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Crucible steel is a term that applies to steel made by two different methods in the modern era, although it has been produced in varying locales throughout history. It is made by melting iron and other materials in a crucible. Crucible steel was produced in South and Central Asia during the medieval era. Techniques for production of high quality steel were developed by Benjamin Huntsman in England in the 18th century; however, Huntsman's process used iron and steel as raw materials rather than direct conversion from cast iron as in the Bessemer process. The homogeneous crystal structure of this cast steel improved its strength and hardness compared to preceding forms of steel.
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. 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. The Central Asian finds are all from excavations and date from the 8th to 12th centuries AD, while the Indian/Sri Lankan material is as early as 300 BC. In addition, India's iron ore had trace vanadium and other rare earths 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, there have been swords discovered in Europe, particularly in Scandinavia. The swords in question have the ambiguous name etched into it, Ulfberht. These swords actually date in a 200-year period from the 9th century to the early 11th century. It is speculated by many[who?] that the process of making the blades originated in the Middle East and subsequently been traded during the Volga Trade Route days.
In the first centuries of the Islamic period, there appear some scientific studies on swords and steel. 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.
South India and Sri Lanka
There are many ethnographic accounts of Indian crucible steel production, however, scientific investigations of crucible steel remains have only been published from four regions: three in India and one in Sri Lanka. Indian/Sri Lankan crucible steel is commonly referred to as wootz, which is generally agreed to be an English corruption of the word ukko or hookoo. 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.
The known sites of crucible steel production in south India, i.e. at Konasamudram and Gatihosahalli, date from at least the late medieval period, 16th century. One of the earliest known sites, which shows some promising preliminary evidence that may be linked to ferrous crucible processes in Kodumanal, near Coimbatore in Tamil Nadu. The site is dated between the third century BC and the third century AD. 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. Wootz from Hyderabad or the Decanni 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. 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.
Recent archaeological investigations have suggested that Sri Lanka also supported innovative technologies for iron and steel production in antiquity. The Sri Lankan system of crucible steel making was partially independent of the various Indian and Middle Eastern systems. 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 AD. In twelfth century the land of Serendib (Sri Lanka) seems to have been the main supplier of crucible steel, but over the centuries slipped back, and by the nineteenth century just a small industry survived in the Balangoda district of the central southern highlands.
A series of excavations at Samanalawewa indicated the unexpected and previously unknown technology of west-facing smelting sites, which are different types of steel production. These furnaces were used for direct smelting to steel. Because of their location on the western sides of hilltops for use of wind in the smelting process they are named west-facing. Sri Lankan furnace steels were known and traded between the 9th and 11th centuries and earlier, but apparently not later. These sites were dated to the 7th–11th centuries. The coincidence of this dating with the 9th century Islamic reference to Sarandib is of great importance. The crucible process existed in India at the same time that the west- facing technology was operating in Sri Lanka.
Central Asia has a rich history of crucible steel production, beginning during the late 1st millennium AD. From the sites in modern Uzbekistan and Merv in Turkmenistan, there exists good archaeological evidence for the large scale production of crucible steel. They all belong in broad terms to the same early medieval period between the late 8th or early 9th and the late 12th century AD Contemporary with the early crusades.
The two most prominent crucible steel sites in eastern Uzbekistan carrying the Ferghana Process are Akhsiket and Pap in the Ferghana Valley, whose position within the Great Silk Road has been historically and archaeologically proved. The material evidence of the sites consists of large number of archaeological finds relating to steel making from 9th–12th centuries AD in the form of hundreds of thousands of fragments of crucibles often with massive slag cakes. Archaeological work at Akhsiket, has identified that the crucible steel process was of the carburization of iron metal. This process appears to be typical of and restricted to the Ferghana Valley in eastern Uzbekistan, and it is therefore called the Ferghana Process. This process lasts in that region for roughly four centuries..
Evidences of the production of crucible steel have been found in Merv, Turkmenistan, a major city on the 'Silk Road'. The Islamic scholar, al-Kindi (AD 801–866) mentions that during the ninth century the region of Khorasan, the area to which the cities Nishapur, Merv, Herat and Balkh belong, is a steel manufacturing centre. Evidence from a metallurgical workshop at Merv, dated to the ninth- early tenth century A.D., provides an illustration of the co-fusion method of steel production in crucibles, about 1000 years earlier than the distinctly different wootz process. The crucible steel process at Merv might be seen as technologically related to what Bronson (1986, 43) calls Heyderabad process, a variation of the wootz process, after the location of the process documented by Voysey in the 1820s.
