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Local Area Info: Crucible steel

Crucible steel is steel made by melting pig iron (cast iron), iron, and sometimes steel, often along with sand, glass, ashes, and other fluxes, in a crucible. In ancient times steel and iron were impossible to melt using charcoal or coal fires, which could not produce temperatures high enough. However, pig iron, having a higher carbon content thus a lower melting point, could be melted, and by soaking wrought iron or steel in the liquid pig-iron for long periods of time, the carbon content of the pig iron could be reduced as it slowly diffused into the iron. Crucible steel of this type was produced in South and Central Asia during the medieval era. This generally produced a very hard steel, but also a composite steel that was inhomogeneous, consisting of a very high-carbon steel (formerly the pig-iron) and a lower-carbon steel (formerly the wrought iron). This often resulted in an intricate pattern when the steel was forged, filed or polished, with possibly the most well-known examples coming from the wootz steel used in Damascus swords. Due to the use of fluxes the steel was often much higher in quality (lacking impurities) and in carbon content compared to other methods of steel production of the time.

Techniques for production of high quality steel were developed by Benjamin Huntsman in England in the 18th century. Huntsman used coke rather than coal or charcoal, achieving temperatures high enough to melt steel and dissolve iron. Huntsman's process differed from some of the wootz processes in that it took a longer time to melt the steel and to cool it down and allowed more time for the diffusion of carbon. Huntsman's process used iron and steel as raw materials, in the form of blister steel, rather than direct conversion from cast iron as in puddling or the later Bessemer process. The ability to fully melt the steel removed any inhomogeneities in the steel, allowing the carbon to dissolve evenly into the liquid steel and negating the prior need for extensive blacksmithing in an attempt to achieve the same result. Similarly, it allowed steel to simply be poured into molds, or cast, for the first time. The homogeneous crystal structure of this cast steel improved its strength and hardness compared to preceding forms of steel. The use of fluxes allowed nearly complete extraction of impurities from the liquid, which could then simply float to the top for removal. This produced the first steel of modern quality, providing a means of efficiently changing excess wrought iron into useful steel. Huntsman's process greatly increased the European output of quality steel suitable for use in items like knives, tools, and machinery, helping to pave the way for the Industrial revolution.

Iron alloys are most broadly divided by their carbon content: cast iron has 2-4% carbon impurities; wrought iron oxidizes away most of its carbon, to less than 0.1%. The much more valuable steel has a delicately intermediate carbon fraction, and its material properties range according to the carbon percentage: high carbon steel is stronger but more brittle than low carbon steel. Crucible steel sequesters the raw input materials from the heat source, allowing precise control of carburization (raising) or oxidation (lowering carbon content). Fluxes, such as limestone, could be added to the crucible to remove or promote sulfur, silicon, and other impurities, further altering its material qualities.