Molds can be made of plaster, wood, bisqued clay, foam board, upholstery foam, and other similar substances. Nearly any substance can be used, but the usage will have limitations. This is due to the fact that clay shrink as it dries. Hump molds must be elastic enough to allow for the clay's contraction. Slump molds have less constraints, since the clay will shrink away from the mold, not toward the mold's surface.
In both types of molds, however, if a non-porous material is used, sheets of newspaper need to be sandwiched in between the clay slab and the mold. Otherwise, the clay may not release from the mold cleanly.
Bowls, plates, and platters can easily be made by slumping a slab into an existing bowl or plate. This works best for shallow forms. Deep bowls will require the slab to be cut, removing excess clay, in order to form smoothly into the mold. Another method would be to piece several different slabs into the mold to complete the form. In either case, joints must be firmly welded.
Another type of slump mold is the open-center mold. These allow the natural curve of the slab itself to shape the form. Molds, often made of foam or plywood, are fairly tall and fashioned with a large hole through the center. An oversized slab is laid over the top, then the mold is raised and gently dropped to encourage the clay to settle downward in the middle.
Draping Clay Over a Mold
Due to the clay's shrinkage, hump molds are best made of porous, flexible material. Besides the materials mentioned above, a one-time mold can be created using wadded newspaper or cloth, kept together inside a plastic sheet or bag.
If the hump mold is able to compress with the clay, it allows for the addition of feet or a foot ring while the clay is still soft. If the mold cannot compress, the clay must be removed from the mold before it has reached the leather-hard stage. Feet cannot be added in this case, since the main part of the pot would collapse around any additions on is bottom.