A matrix for casting metal. Molten metal poured into a concavity will solidify into a corresponding shape. The concavity has only to be given the shape of the required artifact. Such molds can be made of stone, pottery, or metal with a melting point higher than that of the alloy being cast. Molds were also used for making figurines and relief-decorated pottery. The simplest type of mold is a one-piece or open one, from which the casting emerges with one flat face, requiring further hammering to give it a symmetrical form. Two-piece molds allowed bifacial tools and weapons to be cast - a third piece, or core, being added if a socket was required. These technical advances had been made before the end of the Early Bronze Age. Multi-piece molds were used in Shang China. Molds were used to produce the elaborate asymmetrical vessels of the Mochica and Chimú styles. The earliest molds for casting metal were made of stone. During the Late Bronze Age, piece molds began to be formed of clay.