Copper was the first metal used by humans and is second only to iron in its utility over the years. Oxidized ores, such as cuprite and tenorite, can be reduced directly to metallic copper by heating with carbon in a furnace. The sulfide ores, however, require a more complex treatment. In its native state, copper can be so pure that it requires only melting with a flux to produce "lake copper," a method that for many years was the world standard.