In both types of molds, 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. 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 down in the middle. 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.
