The clay used needs to be matched to the final form. Large structures will need more strength. Not only should the piece's walls be thicker, but the clay body should be chosen for strength and also for a lower coefficient of expansion.
Clay bodies that have sand or grog in them tend to be best. Clay bodies that shrink a great deal should be avoided. Preferably, the clay body should have a shrinkage rate of eight percent or lower.
The clay should be soft in consistency. Stiff clay won't weld together as well, which results in weaker joints that can crack apart. Clay should also be thoroughly wedged before use, as well. This will homogenize it and remove air pockets which can cause blow outs in the kiln.
The Coil Pot Floor
The floor or bottom of a coiled form is usually a slab or patty of clay the same thickness as the finished pot will be. The bottom slab should be substantially larger than the piece's diameter will be; the slab will be trimmed down later.
Once the slab is made, it should be placed on a support surface that will allow 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, a puki, a bowl-shaped mold made of plaster, wood, or biqueware, is used.
If a puki isn't available, a regular bowl of the right size and slope can be used. However, it must be lined with several layers of newspaper or cloth before the clay slab is placed into it.
Beginning to Coil
In sculptural terms, coiling is an additive process; the pot is created through adding material to it. Generally speaking, potters do use coils of clay, but additions can actually be made using other shapes, such as small patties.
Working the piece on a turn table, banding wheel, or 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. Once the first coiled row is in place, the excess slab is trimmed off and the outer edge of the slab is welded upward into the coil.
Soft clay can be directly welded row-into-row. The strongest joints are welded on both the interior and exterior of the wall. Generally speaking, the thumb will be used on the inner surface moving downward, with the fingers on the outer surface welding upwards. If the clay has stiffened at all, score each surface to be joined, brush on slurry or slip, then lay the row and weld.
When coils are welded the wall will thin and expand outward. The more aggressive the welding, the more the walls will thin and expand. Because of this, coils may need to be made substantially thicker than the final wall will be. As a general guideline, smaller pots will use coils 3/8 to 1 inch in diameter, and large pots will use coils 3/4 to 1-1/2 inch in diameter.
Taking a Break
Coiled forms can take more time to create than we have in one sitting. When you have to leave your work while it is in progress, place damp paper towels across the top row. Cover the entire piece with plastic.
If you should need 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 support surface. If the humidity is high, add a layer of newspaper or cloth in between the pot and the plastic. This will absorb any condensation, which could otherwise make your pot collapse.
The natural inclination of the clay is to expand and move outward. To move the wall inward, coils need to be applied to the inner surface of the uppermost row. To compliment this direction 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.
At this point your pot's form may be completed, or you may also wish to further refine the form and thin the walls even further. The clay can be worked again at the soft leather-hard stage, using the paddle and anvil or rib and hand techniques.