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Ferrous metallurgy is the metallurgy of iron and alloys. It began far back in prehistory. The earliest surviving iron artifacts, from the 4th millennium BC in Egypt,[1] were made from meteoritic iron-nickel.[2] It is not known when or where the smelting of iron from ores began, but by the end of the 2nd millennium BC iron was being produced from iron ores from at least Greece to India,[3][4][5][6] and more controversially Sub-Saharan Africa.[7][8] The use of wrought iron (worked iron) was known by the 1st millennium BC, and its spread marked the Iron Age. During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.

By the 4th century BC Wootz steel (with a carbon content between pig iron and wrought iron) was being exported from India to ancient China, Africa, the Middle East and Europe. Archaeological evidence of cast iron appears in 5th-century BC China.[9] New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century. During the Industrial Revolution, new methods of producing bar iron by substituting coke for charcoal were devised and these were later applied to produce steel, creating a new era of greatly increased use of iron and steel that some contemporaries described as a new Iron Age.[10] In the late 1850s, Henry Bessemer invented a new steelmaking process, that involved blowing air through molten pig iron to burn off carbon, and so to produce mild steel. This and other 19th-century and later steel making processes have displaced wrought iron. Today, wrought iron is no longer produced on a commercial scale, having been displaced by the functionally equivalent mild or low carbon steel.[11]:145

The largest and most modern underground iron ore mine in the world is located in Kiruna, Norrbotten County, Lapland.[12] The mine which is owned by Luossavaara-Kiirunavaara AB, a large Swedish mining company, has an annual production capacity of over 26 million tonnes of iron ore.

Meteoritic iron

Native iron in the metallic state occurs rarely as small inclusions in certain basalt rocks. Besides meteoritic iron, Thule people of Greenland have used native iron from the Disko region.[2]

Iron smelting and the Iron Age

Iron smelting—the extraction of usable metal from oxidized iron ores—is more difficult than tin and copper smelting. While these metals and their alloys can be cold-worked or melted in relatively simple furnaces (such as the kilns used for pottery) and cast into molds, smelted iron requires hot-working and can be melted only in specially designed furnaces. Iron is a common impurity in copper ores and iron ore was sometimes used as a flux, thus it is not surprising that humans mastered the technology of smelted iron only after several millennia of bronze metallurgy.[13]

The place and time for the discovery of iron smelting is not known, partly because of the difficulty of distinguishing metal extracted from nickel-containing ores from hot-worked meteoritic iron.[2] The archaeological evidence seems to point to the Middle East area, during the Bronze Age in the 3rd millennium BC. However, wrought iron artifacts remained a rarity until the 12th century BC.

The Iron Age is conventionally defined by the widespread replacement of bronze weapons and tools with those of iron and steel.[15] That transition happened at different times in different places, as the technology spread. Mesopotamia was fully into the Iron Age by 900 BC. Although Egypt produced iron artifacts, bronze remained dominant until its conquest by Assyria in 663 BC. The Iron Age began in India about 1200 BC, in Central Europe about 600 BC, and in China about 300 BC.[16][17] Around 500 BC, the Nubians, who had learned from the Assyrians the use of iron and were expelled from Egypt, became major manufacturers and exporters of iron.[18]

Ancient Near East

gold. The early Hittites are known to have bartered iron (meteoritic or smelted) for silver, at a rate of 40 times the iron's weight, with the Old Assyrian Empire in the first centuries of the second millennium BC.[14]

Meteoric iron was also fashioned into tools in the Arctic, about the year 1000, when the Thule people of Greenland began making harpoons, knives, ulus and other edged tools from pieces of the Cape York meteorite. Typically pea-size bits of metal were cold-hammered into disks and fitted to a bone handle.[2] These artifacts were also used as trade goods with other Arctic peoples: tools made from the Cape York meteorite have been found in archaeological sites more than 1,000 miles (1,600 km) distant. When the American polar explorer Robert Peary shipped the largest piece of the meteorite to the American Museum of Natural History in New York City in 1897, it still weighed over 33 tons. Another example of a late use of meteoritic iron is an adze from around 1000 AD found in Sweden.[2]

