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. Crucible steel was first developed in the middle of the 1st millennium BCE in Southern India and Sri Lanka using the wootz process.
In pre-industrial times, it was not possible to produce sufficient temperatures with charcoal or coal fires required to melt iron or steel. However, pig iron, having a higher net alloy content and thus a lower melting point, could be melted. This is due to the effect of Freezing Point Depression. By co-heating wrought iron or steel with pig-iron for a long time, Carbon content in the pig iron could be reduced as it slowly diffused into the iron. In addition, H2O containing organic materials could be mixed into fluxing agents. Oxygen reacted with excess phosphorus, sulfur, and silicon typical of pig iron producing additional heat and low density slag. The additional heat was sufficient to dissolve some or all of Wrought Iron scraps turning the previous raw materials into a medium-carbon steel ingot. This also allowed slag and oxide inclusions in the W.I. to float on the surface, producing superior internal cleanliness. 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 an ingot that might be inhomogeneous, consisting of a very high-carbon steel regions (formerly the pig-iron) and lower-carbon steel regions (formerly the wrought iron).
The crucible steel process might result in an intricate pattern when the steel was filed, polished, then etched. Possibly the most well-known examples coming from the wootz steel used in Damascus swords. Such steel was typically higher in average carbon content (typically ranging in the area of 1.5 to 2.0%) and relatively high in phosphorus, which contributed to the distinctive water pattern. This effect was caused by slowly cooling the cruicibles over several days.
The preceding factors caused an excessively coarse, dendritic crystal grain structure in crucible steel ingots, and alloy segregation at the edges of the grains as they grew. The increased alloy content at the as-cooled grain boundaries caused the steel to be attacked more slowly by etchants, resulting in a frost-like or woodgrain-like pattern after polishing and etching. However the majority of crucible steel ingots did not exhibit this effect. This led items showing the effect to be ascribed mystical properties. Although it was merely a result of excessively coarse grain size and a particular alloy composition.
The steel was usually subjected to a grain refinement heat treatment, then forged at relatively low temperatures to avoid hot-shortness, crumbling, or excess diffusion of carbon.
With a carbon content close to that of cast iron, it usually required minimal heat treatment after finishing other than air cooling amd tempering to achieve the correct hardness, relying on composition alone.
In Europe, crucible steel was developed in parallel during the 1740s, by Benjamin Huntsman, of Sheffield, England. Huntsman used coke, rather than charcoal, achieving temperatures high enough to melt steel and dissolve iron, in combination with a glass shard flux to reduce impurities. Huntsman's process differed from some of the Wootz processes in that it 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, a glass flux to reduce impurities, and the higher temperatures and carbon content of coke, along with a longer time to melt the steel and to cool it down, to thus allowed more time for the diffusion of carbon.
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 be cast by pouring into molds. 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.
Methods of production 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 decarburization (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.
Various methods were used to produce crucible steel. According to Islamic texts such as al-Tarsusi and Abu Rayhan Biruni, three methods are described for indirect production of steel.Feuerbach et al. 1997, 105 The medieval Islamic historian Abu Rayhan Biruni (c. 973–1050) provides the earliest reference of the production of Damascus steel.Feuerbach et al. 1998, 38 The first, and the most common, traditional method is solid state carburization of wrought iron. This is a diffusion process in which wrought iron is packed in crucibles or a hearth with charcoal, then heated to promote diffusion of carbon into the iron to produce steel.Feuerbach et al. 1995, 12 Carburization is the basis for the wootz process of steel. The second method is the decarburization of cast iron by removing carbon from the cast iron. The third method uses wrought iron and cast iron. In this process, wrought iron and cast iron may be heated together in a crucible to produce steel by fusion. In regard to this method Abu Rayhan Biruni states: "this was the method used in Hearth". It is proposed that the Indian method refers to Wootz carburization method; i.e., the Mysore or Tamil processes.Srinivasan 1994, 56
Variations of co-fusion process have been found primarily in Persia and Central Asia but have also been found in Hyderabad, IndiaFeuerbach et al. 1998, 39 called Deccani or Hyderabad process. For the carbon, a variety of organic materials are specified by the contemporary Islamic authorities, including pomegranate rinds, acorns, fruit skins like orange peel, leaves as well as the white of egg and shells. Slivers of wood are mentioned in some of the Indian sources, but significantly none of the sources mention charcoal.
