Iron ore
In cast iron sulfur promotes the formation of white iron. As little as 0.5% can counteract the effects of slow cooling and a high silicon content . White cast iron is more brittle, but also harder. It is generally avoided, because it is difficult to work, except in China where high sulfur cast iron, some as high as 0.57%, made with coal and coke, was used to make bells and chimes . According to , good foundry iron should have less than 0.15% sulfur. In the rest of the world a high sulfur cast iron can be used for making castings, but will make poor wrought iron.
Blacksmith
When iron ore is smelted into usable metal, a certain amount of carbon is usually alloyed with the iron. (Charcoal is almost pure carbon.) The amount of carbon significantly affects the properties of the metal. If the carbon content is over 2%, the metal is called cast iron, because it has a relatively low melting point and is easily cast. It is quite brittle, however, and cannot be forged so therefore not used for blacksmithing. If the carbon content is between 0.25% and 2%, the resulting metal is tool grade steel, which can be heat treated as discussed above. When the carbon content is below 0.25%, the metal is either "wrought iron (wrought iron is not smelted and cannot come from this process) " or "mild steel." The terms are never interchangeable. In preindustrial times, the material of choice for blacksmiths was wrought iron. This iron had a very low carbon content, and also included up to 5% of glassy iron silicate slag in the form of numerous very fine stringers. 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 with modern mild steel, because it welds in a narrower temperature band. The fibrous nature of wrought iron required knowledge and skill to properly form any tool which would be subject to stress. Modern steel is produced using either the blast furnace or arc furnaces. Wrought iron was produced by a labor-intensive process called "puddling", so this material is now a difficult-to-find specialty product. Modern blacksmiths generally substitute mild steel for making objects traditionally of wrought iron. Sometimes they use electrolytic-process pure iron.