The Process, Continued - Throwing
The potter then makes sure that the clay is exactly in the center of the wheel by pushing and guiding it with both hands, wetted so that the clay glides smoothly between them.
The centering process is critical to a successful piece. If the piece is not centered, it will wobble as it spins on the wheel, and the walls will be of uneven thickness. If it's too far off-center, it will collapse. Creating a large pot is virtually impossible unless the clay is well centered.
Once the clay is centered, the potter pushes down on the center of the turning clay to create a concavity. Usually the potter wants the "opening" to reach down to about 1/2 inch to 1 inch from the wheelhead (depending on whether the potter will trim away part of that clay at the bottom to create a foot for the pot). That 1/2 to 1 inch will be the base of the pot. Then, with the wheel spinning, the potter pushes from the inside of the pot with the knuckles or finger of one hand with the other hand doing the same thing just opposite those fingers to the outside of the clay. The potter starts near the base of the hole and simultaneously pushes in with both hands while raising them towards the top of the clay. This squeezes the walls of the clay upwards and begins to form the pot.
The shape the potter wants to create will determine how he or she pulls up the walls. Though there are many "correct" methods of throwing, generally speaking the potter will pull the walls up inward--i.e. so that the opening is narrower at the top than at the base (for a bottle shape), straight up (for a vase shape), and outward (for a bowl shape)