Inventions whose working principle is the extraction, refining, making or treatment of metals: smelting and steelmaking processes, hardening and surface treatments, and the chemical and electrical methods that win metals from their ores. Belongs here: the blast furnace and the Bessemer process, the Hall-Heroult and Kroll processes, nitriding and shot peening, and froth flotation. Does not belong here: finished metal tools and weapons, which sit with mechanical engineering, and materials such as plastics or ceramics, which sit with materials engineering.