Iron technology
In KM2, the bases of 13 intact and 2 disturbed Early Iron Age furnaces were excavated along with a large industrial refuse pit and iron artifacts. The furnace bowls were lined with earth from termite mounds and the furnace shaft was built with bricks. The furnaces have a mean diameter of 93.6 cm (36.9 in), [4] ranging between 86–103 cm (34–41 in) in diameter, with 10 of the 11 furnaces between 93–103 cm (37–41 in). [5] In KM2, large amounts of industrial debris and domestic pottery were dumped into a refuse pit alongside discarded furnace bricks, slag, tuyères, iron fragments, and charcoal. [5]
At the KM3 site are mines, slag dumps, ore-processing areas, industrial refuse dumps, and smelting furnaces. [4] Smelting furnaces were abundant, although recent farming practices in the core of the industrial site had disturbed some furnaces. The KM3 furnaces are more consistent in design and contents than the KM2 furnace pits. The furnace bowls are larger and more regular in size than the KM2 furnaces, with a mean diameter of 113.8 cm (44.8 in), ranging only between 112–116 cm (44–46 in). [5] The mean depth of 28 cm (11 in) among KM3 furnace pits is significantly greater than the mean depth of 18 cm (7.1 in) at KM2, a difference attributable to the more complete preservation at KM3. [6]
The furnace bowls excavated on KM2 and KM3 were filled with a variety of materials, including slag, pieces of partly reduced iron ore, fired bricks used to build the furnace, charcoal, pieces of tuyères, as well as some domestic pottery. [3]
Tuyères
Preheating of air blasts allowed the furnaces to achieve higher combustion temperatures, which dramatically improved fuel efficiency in the iron production process. Peter Schmidt observed that the Haya in northwestern Tanzania employed the practice of preheating by placing tuyères inside their furnaces, which results in hot air blast. [2] The study of KM2 and KM3 sites allowed for the collection of more definitive evidence for prehistoric tuyères, in order to discern how and when preheated iron technology developed in Africa.
The tuyères employed inside the smelting furnace must be made of clay that is highly refractory. The heat transfer through the clay has to be sufficiently high to heat up the air passing through the clay chamber, in order to achieve a higher temperature in the blast zone, which in turns leads to more heat transfer through the clay walls of the tuyère. [3] Many small pieces of tuyère with physical characteristics indicating their presence inside furnaces were recovered at the KM2 site. The majority of tuyère fragments show convincing evidence in their colors to indicate that they were inside the prehistoric smelting furnaces of KM2. [3]
The hypothesis that preheating had developed at least by the 400 AD in Africa has been confirmed by the discovery of a 36 cm (14 in) whole or long section of tuyère uncovered in furnace 9 of KM3. The tuyère is 40% (14.5 cm (5.7 in)) slag-wetted and vitrified, and the slag-wetted section is gray in color, suggesting exposure to oxidation. [5] Other physical properties of the tuyère, such as the reduction of clay, also suggest that tuyères had been placed inside Early Iron Age furnaces for the preheating of air blast.
Swamp grass
A small pile of charred reeds was found adjacent to furnace 3 in KM2, and since furnace bowls tend to be cleared out after smelting, this serves as the most direct evidence for the use of swamp grass in the iron production process. Ethnographic study of the Haya people in Northwestern Tanzania shows that swamp grass is burned in the furnace bowl until the bowl is filled with the charred swamp reeds. When the iron ore melts above the tuyères, it forms a molten slag with carbon inclusions. The slag then interacts with the fibrous, carbonaceous bed provided by the burned swamp grass, which in turn provides an extremely high carbon-slag contact area for the subsequent carburization of the bloom. [2] The charred reeds also allow the draining of slag from the blast zone, as the slag runs down the inside of the grass stalks and through the spaces between the reeds to the bottom of the furnace pit. [4]
Termite linings
60% of the KM2 furnaces were lined with termite earth, but only one of the KM3 furnaces had traces of an artificial liner. It is uncertain if furnace liners were not as common at KM3 or if they had been removed from furnaces. [3] The earth of a termite mound is more refractory than normal soil because termites select quartz sand grains from the subsoil to build their mounds. Termite mound soil is used as insulation, and employed for its resistance to fusion with slag. [4]
The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age, and the Bronze Age (Chalcolithic). The concept has been mostly applied to Europe and the Ancient Near East, but also, by analogy, to other parts of the Old World.