Native iron in the metallic state occurs rarely as small inclusions in certain basalt rocks. Besides meteoritic iron, Thule people of Greenland have used native iron from the Disko region.[2]

Iron smelting and the Iron Age

Iron smelting—the extraction of usable metal from oxidized iron ores—is more difficult than tin and copper smelting. While these metals and their alloys can be cold-worked or melted in relatively simple furnaces (such as the kilns used for pottery) and cast into molds, smelted iron requires hot-working and can be melted only in specially designed furnaces. Iron is a common impurity in copper ores and iron ore was sometimes used as a flux, thus it is not surprising that humans mastered the technology of smelted iron only after several millennia of bronze metallurgy.[13]

The place and time for the discovery of iron smelting is not known, partly because of the difficulty of distinguishing metal extracted from nickel-containing ores from hot-worked meteoritic iron.[2] The archaeological evidence seems to point to the Middle East area, during

The place and time for the discovery of iron smelting is not known, partly because of the difficulty of distinguishing metal extracted from nickel-containing ores from hot-worked meteoritic iron.[2] The archaeological evidence seems to point to the Middle East area, during the Bronze Age in the 3rd millennium BC. However, wrought iron artifacts remained a rarity until the 12th century BC.

The Iron Age is conventionally defined by the widespread replacement of bronze weapons and tools with those of iron and steel.[15] That transition happened at different times in different places, as the technology spread. Mesopotamia was fully into the Iron Age by 900 BC. Although Egypt produced iron artifacts, bronze remained dominant until its conquest by Assyria in 663 BC. The Iron Age began in India about 1200 BC, in Central Europe about 600 BC, and in China about 300 BC.[16][17] Around 500 BC, the Nubians, who had learned from the Assyrians the use of iron and were expelled from Egypt, became major manufacturers and exporters of iron.[18]

One of the earliest smelted iron artifacts, a dagger with an iron blade found in a Hattic tomb in Anatolia, dated from 2500 BC.[19] About 1500 BC, increasing numbers of non-meteoritic, smelted iron objects appeared in Mesopotamia, Anatolia and Egypt.[2] Nineteen meteoric iron objects were found in the tomb of Egyptian ruler Tutankhamun, who died in 1323 BC, including an iron dagger with a golden hilt, an Eye of Horus, the mummy's head-stand and sixteen models of an artisan's tools.[20] An Ancient Egyptian sword bearing the name of pharaoh Merneptah as well as a battle axe with an iron blade and gold-decorated bronze shaft were both found in the excavation of Ugarit.[19]

Although iron objects dating from the Bronze Age have been found across the Eastern Mediterranean, bronzework appears to have greatly predominated during this period.[21] By the 12th century BC, iron smelting and forging, of weapons and tools, was common from Sub-Saharan Africa through India. As the technology spread, iron came to replace bronze as the dominant metal used for tools and weapons across the Eastern Mediterranean (the Levant, Cyprus, Greece, Crete, Anatolia and Egypt).[15]

Iron was originally smelted in bloomeries, furnaces where bellows were used to force air through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduced the iron oxide from the ore to metallic iron. The bloomery, however, was not hot enough to melt the iron, so the metal collected in the bottom of the furnace as a spongy mass, or bloom. Workers then repeatedly beat and folded it to force out the molten slag. This laborious, time-consuming process produ

Although iron objects dating from the Bronze Age have been found across the Eastern Mediterranean, bronzework appears to have greatly predominated during this period.[21] By the 12th century BC, iron smelting and forging, of weapons and tools, was common from Sub-Saharan Africa through India. As the technology spread, iron came to replace bronze as the dominant metal used for tools and weapons across the Eastern Mediterranean (the Levant, Cyprus, Greece, Crete, Anatolia and Egypt).[15]

Iron was originally smelted in bloomeries, furnaces where bellows were used to force air through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduced the iron oxide from the ore to metallic iron. The bloomery, however, was not hot enough to melt the iron, so the metal collected in the bottom of the furnace as a spongy mass, or bloom. Workers then repeatedly beat and folded it to force out the molten slag. This laborious, time-consuming process produced wrought iron, a malleable but fairly soft alloy.

