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. 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. 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. 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. 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. 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.