Steel is an alloy made up of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, weapons, and rockets. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.
Smelting is a process of applying heat to ore in order to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slag and leaving the metal base behind. The reducing agent is commonly a fossil fuel source of carbon, such as coke—or, in earlier times, charcoal. The oxygen in the ore binds to carbon at high temperatures due to the lower potential energy of the bonds in carbon dioxide. Smelting most prominently takes place in a blast furnace to produce pig iron, which is converted into steel.
Wrought iron is an iron alloy with a very low carbon content in contrast to that of cast iron. It is a semi-fused mass of iron with fibrous slag inclusions, which gives it a "grain" resembling wood that is visible when it is etched, rusted, or bent to the point of failure. Wrought iron is tough, malleable, ductile, corrosion resistant, and easily forge welded, but is more difficult to weld electrically.
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being "forced" or supplied above atmospheric pressure.
Bog iron is a form of impure iron deposit that develops in bogs or swamps by the chemical or biochemical oxidation of iron carried in solution. In general, bog ores consist primarily of iron oxyhydroxides, commonly goethite.
The Wealden iron industry was located in the Weald of south-eastern England. It was formerly an important industry, producing a large proportion of the bar iron made in England in the 16th century and most British cannon until about 1770. Ironmaking in the Weald used ironstone from various clay beds, and was fuelled by charcoal made from trees in the heavily wooded landscape. The industry in the Weald declined when ironmaking began to be fuelled by coke made from coal, which does not occur accessibly in the area.
A bloomery is a type of metallurgical furnace once used widely for smelting iron from its oxides. The bloomery was the earliest form of smelter capable of smelting iron. Bloomeries produce a porous mass of iron and slag called a bloom. The mix of slag and iron in the bloom, termed sponge iron, is usually consolidated and further forged into wrought iron. Blast furnaces, which produce pig iron, have largely superseded bloomeries.
The Haya are a Bantu ethnic group based in Kagera Region, northwestern Tanzania, on the western side of Lake Victoria. With over one million people, it is estimated the Haya make up approximately 2% of the population of Tanzania. Historically, the Haya have had a complex kingship-based political system. Agriculture, particularly banana farming, is central to Haya economic life.
Ferrous metallurgy is the metallurgy of iron and its alloys. The earliest surviving prehistoric iron artifacts, from the 4th millennium BC in Egypt, were made from meteoritic iron-nickel. 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 in the region from Greece to India, and Sub-Saharan Africa. The use of wrought iron was known by the 1st millennium BC, and its spread defined the Iron Age. During the medieval period, smiths in Europe found a way of producing wrought iron from cast iron using finery forges. All these processes required charcoal as fuel.
Cornwall Iron Furnace is a designated National Historic Landmark that is administered by the Pennsylvania Historical and Museum Commission in Cornwall, Lebanon County, Pennsylvania in the United States. The furnace was a leading Pennsylvania iron producer from 1742 until it was shut down in 1883. The furnaces, support buildings and surrounding community have been preserved as a historical site and museum, providing a glimpse into Lebanon County's industrial past. The site is the only intact charcoal-burning iron blast furnace in its original plantation in the western hemisphere. Established by Peter Grubb in 1742, Cornwall Furnace was operated during the Revolution by his sons Curtis and Peter Jr. who were major arms providers to George Washington. Robert Coleman acquired Cornwall Furnace after the Revolution and became Pennsylvania's first millionaire. Ownership of the furnace and its surroundings was transferred to the Commonwealth of Pennsylvania in 1932.
