Smelted iron appears sporadically in the archeological record from the middle Bronze Age. While terrestrial iron is naturally abundant, its high melting point of 1,538 °C placed it out of reach of common use until the end of the second millennium BC. Tin's melting point of 231.9 °C and copper's still moderate melting point of 1,085 °C placed them within the capabilities of the Neolithic pottery kilns, which date back to 6000 BC and were able to produce temperatures greater than 900 °C. In addition to specially designed furnaces, ancient iron production needed to develop complex procedures for the removal of impurities, for regulating the admixture of carbon in combination with hot-working to achieve a useful balance of hardness and strength (steel), and for adding alloys to prevent rust; s