Crucible steel
Crucible steel was first developed in the middle of the 1st millennium BCE in Southern India and Sri Lanka using the wootz process.
[1][2][3][4] In ancient times, it was not possible to produce very high temperatures with charcoal or coal fires, which were required to melt iron or steel.
The steel was usually worked very little and at relatively low temperatures to avoid any decarburization, hot short crumbling, or excess diffusion of carbon.
With a carbon content close to that of cast iron, it usually required no heat treatment after shaping other than air cooling to achieve the correct hardness, relying on composition alone.
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.
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.
[7] The medieval Islamic historian Abu Rayhan Biruni (c. 973–1050) provides the earliest reference of the production of Damascus steel.
[10] 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.
[12] 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.
[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.
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.
[20][21] 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.
[22] 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.
[23] 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.
[28][29] Recent archaeological investigations have suggested that Sri Lanka also supported innovative technologies for iron and steel production in antiquity.
[30] 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.
[33] 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.
[39] From the sites in modern Uzbekistan and Merv in Turkmenistan, there is good archaeological evidence for the large scale production of crucible steel.
[42] The material evidence consists of large number of archaeological finds relating to steel making from 9th–12th centuries CE in the form of hundreds of thousands of fragments of crucibles, often with massive slag cakes.
[44] Evidence from a metallurgical workshop at Merv, dated to the ninth- early tenth century CE, provides an illustration of the co-fusion method of steel production in crucibles, about 1000 years earlier than the distinctly different wootz process.
From the mid-17th century onwards, European travellers to the Indian subcontinent wrote numerous vivid eyewitness accounts of the production of steel there.
It was probably the investigations of George Pearson, reported at the Royal Society in 1795, which had the most far-reaching impact in terms of kindling interest in wootz amongst European scientists.
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.
Sheffield's Abbeydale Industrial Hamlet operates for the public a scythe-making works, which dates from Huntsman's times and is powered by a water wheel, using crucible steel made at the site.
The resulting billet could then be hammered flat, cut into plates, which were stacked and welded again, thinning and compounding the layers, and evening out the carbon more as it slowly diffused out of the high-carbon steel into the lower-carbon iron.
Thus, further attempts to homogenize the steel resulted in a carbon content too low for use in items like springs, cutlery, swords, or tools.
Therefore, steel intended for use in such items, especially tools, was still being made primarily by the slow and arduous bloomery process in very small amounts and at high cost, which, albeit better, had to be manually separated from the wrought iron and was still impossible to fully homogenize in the solid state.
Huntsman was one of the first to begin experimenting with the addition of alloying agents like manganese to help remove impurities such as oxygen from the steel.
