Blast furnaces

In what is called ‘indirect smelting’ of iron, wrought iron is produced from ore in a two-step process. In the first step, ore is smelted in a blast furnace, producing cast iron, typically containing 3–4% carbon. In the second step, all or most of this carbon is removed. In early times, both in China and the West, the removal of carbon was done in a finery, which will be discussed on other pages: here, here, and here.

A blast furnace produces molten cast iron from ore in a process which operates continuously over a long period. This period is, in modern times, measured in years. In earlier times, the operation may have continued over days or weeks before it was necessary to stop for repairs.

In a blast furnace, iron ore, fuel (charcoal or coke), and a flux (typically limestone) are charged continually at the top of a shaft and an air blast is blown in at the bottom. The burning fuel heats the furnace contents (called its burden) to a high temperature, and also provides an atmosphere rich in carbon monoxide (CO), which reduces iron oxides to metallic iron and carburizes the iron to 3–4% carbon. Molten iron and molten slag are tapped periodically from the bottom.

Blast furnace operation

This little cartoon shows how a modern blast furnace operates. Source: www.technologystudent.com/joints/iron3.html.

The blast furnace is the largest machine used in modern industry, for its efficiency increases greatly with size. The largest blast furnace in the world today is in Beijing. Another very large blast furnace is in Chiba, Japan.

Blast Furnace no. 5, JFE Steelworks, Chiba, Japan 日本千叶县第五高炉	Blast furnace at Capital Steelworks, Beijing 首钢京唐钢铁联合有限公司 Height 128 m, production 4.5 million t/year

Modern blast furnaces are embedded in so many auxiliary constructions that it is difficult, as in the photographs above, to see the blast furnace itself and its operation. This is easier in the following films of earlier blast furnace operation.

This film was made in 1954 to advertise the Stanton Ironworks Company Ltd in Derbyshire, England. It gives a very good description of blast furnace iron operation as it was practised at that time. It also tells of the wonderful quality of Stanton pig iron, and of the company’s great corporate responsibility, but we need not take notice of advertising claims.

This film cannot be shown here due to restrictions by the owners. The DVD must be rented from the local archive in Falun, Sweden. It combines films made in 1918 and 1926–27, showing the process of iron production in two very old-fashioned Swedish ironworks. It shows charcoal production, smelting in a blast furnace, and fining of pig iron to wrought iron.

Diagram of the Yangcheng blast furnace (Wu Kunyi & Miao Changxing 1994)

This Japanese television film, probably made in the 1970’s, shows the operation of a very old-fashioned Chinese blast furnace, called here a lilu 犁炉, at the Henghe Lijing Chang 横河犁镜厂 in Yangcheng County 阳城县, high in the mountains of Shanxi. Iron is cast directly from the furnace into moulds.

More technical information on this type of furnace is given by Wu Kunyi and Miao Changxing (1994). See also Liu Peifeng et al. (2015: 60–61).

Operation at this site ended at the end of the 20th century, but it has been proposed as a World Heritage Site, and it is therefore well preserved (Liu Peifeng 2014: 37). More on the site can be found through a Google or Yahoo or Baidu search; here is one interesting web page, with links to more:
源远流长的生铁冶炼技艺

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A traditional blast furnace in Mazandaran, northern Iran.

Photograph and sketch of a blast furnace in ‘Lawitschtängä’, Mazandaran, northern Iran, in 1927. Reproduced from Böhne (1928: 1579). Height ca. 3 m. Translation of Böhne’s description.

Traditional Chinese blast furnaces

From the 19th and early 20th centuries we have numerous descriptions of blast furnaces in various parts of China. Foreign and Chinese travellers described some as mere curiosities, but after the Japanese invasion of 1937, when China was isolated and imports of iron and steel were stopped, serious technical studies of the traditional iron industry were initiated, both in the Guomindang base in Sichuan-Chongqing and in the Communist base in Yan’an.

During the period of the Great Leap Forward, 1958–59, numerous technical studies of local traditional iron-production techniques were published. Some of these are nonsense, but many more are excellent serious accounts by engineers that can help archaeologists to understand the early techniques.

Traditional blast furnaces in Sichuan and Chongqing
Traditional blast furnaces in Dabieshan

The first aspect to be noticed about the traditional techniques is their great variation from place to place. This is no doubt because they have developed separately in master–apprentice traditions, leading gradually, in each locality, through small variations in each generation, to technologies that were quite different.

And the second matter to be noticed is that the largest and most efficient technologies had largely died out by 1800, due among other things to competition with cheap imported iron. We therefore do not have technical studies of the most advanced Chinese blast furnaces. In isolated regions, where there was less competition from foreign imports, the local traditional technologies survived well into the 20th century, as we can see for example in the Japanese film above. It is these that we have good technical documentation for.