Concurrent with the transition from bronze to iron was the discovery of carburization, the process of adding carbon to wrought iron. While the iron bloom contained some carbon, the subsequent hot-working oxidized most of it. Smiths in the Middle East discovered that wrought iron could be turned into a much harder product by heating the finished piece in a bed of charcoal, and then quenching it in water or oil. This procedure turned the outer layers of the piece into steel, an alloy of iron and iron carbides, with an inner core of less brittle iron.

The development of iron smelting was traditionally attributed to the Hittites of Anatolia of the Late Bronze Age.[22] It was believed that they maintained a monopoly on iron working, and that their empire had been based on that advantage. According to that theory, the ancient Sea Peoples, who invaded the Eastern Mediterranean and destroyed the Hittite empire at the end of the Late Bronze Age, were responsible for spreading the knowledge through that region. This theory is no longer held in the mainstream of scholarship,[22] since there is no archaeological evidence of the alleged Hittite monopoly. While there are some iron objects from Bronze Age Anatolia, the number is comparable to iron objects found in Egypt and other places of the same time period, and only a small number of those objects were weapons.[21]

A more recent theory claims that the development of iron technology was driven by the disruption of the copper and tin trade routes, due to the collapse of the empires at the end of the Late Bronze Age.[22] These metals, esp

A more recent theory claims that the development of iron technology was driven by the disruption of the copper and tin trade routes, due to the collapse of the empires at the end of the Late Bronze Age.[22] These metals, especially tin, were not widely available and metal workers had to transport them over long distances, whereas iron ores were widely available. However, no known archaeological evidence suggests a shortage of bronze or tin in the Early Iron Age.[23] Bronze objects remained abundant, and these objects have the same percentage of tin as those from the Late Bronze Age.

The history of ferrous metallurgy in the Indian subcontinent began in the 2nd millennium BC. Archaeological sites in Gangetic plains have yielded iron implements dated between 1800 and 1200 BC.[24] By the early 13th century BC, iron smelting was practiced on a large scale in India.[24] In Southern India (present day Mysore) iron was in use 12th to 11th centuries BC.[5] The technology of iron metallurgy advanced in the politically stable Maurya period[25] and during a period of peaceful settlements in the 1st millennium BC.[5]

Iron artifacts such as spikes, knives, daggers, arrow-heads, bowls, spoons, saucepans, axes, chisels, tongs, door fittings, etc., dated from 600 to 200 BC, have been discovered at several archaeological sites of India.[16] The Greek historian Herodotus wrote the first western account of the use of iron in India.[16] The Indian mythological texts,

Iron artifacts such as spikes, knives, daggers, arrow-heads, bowls, spoons, saucepans, axes, chisels, tongs, door fittings, etc., dated from 600 to 200 BC, have been discovered at several archaeological sites of India.[16] The Greek historian Herodotus wrote the first western account of the use of iron in India.[16] The Indian mythological texts, the Upanishads, have mentions of weaving, pottery and metallurgy, as well.[26] The Romans had high regard for the excellence of steel from India in the time of the Gupta Empire.[27]

Perhaps as early as 500 BC, although certainly by 200 AD, high-quality steel was produced in southern India by the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in a crucible and heated until the iron melted and absorbed the carbon.[28] Iron chain was used in Indian suspension bridges as early as the 4th century.[29]

