The blacksmith's materials
When iron ore is smelted into usable metal, a certain amount of carbon is usually alloyed with the iron. The amount of carbon has extreme effects on the properties of the metal. If the carbon content is over 2%, the metal is called cast iron. Cast iron is so called because it has a relatively low melting point and is easily cast. It is quite brittle however, and therefore not used for blacksmithing. If the carbon content is between .25% and 2%, the resulting metal is tool steel, which can be heat treated as discussed above. When the carbon content is below .25%, the metal may be called either "wrought iron" or "mild steel."
The distinction between wrought iron and mild steel is one of the manufacturing process and the end use, and the terms are sometimes interchangeable. In pre-industrial times, the material of choice for blacksmiths was puddled iron. This iron had a very low carbon content, and also included up to 5% of glassy slag. This slag content made the iron very tough, gave it considerable resistance to rusting, and allowed it to be more easily "forge welded," a process in which the blacksmith permanently joins two pieces of iron, or a piece of iron and a piece of steel, by heating them nearly to a white heat and hammering them together. Forge welding is more difficult to do with modern mild steel.
Modern steel production, using the blast furnace, cannot produce true wrought iron, so this material is now a difficult-to-find specialty product. Modern blacksmiths generally substitute mild steel for making objects that were traditionally of wrought iron.