The two key ingredients that enabled the mass production of steel. We travel to Sheffield - the birthplace of modern steelmaking - to get up close to the technological mid-wife, a Bessemer converter. Plus we visit the modern Forgemasters steelworks, to get a taste of just how hard it is to produce unbreakable parts for nuclear power stations and oil rigs. (Photo: Steel component at Forgemasters Steelworks, Credit: Laurence Knight/BBC)