Wootz steel was produced in India and Sri Lanka from around 300 BC.[28] Wootz steel is famous from Classical Antiquity for its durability and ability to hold an edge. When asked by King Porus to select a gift, Alexander is said to have chosen, over gold or silver, thirty pounds of steel.[27] Wootz steel was originally a complex alloy with iron as its main component together with various Wootz steel was produced in India and Sri Lanka from around 300 BC.[28] Wootz steel is famous from Classical Antiquity for its durability and ability to hold an edge. When asked by King Porus to select a gift, Alexander is said to have chosen, over gold or silver, thirty pounds of steel.[27] Wootz steel was originally a complex alloy with iron as its main component together with various trace elements. Recent studies have suggested that its qualities may have been due to the formation of carbon nanotubes in the metal.[30] According to Will Durant, the technology passed to the Persians and from them to Arabs who spread it through the Middle East.[27] In the 16th century, the Dutch carried the technology from South India to Europe, where it was mass-produced.[31]

Steel was produced in Sri Lanka from 300 BC[28] by furnaces blown by the monsoon winds. The furnaces were dug into the crests of hills, and the wind was diverted into the air vents by long trenches. This arrangement created a zone of high pressure at the entrance, and a zone of low pressure at the top of the furnace. The flow is believed to have allowed higher temperatures than bellows-driven furnaces could produce, resulting in better-quality iron.[32][33][34] Steel made in Sri Lanka was traded extensively within the region and in the Islamic world.

One of the world's foremost metallurgical curiosities is an iron pillar located in the Qutb complex in Delhi. The pillar is made of wrought iron (98% Fe), is almost seven meters high and weighs more than six tonnes.[35] The pillar was erected by Chandragupta II Vikramaditya and has withstood 1,600 years of exposure to heavy rains with relatively little corrosion.

China

Greece in the late 10th century BC.[49] The earliest marks of Iron Age in Central Europe are artifacts from the Hallstatt C culture (8th century BC). Throughout the 7th to 6th centuries BC, iron artifacts remained luxury items reserved for an elite. This changed dramatically shortly after 500 BC with the rise of the La Tène culture, from which time iron metallurgy also became common in Northern Europe and Britain. The spread of ironworking in Central and Western Europe is associated with Celtic expansion. By the 1st century BC, Noric steel was famous for its quality and sought-after by the Roman military.

The annual iron output of the Roman Empire is estimated at 84,750 t.[50]

Africa south of the Sahara

Iron Age finds in East and Southern Africa, corresponding to the early 1st millennium AD Bantu expansion

Though there is some uncertainty, some archaeologists believe that iron metallurgy was developed independently in sub-Saharan Africa (possibly in West Africa).[51][52]

Inhabitants of Termit, in eastern Niger, smelted iron around 1500 BC.[53]

In the region of the Aïr Mountains in Niger there are also signs of independent copper smelting between 2500 and 1500 BC. The process was not in a developed state, indicating smelting was not foreign. It became mature about 1500 BC.[54]

Archaeological sites containing iron smel

The annual iron output of the Roman Empire is estimated at 84,750 t.[50]

Though there is some uncertainty, some archaeologists believe that iron metallurgy was developed independently in sub-Saharan Africa (possibly in West Africa).[51][52]

Inhabitants of Termit, in eastern Niger, smelted iron around 1500 BC.[53]

In the region of the Aïr Mountains in Niger there are also signs of independent copper smelting between 2500 and 1500 BC. The process was not in a developed state, indicating smelting was not foreign. It became mature about 1500 BC.[54]

Archaeological sites containing iron smelting furnaces and slag have also been excavated at sites in the Nsukka region of southeast Nigeria in what is now Igboland: dating to 2000 BC at the site of Lejja (Eze-Uzomaka 2009)[55][52] and to 750 BC and at the site of Opi (Holl 2009).[52] The site of Gbabiri (in the Central African Republic)

Inhabitants of Termit, in eastern Niger, smelted iron around 1500 BC.[53]

In the region of the Aïr Mountains in Niger there are also signs of independent copper smelting between 2500 and 1500 BC. The process was not in a developed state, indicating smelting was not foreign. It became mature about 1500 BC.[54]