Early modern accounts
The first European references to crucible steel seem to be no earlier than the Post Medieval period. European experiments with “Damascus” steels go back to at least the sixteenth century, but it was not until the 1790s that laboratory researchers began to work with steels that were specifically known to be Indian/wootz. At this time, Europeans knew of India's ability to make crucible steel from reports brought back by travellers who had observed the process at several places in southern India.
From the mid-17th century onwards, there are numerous vivid eyewitness accounts of the production of steel by European travellers to the Indian subcontinent. These include accounts by Jean Baptist Tavernier in 1679, Francis Buchanan in 1807, and H.W. Voysey in 1832. The 18th, 19th and early 20th century saw a heady period of European interest in trying to understand the nature and properties of wootz steel. Indian wootz engaged the attention of some of the best-known scientists. One was Michael Faraday who was fascinated by wootz steel. It was probably the investigations of George Pearson in 1795 reported at the Royal Society, which had the most far-reaching impact in terms of kindling interest in wootz amongst European scientists. He was the first of these scientists to publish his results and, incidentally, the first to use the word "wootz" in print.
Another investigator, David Mushet, was able to infer that wootz was made by fusion. David Mushet patented his process in 1800. He made his report in 1805. But the first successful European process had been developed by Benjamin Huntsman some 50 years previously in the 1740s.
History of production in England
Benjamin Huntsman was a clockmaker in search of a better steel for clock springs. In Handsworth near Sheffield, he began producing steel in 1740 after years of experimenting in secret. Huntsman's system used a coke-fired furnace capable of reaching 1,600 °C, into which up to twelve clay crucibles, each capable of holding about 15 kg of iron, were placed. When the crucibles or "pots" were white-hot, they were charged with lumps of blister steel, an alloy of iron and carbon produced by the cementation process, and a flux to help remove impurities. The pots were removed after about 3 hours in the furnace, impurities in the form of slag skimmed off, and the molten steel poured into moulds to end up as cast ingots. Complete melting of the steel produced a highly uniform crystal structure upon cooling, which gave the metal increased tensile strength and hardness compared to other steels being made at the time.
Before the introduction of Huntsman's technique, Sheffield produced about 200 tonnes of steel per year from Swedish wrought iron (see Oregrounds iron). The introduction of Huntsman's technique changed this radically: one hundred years later the amount had risen to over 80,000 tonnes per year, or almost half of Europe's total production. Sheffield developed from a small township into one of Europe's leading industrial cities.
The steel was produced in specialised workshops called 'crucible furnaces', which consisted of a workshop at ground level and a subterranean cellar. The furnace buildings varied in size and architectural style, growing in size towards the latter part of the 19th century as technological developments enabled multiple pots to be "fired" at once, using gas as a heating fuel. Each workshop had a series of standard features, such as rows of melting holes, teaming pits[clarification needed], roof vents, rows of shelving for the crucible pots and annealing furnaces to prepare each pot before firing. Ancillary rooms for weighing each charge and for the manufacture of the clay crucibles were either attached to the workshop, or located within the cellar complex. The steel, originally intended for making clock springs, was later used in other applications such as scissors, axes and swords.
19th and 20th century production
In another method, developed in the United States in the 1880s, iron and carbon were melted together directly to produce crucible steel. Throughout the 19th century and into the 1920s a large amount of crucible steel was directed into the production of cutting tools, where it was called tool steel.
The crucible process continued to be used for specialty steels, but is today obsolete. Similar quality steels are now made with an electric arc furnace. Some uses of tool steel were displaced, first by high speed steel  and later by materials such as tungsten carbide.
Crucible steel elsewhere
Another form of crucible steel was developed in 1837 by the Russian engineer, Pavel Anosov. His technique relied less on the heating and cooling, and more on the quenching process of rapidly cooling the molten steel when the right crystal structure had formed within. He called his steel bulat; its secret died with him. In the United States crucible steel was pioneered by William Metcalf.
Methods of crucible steel production
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. The medieval Islamic historian Abu Rayhan Biruni (c. 973–1050) provides the earliest reference of the production of Damascus steel. He describes only three methods for producing steel. The first method 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. 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.
Variations of co-fusion process have been found preliminary in Persia and Central Asia but have also been found in Hyderabad, India 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.
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