There are two families of metals: Ferrous and non ferrous. All ferrous metals contain iron. Non ferrous metals include aluminium, zinc and copper and its alloys, for example bronze. The use of bronze for making cast sculpture is very ancient. From the early twentieth century artists such as Pablo Picasso and the Russian Constructivists began to explore the use of other metals, and Julio González introduced welded metal sculpture. The use of a range of metals and of industrial making techniques became widespread in Minimal art and New Generation sculpture for example.