The term bronze is deceptive. Because metallic content cannot be determined from appearance, cast sculptures made from a variety of metal alloys are often all referred to as bronzes. Copper, which is easily worked through smithing, is particularly difficult to cast without the addition of other metals that slow its oxidation rate and improve the flow of the molten metal. Different alloys of copper mixed with zinc, tin, and lead result in what are more accurately referred to as bronzes (copper and tin) and brasses (copper and zinc). The metals used in West African sculptures come from a variety of sources. Tin is plentiful in Nigeria, and in the 1980s copper and lead sources that appear to have been mined were identified in southeast Nigeria. Certainly, however, the greatest periods of casting coincide with the influx of metals into the region from outside. The metals used in Ife bronzeworks were from brass brought across the Sahara by Arab caravans beginning in the twelfth century. In the fifteenth century, copper and brass were brought by Portuguese trading ships, contributing to another increase in metalwork.