When working with a mold what is best clay to use?

There are different types of molds and so different types of clay work best in specific molds. Plaster molds are generally used with clay in liquid form, known as slip. In this type of mold, the porous plaster absorbs water and the solid clay is deposited on the inside of the mold, allowed to stiffen and then the mold is disassembled and the casting removed and trimmed. Slump molds or hump molds are generally used with clay slabs and the clay depends on what you are looking for in the end results: porcelain, stoneware, low-fire clay etc.