I use plaster molds to make pottery bowls dishes and platters.
I mainly use Hump and Press molds. Pictures of these molds are shown below. This is a good method for learning how to make pottery.
A plaster hump mold is a mold created in a shape of the inside of a dish / pot/ platter. Clay is rolled out flat and then laid over the mold. The clay is trimmed and then dried. When the clay is removed from the mold. The clay has dried with the inside of the dish being the shape of the mold.
A press mold is a mold made in the shape of the outside of a dish/ pot/ platter. Taking the same approach as for the hump mold. Clay is rolled out flat and then laid over the mold. This time the clay is pressed in to the shape of the mold and then trimmed. When dry it is removed and the outside of the dish is in the shape of the mold.
These molds are one piece molds.
For items that are more complex then a mold with multiple sections is used. I have seen large press molds in a pottery in Mexico. Vases are made with a height of over 1.5 meters.
Notice the blue sections on the mold. These are in place in order to make sure that the three pieces interlock tightly in order to avoid any slip getting caught in the mold joints.
The more common use for multi part molds is with clay slip. Clay slip is liquid clay. When poured in to the plaster mold it dries quickest next to the plaster. The clay is left for approx 10 minutes until a layer of clay has dried. The liquid clay is poured out of the mold and the dried layer next to the mold is left behind to dry. The thickness of this layer is dependent on the time you let the liquid clay stay in the mold.
Once the clay is dry the mold is removed and you are left with the remaining clay item. The more complex the item you are trying to make the more sections there are in the mold.
See some examples of my pots made with plaster molds.