Hump moulds are commonly used by potters who do slab work. It is an alternative method of creating bowl-shaped pottery without the use of a potter's wheel. Hump moulds can be made in any size using plaster or unfired clay. They work by filling the space inside the bowl and are usually the shape of an upside-down bowl. A sheet or slab of clay is prepared and draped over the hump. After some drying time, the bowl-shaped slab can then be lifted from the hump mould.
Decide on the shape and size of the bowl you want to make, to establish the required size of the hump mould. Pick a metal or plastic bowl, for example, from which to make a plaster hump mould. Fill the bowl with water, and pour it into a mixing bucket for the plaster. Put on a dust mask, and slowly pour the powdered plaster into the bucket until it forms an island above the water surface. Let it sit for one to two minutes, and then gently stir the plaster mix.
Brush soapy water on the inside of the bowl to make the plaster release. Pour the plaster into the bowl until it is full. Shake the bowl a little to get out the air bubbles in the plaster. Let the plaster cure until hard. Turn the bowl over and hold the plaster with the other hand until it releases. Place the mould on a piece of wood.
Shave off any sharp edges with the knife and, if necessary, use sandpaper to create a smooth surface on the hump mould. Remember that this surface represents the inside of the bowl. Dry the mould completely for a few days. If it is a very large mould, dry it for several weeks.
Roll out a slab of clay about 3/4-inch thick, depending on your preference. Roll it to a size that will completely cover your mould. Raise the mould up from the table so that the clay can drape down over the edge of the mould. Carefully lift the clay slab and drape it onto the hump mould.
Cut the excess clay from the bottom of the mould using a knife. Cut the clay in an even line to create a good rim for the bowl. Use a sponge to smooth the edge along the cut.
Wait for half an hour to an hour, and try to gently lift the bowl off the mould. If it begins to warp, place it back on the mould and let it dry a little longer. Re-use this mould indefinitely to make more bowls in this manner.