Metals that have been among the main additions to the materials used in sculpture in the 20th century. Steel is a purified form of iron, but the two metals are not always precisely distinguished: up to about the Second World War, the material used in sculpture was generally referred to as iron, but much of it could probably more accurately be described as steel. Iron is one of the most widely distributed of metals and has been used in ornamental work since prehistoric times. Its malleability varies with the impurities present, especially the amount of carbon. The type that has been most used in decorative work is wrought iron, which has a very low carbon content and can be hammered into elaborate shapes. In the 19th century it was superseded for many purposes by cast iron; this contains more carbon and is consequently more brittle, but it had the advantages of being cheaper to produce and less subject to corrosion. Steel combines something of the strength of wrought iron with the malleability of cast iron. ‘Cor‐Ten’ steel is a proprietary name for a type of ‘self‐weathering’ steel popular with some contemporary sculptors. It contains a small amount of copper and acquires a patina that resists corrosion.
The first modern sculptor to use iron was perhaps Pablo Gargallo, who made hammered masks in the material from about 1907. His work helped to inspire Picasso to create what has been described as the first steel sculpture, Guitar (1912, MoMA, New York,), made of sheet metal and wire. Picasso's sculptural experimentation was an inspiration to Tatlin, the founder of Constructivism, in which steel (along with other modern materials) played a large part, its association with engineering making it appropriate to the creation of forms expressing the machine age. Picasso also played an important role in the development of welded sculpture, collaborating from 1928 to 1931 with Julio González, the main pioneer of the technique. González (who came from a family of metalworkers) taught Picasso welding, in which pieces of metal are joined by melting them together with a blowtorch. This made possible such free‐flowing openwork constructions as Picasso's Woman in a Garden (1929–30, Musée Picasso, Paris), which Timothy Hilton describes as ‘one of the great sculptures of the twentieth century’ and as ‘much more “modern” than anything he painted at this time’ (Picasso, 1975). Among the many sculptors influenced by such ‘drawing in space’ was David Smith, who said after seeing reproductions of González's work: ‘I learned that art was being made with steel—the material and machines that had previously meant only labor and earning power.’ Like González, Smith was highly influential on sculpture after the Second World War, and more than anyone else he established steel as a material with its own expressive qualities, notably by grinding and polishing the surfaces of his work. Smith also helped encourage the use of scrap metal and prefabricated industrial parts in sculpture. Scrap has been much used in Junk art, for example, and sheet steel in the work of Anthony Caro, who inspired a generation of British abstract sculptors. Minimal artists, too, have made much use of steel, valuing its impersonal qualities.