Blast furnaces in Chinese archaeology

A Han blast furnace at Guxingzhen, Zhengzhou, Henan

Of numerous excavations of Han ironworks, the excavation at Guxingzhen 古荥镇 in Zhengzhou, Henan, is the most relevant for our purposes. A museum has been built over the site, and I visited there together with Dr. Li Jinghua 李京华 in 1987.

A large blast-furnace ‘bear’ of sintered iron at the Guxingzhen ironworks museum. Dr Li Jinghua 李京华 points out where the blast-furnace tuyère would have been. In the foreground is his assistant, Huang Keying 黄克映. Photo by DBW, September 1987.

The huge iron ‘bear’ seen here, the product of some sort of accident in furnace operation, shows the shape of the lower part of the furnace and the location of one of the tuyères. (‘Bear material’ is the sintered mass of iron and slag that accumulates at the bottom of a blast furnace.)

The furnace is reconstructed by Li Jinghua and others as shown below.

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Speculative reconstruction of the Guxingzhen blast furnace by Liu Yuncai 刘云彩, Li Jinghua, and others, modified by DBW. Dimensions in millimetres.

Sketch by Li Jinghua of the operation of a Han blast furnace, redrawn by R. F. Tylecote (1983).

The blast furnace was shored up with tamped earth earth as shown in this sketch.

A Song blast furnace at Kuangshancun 矿山村, near Handan, Hebei

Blast furnace at Kuangshancun 矿山村 in Wuan County 武安县, near Handan, Hebei. Reproduced by Liu Yuncai (1978: 23) from Guangming ribao 光明日报, 1959.12.13.	Liu Yuncai’s reconstruction of the furnace on the basis of the photograph. Dimensions in millimetres.	A later photograph of the same furnace, from Li Yanxiang et al. 2016: 56.

Several ancient blast furnaces were still standing in Kuangshancun as late as the 1950’s. Two still remained in 2012, but today only this one remains today. A calibrated radiocarbon date gives, at the 95% confidence level, 810–990 CE, i.e. the late Tang or early Song. In the Song period this was in the province of Cizhou 磁州, which was one of the most important iron-producing regions of the time.

Diagram of a blast furnace in Sichuan, ca. 1958, from Yang Kuan 1982: 185, fig. 47.

‘An ancient charcoal blast furnace in the interior of China’, from Gottwald 1938: 109, fig. 1.

What we see in Kuang­shan­cun may have been the inner shaft of a blast furnace like the traditional furnaces shown here, with an outer framework of wood and and a tamped-earth fill between the frame and the shaft.

Jin 金 dynasty blast furnaces in Heilongjiang

The illustrations below show two blast furnaces of a type that is known from several places in Henan, Jiangxi, and Heilongjiang. They are built directly into a hillside in order to obviate the need for a strong outer construction.

Blast furnaces no. 5 and 4 at Dongchuan 东川 in Acheng County 阿城县, Heilongjiang, from Wang Yongxiang (1965). Left: both furnaces; middle: no. 4; right: diagram of no. 4.

This type seems to be best suited to the loess regions, with their cloven topography. At a level place above a sheer cliff a few metres high a shaft was dug, 2–3 m in depth. From the side of the cliff a horizontal tunnel was dug to the shaft and reinforced with granite slabs. The whole was lined with smaller stones, mortared with clay, then plastered with a refractory clay. A second tunnel was often dug under the bottom, probably to allow heat to escape and to alleviate cracking of the furnace bottom. In furnace operation the high heat has baked the surrounding untouched loess soil to a hard red layer up to a half metre thick. 

See also my “Blast furnaces in Song–Yuan China”

Ming–Qing blast furnaces in Guangdong

In about 1680 Qu Dajun 屈大均 wrote in his Guangdong xinyu 广东新语 that the best iron comes from Guangdong (铁莫良于广铁) . His book gives a description of the industry and its technology.

广东新语•货语•铁

Guangdong xinyu: ‘Goods’: ‘Iron’

And in Guangdong the best iron comes from Luoding 罗定. Archaeological teams from Guangdong Provincial Museum 广东省博物馆 investigated iron-production sites in Luoding County in 1978 and 1982. Here they found numerous villages with the character lu 炉, ‘furnace’, in their names. In each, remains of iron production were found.

One possible reconstruction of the Luxia furnace.

A blast furnace was excavated in a village called Luxia 炉下, ‘Below the Furnace’. The excavation report gives the sketch shown here, which does not entirely match the verbal description. According to the description, the entire north wall of the furnace above the taphole is missing, the remaining height of the south wall is 271 cm, and the original height was 6–8 m. The measurements shown in the sketch are those of the remaining upper part of the furnace as excavated.

Excavation of an English blast furnace from about 1600 CE

This film is an episode of “Time Team”, a popular British television series about archaeology. In each episode the archaeologists attempt to answer a particular question in a three-day excavation. This episode concerns a blast furnace that was known from documents. Some of the most important British archaeometallurgists participated: David Cranstone, Gerry McDonnell, and Tim Young. The original film is at youtu.be/4dI8r4JPM-s.

References

Last edited by DBW 23 February 2023