Early history
Crucible steel is generally attributed to production centres in India and Sri Lanka where it was produced using the so-called "wootz" process, and it is assumed that its appearance in other locations was due to long-distance trade.Feuerbach 2002, 13 Only recently it has become apparent that places in Central Asia like Merv in Turkmenistan and Akhsiket in Uzbekistan were important centres of production of crucible steel.Ranganathan and Srinivasan 2004, 126 The Central Asian finds are all from excavations and date from the 8th to 12th centuries CE, while the Indian/Sri Lankan material is as early as 300 BCE. India's iron ore had trace vanadium and other alloying elements leading to increased hardenability in Indian crucible steel which was famous throughout the middle east for its ability to retain an edge.
While crucible steel is more attributed to the Middle East in early times, pattern welded swords, incorporating high-carbon, and likely crucible steel, have been discovered in Europe, from the 3rd century CE, particularly in Scandinavia. Swords bearing the brand name Ulfberht, and dating to a 200-year period from the 9th century to the early 11th century, are prime examples of the technique. It is speculated by many that the process of making these blades originated in the Middle East and subsequently had been traded during the Volga Trade Route days.See: Williams, Alan (2009) "A metallurgical study of some Viking swords," Gladius, 29 : 124–189; see p. 143. National Physical Laboratory (U.K.) uncovers Viking trade routes (2009 January 12)
In the first centuries of the Islamic period, some scientific studies on swords and steel appeared. The best known of these are by Jabir ibn Hayyan 8th century, al-Kindi 9th century, Al-Biruni in the early 11th century, al-Tarsusi in the late 12th century, and Fakhr-i-Mudabbir 13th century. Any of these contains far more information about Indian and damascene steels than appears in the entire surviving literature of classical Greece and Rome.Bronson 1986, 19
South India and Sri Lanka
There are many ethnographic accounts of Indian crucible steel production; however, scientific investigations of the remains of crucible steel production have only been published for four regions: three in India and one in Sri Lanka.Feuerbach 2002, 164 Indian/Sri Lankan crucible steel is commonly referred to as wootz, which is generally agreed to be an English corruption of the word ukko (in the Canarese language) or hookoo (in the Telugu language).Feuerbach 2002, 163
European accounts from the 17th century onwards have referred to the repute and manufacture of "wootz", a traditional crucible steel made specially in parts of southern India in the former provinces of Golconda, Mysore and Salem. As yet the scale of excavations and surface surveys is too limited to link the literary accounts to archaeometallurgical evidence.Griffiths and Srinivasan 1997, 111
The proven sites of crucible steel production in south India, e.g. at Konasamudram and Gatihosahalli, date from at least the late medieval period, 16th century.Srinivasan 1994, 52 One of the earliest known potential sites, which shows some promising preliminary evidence that may be linked to ferrous crucible processes in Kodumanal, near Coimbatore in Tamil Nadu.Ranganathan and Srinivasan 2004, 117 The site is dated between the third century BCE and the third century CE.Craddock 2003, 245 By the seventeenth century the main centre of crucible steel production seems to have been in Hyderabad. The process was apparently quite different from that recorded elsewhere.Craddock 1995, 281 Wootz from Hyderabad or the Deccani process for making watered blades involved a co-fusion of two different kinds of iron: one was low in carbon and the other was a high-carbon steel or cast iron.Moshtagh Khorasani 2006, 108 Wootz steel was widely exported and traded throughout ancient Europe, China, the Arab world, and became particularly famous in the Middle East, where it became known as Damascus steel.Srinivasan 1994Srinivasan & Griffiths
Recent archaeological investigations have suggested that Sri Lanka also supported innovative technologies for iron and steel production in antiquity.Ranganathan and Srinivasan 2004, 125 The Sri Lankan system of crucible steel making was partially independent of the various Indian and Middle Eastern systems.Bronson 1986, 43 Their method was something similar to the method of carburization of wrought iron. The earliest confirmed crucible steel site is located in the Knuckles range in the northern area of the Central Highlands of Sri Lanka dated to 6th–10th centuries CE.Feuerbach 2002, 168 In the twelfth century the land of Serendib (Sri Lanka) seems to have been the main supplier of crucible steel, but over the centuries production slipped back, and by the nineteenth century just a small industry survived in the Balangoda district of the central southern highlands.Craddock 1995, 279