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A bloomery is a type of metallurgical furnace once used widely for smelting iron from its iron 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. , which produce pig iron, have largely superseded bloomeries.
The first step taken before the bloomery can be used is the preparation of the charcoal and the iron ore. Charcoal is nearly pure carbon, which, when burned, both produces the high temperature needed for the smelting process and provides the carbon monoxide needed for Redox of the metal.
The ore is broken into small pieces and usually roasted in a fire, to make rock-based ores easier to break up, bake out some impurities, and (to a lesser extent) to remove any moisture in the ore. Any large impurities (as silica) in the ore can be removed as it is crushed. The desired particle size depends primarily on which of several ore types may be available, which will also have a relationship to the layout and operation of the furnace, of which a number of regional, historic/traditional forms exist. Natural iron ores can vary considerably in oxide form ( / / ), and importantly in relative iron content. Since slag from previous blooms may have a high iron content, it can also be broken up and may be recycled into the bloomery with the new ore.
In operation, after the bloomery is heated typically with a wood fire, shifting to burning sized charcoal, iron ore and additional charcoal are introduced through the top. Again, traditional methods vary, but normally smaller charges of ore are added at the start of the main smelting sequence, increasing to larger amounts as the smelt progresses. Overall, a typical ratio of total charcoal to ore added is in a roughly one-to-one ratio. Inside the furnace, carbon monoxide from the incomplete combustion of the charcoal redox the iron oxides in the ore to Iron without melting the ore; this allows the bloomery to operate at lower temperatures than the melting temperature of the ore. As the desired product of a bloomery is iron that is easily Forging, it requires a low carbon content. The temperature and ratio of charcoal to iron ore must be carefully controlled to keep the iron from absorbing too much carbon and thus becoming unforgeable. Cast iron occurs when the iron absorbs 2% to 4% carbon. Because the bloomery is self-fluxing, the addition of limestone is not required to form a slag.
The small particles of iron produced in this way fall to the bottom of the furnace, where they combine with molten slag, often consisting of fayalite, a compound of silicon, oxygen, and iron mixed with other impurities from the ore. The hot liquid slag, running to the bottom of the furnace, cools against the base and lower side walls of the furnace, effectively forming a bowl still containing fluid slag. As the individual iron particles form, they fall into this bowl and Sintering together under their own weight, forming a spongy mass referred to as the bloom. Because the bloom is typically porous, and its open spaces can be full of slag, the extracted mass must be beaten with heavy hammers to both compress voids and drive out any molten slag remaining. This process may require several additional heating and compaction cycles, working at high 'welding' temperatures. Iron treated this way is said to be wrought (worked), and the resulting iron, with reduced amounts of slag, is called wrought iron or bar iron. Because of the creation process, individual blooms can often have differing carbon contents between the original top and bottom surfaces, differences that will also be somewhat blended together through the flattening, folding, and hammer-welding sequences. Intentionally producing blooms that are coated in steel (i.e. iron with a higher carbon content) by manipulating the charge of and air flow to the bloomery is also possible.
As the era of modern commercial steelmaking began, the word "bloom" was extended to another word sense referring to an intermediate-stage piece of steel, of a size comparable to many traditional iron blooms, that was ready to be further worked into billet.
Wrought iron was used in the construction of monuments such as the iron pillar of Delhi, built in the third century AD during the Gupta Empire. The latter was built using a towering series of disc-shaped iron blooms. Similar to China, high-carbon steel was eventually used in India, although cast iron was not used for architecture until modern times.
As a bloomery's size is increased, the iron ore is exposed to burning charcoal for a longer time. When combined with the strong air blast required to penetrate the large ore and charcoal stack, this may cause part of the iron to melt and become saturated with carbon in the process, producing unforgeable pig iron, which requires finery forge to be reduced into cast iron, steel, and iron. This pig iron was considered a waste product detracting from the largest bloomeries' yield, and early , identical in construction, but dedicated to the production of molten iron, were not built until the 14th century.Douglas Alan Fisher, The Epic of Steel, Harper & Row 1963, p. 26–29 Blast furnace, theory and practice, American Institute of Mining, Metallurgical, and Petroleum Engineers, Gordon and Breach Science 1969, pp. 4–5
Bloomery type furnaces typically produced a range of iron products from very low-carbon iron to steel containing around 0.2–1.5% carbon. The master smith had to select pieces of low-carbon iron, carburizing them, and pattern welding them together to make steel sheets. Even when applied to a noncarburized bloom, this pound, fold, and weld process resulted in a more homogeneous product and removed much of the slag. The process had to be repeated up to 15 times when high-quality steel was needed, as for a sword. The alternative was to case hardening of a finished product. Each welding's heat oxidises some carbon, so the master smith had to make sure enough carbon was in the starting mixture.Alan R. Williams, Methods of manufacture of swords in medieval Europe, Gladius 1977, p. 70–77
In England and Wales, despite the arrival of the blast furnace in the Weald in about 1491, bloomery forges, probably using waterpower for the hammer and the bellows, were operating in the West Midlands region beyond 1580. In Furness and Cumberland, they operated into the early 17th century and the last one in England (near Garstang) did not close until about 1770.H. R. Schubert, History of the British Iron and Steel Industry (1957).R. F. Tylecote, History of Metallurgy (1991).