A cupola or cupola furnace is a melting device used in foundries that can be used to melt cast iron, Ni-resist iron and some bronzes. The cupola can be made almost any practical size. The size of a cupola is expressed in diameters and can range from 1.5 to 13 feet. The overall shape is cylindrical and the equipment is arranged vertically, usually supported by four legs. The overall look is similar to a large smokestack.
The topic of early iron-metallurgy in Africa encompasses both studies of the technology and archaeology of indigenous iron-production.
Archaeometallurgical slag is slag discovered and studied in the context of archaeology. Slag, the byproduct of iron-working processes such as smelting or smithing, is left at the iron-working site rather than being moved away with the product. As it weathers well, it is readily available for study. The size, shape, chemical composition and microstructure of slag are determined by features of the iron-working processes used at the time of its formation.
Opi is a community in Enugu State of South-Eastern Nigeria. It is populated by the Igbo people and located in Nsukka region. It is the location of a prehistoric archaeological site which contains iron smelting furnaces and slag dated to 750 BC.. Iron ore was smelted in natural draft furnaces and molten slag was drained through shallow conduits to collecting pits forming huge slag blocks weighing up to 47 kg. The operating temperatures are estimated to have varied between 1,155 and 1,450 °C.
Munsa is an archaeological site in Uganda, located in the south-eastern part of Bunyoro, and is commonly recognized by a rocky hill known by the locals as "Bikegete", which is enclosed within an earthworks system of ancient ditches. The site is approximately 5 kilometres (3.1 mi) north-west of Kakumiro township in Bugangaizi County, Kakumiro District. "Munsa" is a Runyoro(Lunyoro/Runyoro Edited by Nicholas Aliganyira Nkuuna) name that means "in the trenches". The architects of the earthworks are unknown, although it has been speculated that the site can be linked to the Bachwezi. There is no evidence for this, however, and it seems likely that association of Munsa with the Bachwezi or Chwezi is a recent development.
The Yokodaidō Steel Production Site is an archaeological site in the city of Minamisōma, Fukushima Prefecture, in the southern Tōhoku region of northern Japan containing the ruins of a large-scale a late-Nara period to early Heian period bloomery (ironworks). The site was designated a National Historic Site of Japan in 2011 by the Japanese government.
Kissi is an Burkinabe archaeological site located in the Oudalan Province of Burkina Faso, near the lake Mare de Kissi and near the borders of Mali, Niger, and the Niger River. Occupied during the Iron Age, Kissi provides evidence for Iron Age textiles, beads, and mortuary practices. The site also has unique ceramic and settlement sequences to it, with clusters of mounds located throughout the site. Radiocarbon dating dates the specific occupation of the site from 1000 BC to 1300 AD.
The Mlambalasi Rock Shelter is located in the south-central highlands of the Iringa Region in southern Tanzania, 50 km away from Iringa City. Excavations in 2006 and 2010 by the Iringa Region Archaeological Project uncovered artifactual deposits from the Later Stone Age (LSA), the Iron Age, and the historic periods, as well as external artifacts from the Middle Stone Age (MSA). Direct dating on Achatina shell and ostrich eggshell beads indicates that the oldest human burials at Mlambalasi are from the terminal Pleistocene. Mlambalasi is characterized by interment LSA and Iron Age periods, as well as by cycles of use and abandonment.
Rwanda's prehistory is a relatively unexplored concept as compared to other regions of Africa. Most archaeological works regarding Rwanda past 1994 are associated with conflict and ethnic violence. However more recently, archaeologists have been attempting to focus on archaeological works from the first and second millennia A.D. For example, some archaeological research has been focusing on the Nyiginya Kingdom, which is the pre-colonial predecessor of the current Rwandan state. Other research has been focusing on the excavations of the earliest agricultural sites, likely from the Iron Age, as well as ceramics to indicate chronology of when certain agricultural groups migrated to Rwanda.
Tanzania