Two links of chain, with cast iron stubs. Puddling is a technique, perfected by Henry Cort in 1784, for making pure iron from pig iron. The pig iron is melted in a furnace, with powdered iron oxide stirred in to drive off the slag. As the purity of the iron increases, its melting point rises. It eventually forms a viscous mas, which is then removed and hammered or rolled into the required shape. Puddling superseded wrought iron working by hand and the introduction of the technique made a significant contribution to the industrial revolution. The method is rarely used today. © Science Museum / Science & Society Picture Library