Archaeological sites containing iron smelting furnaces and slag have also been excavated at sites in the Nsukka region of southeast Nigeria in what is now Igboland: dating to 2000 BC at the site of Lejja (Eze-Uzomaka 2009)[55][52] and to 750 BC and at the site of Opi (Holl 2009).[52] The site of Gbabiri (in the Central African Republic) has yielded evidence of iron metallurgy, from a reduction furnace and blacksmith workshop; with earliest dates of 896-773 BC and 907-796 BC respectively.[56] Similarly, smelting in bloomery-type furnaces appear in the Nok culture of central Nigeria by about 550 BC and possibly a few centuries earlier.[7][8][57][58][59]

There is also evidence that carbon steel was made in Western Tanzania by the ancestors of the Haya people as early as 2,300-2,000 years ago (about 300 BC or soon after) by a complex process of "pre-heating" allowing temperatures inside a furnace to reach 1300 to 1400° C.[60][61][62][63][64][65]

Iron and copper working spread southward through the continent, reaching the Cape around AD 200.[7][8] The widespread use of iron revolutionized the Bantu-speaking farming communities who adopted it, driving out and absorbing the rock tool using hunter-gatherer societies they encountered as they expanded to farm wider areas of savanna. The technologically superior Bantu-speakers spread across southern Africa and became wealthy and powerful, producing iron for tools and weapons in large, industrial quantities.[7][8]

The earliest records of bloomery-type furnaces in East Africa are discoveries of smelted iron and carbon in Nubia that date back between the 7th and 6th centuries BC,[66][67][68] particularly in Meroe where there are known to have been ancient bloomeries that produced metal tools for the Nubians and Kushites and produced surplus for their economy.

Iron technology was further advanced by several inventions in medieval Islam, during the Islamic Golden Age. These included a variety of water-powered and wind-powered industrial mills for metal production, including geared gristmills and forges. By the 11th century, every province throughout the Muslim world had these industrial mills in operation, from Islamic Spain and North Africa in the west to the Middle East and Central Asia in the east.[69] There are also 10th-century references to cast iron, as well as archeological evidence of blast furnaces being used in the Ayyubid and Mamluk empires from the 11th century, thus suggesting a diffusion of Chinese metal technology to the Islamic world.[70]

Geared gristmills[71] were invented by Muslim engineers, and were used for crushing metallic ores before extraction. Gristmills in the Islamic world were often made from both watermills

Geared gristmills[71] were invented by Muslim engineers, and were used for crushing metallic ores before extraction. Gristmills in the Islamic world were often made from both watermills and windmills. In order to adapt water wheels for gristmilling purposes, cams were used for raising and releasing trip hammers.[72] The first forge driven by a hydropowered water mill rather than manual labour was invented in the 12th century Islamic Spain.[73][73]

One of the most famous steels produced in the medieval Near East was Damascus steel used for swordmaking, and mostly produced in Damascus, Syria, in the period from 900 to 1750. This was produced using the crucible steel method, based on the earlier Indian wootz steel. This process was adopted in the Middle East using locally produced steels. The exact process remains unknown, but it allowed carbides to precipitate out as micro particles arranged in sheets or bands within the body of a blade. Carbides are far harder than the surrounding low carbon steel, so swordsmiths could produce an edge that cut hard materials with the precipitated carbides, while the bands of softer steel let the sword as a whole remain tough and flexible. A team of researchers based at the Technical University of Dresden that uses X-rays and electron microscopy to examine Damascus steel discovered the presence of cementite nanowires[74] and carbon nanotubes.[75] Peter Paufler, a member of the Dresden team, says that these nanostructures give Damascus steel its distinctive properties[76] and are a result of the forging process.[76][77]

There was no fundamental change in the technology of iron production in Europe for many centuries. European metal workers continued to produce iron in bloomeries. However, the Medieval period brought two developments—the use of water power in the bloomery process in various places (outlined above), and the first European production in cast iron.

Powered bloomeries