One of the oldest-known blast furnaces in Europe has been found in Lapphyttan in Sweden, Carbon dating to be from the 12th century. The oldest bloomery in Sweden, also found in the same area, has been carbon-14 dated to 700 BC.Magnusson, G. (2015). Järnet och Sveriges medeltida modernisering. Jernkontoret, Stockholm.
Bloomeries survived in Spain and southern France as Catalonia forges into the mid-19th century, and in Austria as the to 1775.
Excavations at L'Anse aux Meadows, Newfoundland, have found considerable evidence for the processing of bog iron and the production of iron in a bloomery by the Norse. The cluster of Viking Age (–1022 AD) at L'Anse aux Meadows are situated on a raised marine terrace, between a sedge peat bog and the ocean. Estimates from the smaller amount of slag recovered archaeologically suggest 15 kg of slag was produced during what appears to have been a single smelting attempt. By comparing the iron content of the primary bog iron ore found in the purpose built 'furnace hut' with the iron remaining in that slag, an estimated 3 kg iron bloom was produced. At a yield of at best 20% from what is a good iron rich ore, this suggests the workers processing the ore had not been particularly skilled. This supports the idea that iron processing knowledge was widespread and not restricted to major centers of trade and commerce. Archaeologists also found 98 nail, and importantly, ship rivet fragments, at the site as well as considerable evidence for woodworking – which points to boat or possibly ship repairs being undertaken at the site.Lewis-Simpson, Shannon (2026). 091973507X, Historic Sites Association of Newfoundland and Labrador. 091973507X
In the Spanish colonization of the Americas, bloomeries or "Catalan forges" were part of "self-sufficiency" at some of the missions, , and . As part of the Franciscan Spanish missions in Alta California, the "Catalan forges" at Mission San Juan Capistrano from the 1790s are the oldest existing facilities of their kind in the present day California. The bloomeries' sign proclaims the site as being "part of Orange County's first industrial complex".
The archaeology at Jamestown Virginia ( circa 1610–1615) had recovered the remains of a simple short-shaft bloomery furnace, likely intended as yet another "resource test" like the one in Vinland much earlier.
In fact, there is a long history of colonial ironworks in Braintree and Saugus, Massachusetts and, notably, by the extended Leonard family in and around Taunton, Massachusetts. Taunton Iron Works was first organized in 1652 by James Leonard and 45 investors and by 1656 produced 20 to 30 tons of iron per year. James was the scion of an early industrial family of Massachusetts—in the 17th and 18th century New England colonists said “Where you find ironworks, there you will find a Leonard.”Rev. Perez Forbes, “Topographical Description of Raynham” (Collections of the Massachusetts Historical Society, 1810), 3:175.] Forbes was a Leonard.
While some may contend that the iron industry was contested by the British, the Crown may have cultured the industry in favor for Loyalist to the crown. A case in fact would be James Leonard's grandson Daniel Leonard, a Loyalist raised by slaves after his mother died in childbirth who was made judge Mandamus during the American Revolution. In a historical exchange of opinion columns published in competing Boston newspapers Daniel Leonard, writing under the pen name " Massachusettensis" argued in support of the King of England against the letters of John Adams who wrote as "Novanglus" in support of revolution. Daniel's father Col. Ephraim Leonard "who, in 1734, erected his iron works on the Canoe River. The works were assessed for five hundred pounds.He was, without doubt, the wealthiest man in the North Precinct" and owned and gifted over 12 slaves and lived on a 1,200 acre estate near the East Mansfield Common in Massachusetts.
The earliest iron forge in colonial Pennsylvania was Thomas Rutter's bloomery near Pottstown, founded in 1716. In the Adirondacks, New York, new bloomeries using the hot blast technique were built in the 19th century.Gordon C. Pollard, "Experimentation in 19th century bloomery production: evidence from the Adirondacks of New York", Historical Metallurgy 32(1) (1998), 33–40.
In New Zealand, a sophisticated direct-reduction furnace was used to smelt iron sand, at the Onehunga Ironworks, in 1883.
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