bottom, use a puki, which is a bowl-shaped mold made of plaster, wood, or bisqueware.
If a puki isn't available, you can use a regular bowl of the right size and slope. Line the bowl with several layers of newspaper or cloth before placing the clay slab into it.
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Beginning to Coil
In sculptural terms, coiling is an additive process, meaning the pot is created by adding material to it. Typically, potters use coils of clay, but additions can also be made with other shapes, such as small patties.
Working the piece on a turntable, banding wheel, or a kickwheel can make the coiling process much faster and easier.
Coiling begins on the upper surface of the pot's floor, not at the sides. Doing it this way results in a much stronger joint. Place the first coil onto the slab floor, then trim the excess slab, and weld the outer edge of the slab upward into the coil.
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Adding Coils to Your Pot
Soft clay can be directly welded row-into-row. The strongest joints are welded on both the interior and exterior of the wall. The standard welding technique is to use one thumb on the inner surface moving downward, while your fingers on the outer surface weld the clay in an upward motion. If the clay has stiffened at all, score each surface to be joined, brush on slurry or slip, then lay the coil row and weld it.
When coils are welded, the wall becomes thinner and expands outward. The more aggressive the welding, the more the walls thin and expand. Coils may need to start substantially thicker than the finished wall will be. As a general guideline, smaller pots use coils 3/8 to 1 inch in diameter; large pots use coils 3/4 inch to 1 1/2 inches in diameter.Continue to 5 of 6 below.
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Taking a Break
Coiled forms can take more time to create than you have available in one sitting. When you have to leave your work while it is in progress, place damp paper towels across the top row, then cover the entire piece with plastic.
If you want the bottom of the pot to stiffen slightly, leave some gaps where the plastic meets the table or shelf. Otherwise, tuck the plastic under the supporting surface. If the humidity is high, add a layer of newspaper or cloth between the pot and the plastic. This will absorb any condensation, which could otherwise make your pot collapse.
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The natural inclination of the clay is to expand and move outward. To move the wall inward, apply a coil to the inner surface of the uppermost row. To complement this directional change, it is sometimes helpful to weld the top of the applied coil inward and the interior side downward.
As you reach the final row of your pot, you may find that the welding process has left your rim too thin. If so, simply add another coil, either to the outer or inner surface of the rim, and weld it.
At this point, your pot's form may be completed, or you can further refine the form and thin the walls even more if desired. You can work the clay again during the soft leather-hard stage, using a paddle and anvil or rib and hand techniques.