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 Blast furnaces, which produce pig iron, have largely superseded bloomeries.
Process
A bloomery consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. Near the bottom, one or more pipes (made of clay or metal) enter through the side walls. These pipes, called tuyeres, allow air to enter the furnace, either by natural draught or forced with bellows or a trompe. An opening at the bottom of the bloomery may be used to remove the bloom, or the bloomery can be tipped over and the bloom removed from the top.
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 reduction 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 reduces the iron oxides in the ore to metallic 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 forgeable, 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 sinter 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 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.
History
The onset of the Iron Age in most parts of the world coincides with the first widespread use of the bloomery. While earlier examples of iron are found, their high nickel content indicates that this is meteoric iron. Other early samples of iron may have been produced by accidental introduction of iron ore in copper-smelting operations. Iron appears to have been smelted in the Middle East as early as 3000 BC, but coppersmiths, not being familiar with iron, did not put it to use until much later. In the West, iron began to be used around 1200 BC.
East Asia Earlier scholarship sometimes assumed that iron production in China began with cast iron and the blast furnace, without an earlier bloomery tradition. New archaeological evidence led Wagner to reject this view. He concludes that wrought bloomery iron was the earliest smelted iron used in China and that iron-smelting technology was introduced into northwestern China from the west, possibly around the 8th century BC.
Iron casting, by contrast, appears to have developed independently in China, probably in the south. Wagner suggests that early iron casting may initially have involved carburizing and melting bloomery iron in cupola furnaces rather than production in blast furnaces. Blast furnaces and fining were in use by the 1st century BC, although the precise chronology and location of these developments remain uncertain. By the early Han period, bloomery production appears to have largely disappeared in China. Wagner argues that its disappearance is more likely to have resulted from Han state control than from competition with blast furnace technology.
Sub-Saharan Africa The earliest records of bloomery-type furnaces in East Africa are discoveries of smelted iron and carbon in Nubia in ancient Sudan dated at least to the seventh to the sixth century BC. The ancient bloomeries that produced metal tools for the Nubians and Kushites produced a surplus for sale. All traditional sub-Saharan African iron-smelting processes are variants of the bloomery process.Cline, W. W. (1937) Mining and Metallurgy in Negro Africa.Meroe become the iron smelting center of East Africa Menasha, Wisconsin: George Banta There is considerable discussion about the origins of iron metallurgy in Africa. Smelting in bloomery type furnaces in West Africa and forging of tools appeared in the Nok culture of central Nigeria by at least 550 BC and possibly several centuries earlier. Also, evidence indicates iron smelting with bloomery-style furnaces dated to 750 BC in Opi (Augustin Holl 2009) and Lejja dated to 2,000 BC (Pamela Eze-Uzomaka 2009), both sites in the Nsukka region of southeast Nigeria in what is now Igboland. The site of Gbabiri, in the Central African Republic, has also yielded evidence of iron metallurgy, from a reduction furnace and blacksmith workshop, with earliest dates of 896–773 and 907–796 BC, respectively.
South Asia During a hydroelectric plant project, in the southern foothills of the Central Highlands, Samanalawewa, in Sri Lanka, a wind-driven furnace was found in an excavation site. Such furnaces were powered by the monsoon winds and have been dated to 300 BC using radiocarbon-dating techniques. These ancient Lankan furnaces might have produced the best-quality steel for legendary Damascus swords as referred in earlier Syrian records. Field trials using replica furnaces confirmed that this furnace type uses a wind-based air-supply principle that is distinct from either forced or natural draught, and show also that they are capable of producing high-carbon steel.
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.
Early to medieval Europe
Early European bloomeries were relatively small, primarily due to the mechanical limits of human-powered bellows and the amount of force possible to apply with hand-driven sledge hammers. Those known archaeologically from the pre-Roman Iron Age tend to be in the 2 kg range, produced in low shaft furnaces. Roman-era production often used furnaces tall enough to create a natural draft effect (into the range of 2m tall), and increasing bloom sizes into the range of 10–15 kg.Radomir Pliener, Iron in Archaeology - the European Bloomery Smelters, chapter XII, 2000 Contemporary experimenters had routinely made blooms using Northern European-derived "short-shaft" furnaces with blown air supplies in the 5–10 kg range The use of waterwheels, spreading around the turn of the first millennium and used to power more massive bellows, allowed the bloomery to become larger and hotter, with associated trip hammers allowing the consolidation forging of the larger blooms created. Progressively larger bloomeries were constructed in the late 14th century, with a capacity of about 15 kg on average, though exceptions did exist. European average bloom sizes quickly rose to 300 kg, where they levelled off until the demise of the bloomery.
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 oxidation 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 blast furnaces, 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. 26–29Blast 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, carburize them, and pattern-weld 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 carburize the surface 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