If you’ve got a big piece which does not allow an easy stripping of casting, mold it with some wax, either on the hollow side or in relief, because it is soft and hollow and so can easily be separated from the piece without leaving anything. But be sure that the melted wax is neither too hot nor too cold. Some people use for molding some wax warmed in boiled water, but while pressing the mold it bends and becomes unusable. Once you have made your wax cavity, you can cast in some very soft and clear clay, and let it dry. Then, upon it you will make a lead or tin cavity, in which what you will make will easily be stripped.