Direct iron smelting

A good first introduction to direct iron smelting is this film:

The producer made a short preview:

The whole DVD is now available on Youtube. I have copied it and added English subtitles, here.

The central person of this film is Lee Sauder. He is one of a small number of experts world-wide who have spent years experimenting with the design of bloomery furnaces, basing their work on ethnographic accounts and modern scientific knowledge. This film is one of the best on the subject, but there are numerous others (of greatly varying quality) to be seen on Youtube. I have copied a few of these here:

What happens in a bloomery?

The fundamental processes involved in bloomery smelting are quickly told. A typical iron ore is a mixture of iron oxides – the useful part – and various useless parts, collectively called gangue (Chinese maishi 脉石). In most cases the gangue is largely composed of silica (SiO₂), with various other minerals in smaller amounts.

In the bloomery, hot carbon monoxide gas (CO) from burning charcoal reacts with iron oxides (Fe₂O₃, Fe₃O₄, FeO) to produce metallic iron in the solid state. At the same time, some of the iron oxide combines with silica in the gangue to produce fayalite (2FeO·SiO₂, also written Fe₂SiO₄), which has a sufficiently low melting point that it can melt and flow out as liquid slag. The slag thus carries away a significant part of the iron in the ore, so it is desirable to limit its amount. On the other hand, the slag also has positive functions, including protection of the metallic iron from being re-oxidized. The heaps of slag found at ancient ironworks sites are of great importance in the interpretation of their technology.

The raw product of the bloomery is most often a bloom, a lump of metallic iron intermixed with large amounts of slag. As can be seen in Lee Sauder’s film, this is then hammered to drive out as much of the slag as possible. It is ‘squeezed’ like a sponge – and one Chinese word for bloomery iron is haimiantie 海绵铁 (sponge iron). Some of the slag remains as inclusions in the iron, and shows up in metallographic studies of artefacts made of this iron.

The furnace

Building a furnace in which these processes can proceed efficiently is a complicated matter, and traditions in different parts of the world have developed extremely different designs. Accounts of bloomery smelting in parts of the world where the technology continued in use into modern times gives us important help in understanding archaeological remains of furnaces and slag.

Some of the most useful reports come from

Africa in the 20th century

Other reports come from

South Asia in the 19th century
Scandinavia in the 18th century
And Southeast Asia	;

These descriptions seem to be the most useful for our purposes, but there are many more from all over the world. One important point for students to take away from them is the enormous variation seen here in bloomery construction and operation. The archaeology of bloomery iron production shows even more variants.

Experimental smelting

Archaeologists sometimes do experimental smelts with furnaces that reproduce aspects of the furnace remains that they excavate. These experiments help in the interpretation of the remains. One very fine example of this kind of experiment was done at the site of the ancient city of Meroe in modern Sudan – this film tells the story:

I have added English subtitles to this version. The original is at www.youtube.com/watch?v=SPU8Uwa-jBQ.

The two expert smelters in the film are Lee Sauder and Jake Keen.

Some archaeological reports on the ironworking remains at the site are Tylecote 1970; 1977; Humphris & Rehren 2014; and Charlton & Humphris 2017.

References

Bohr, Romain, and Oliver Metz. 2015. ‘Theoretical and practical aspects of iron smelting in a bloomery furnace’. In Yearbook of the Institute Europa Subterranea, edited by J. Silvertant. Silvertant Erfgoedprojecten, pp. 1–15.

Charlton, Michael, and Jane Humphris. 2017. ‘Exploring ironmaking practices at Meroe, Sudan—a comparative analysis of archaeological and experimental data’. Archaeological and Anthropological Sciences, 11: 895–912.

Gordon, Robert B., and David J. Killick. 1993. ‘Adaptation of Technology to Culture and Environment: Bloomery Iron Smelting in America and Africa’. Technology and Culture 34.2: 243–270. http://www.jstor.org/stable/3106536

Humphris, Jane, and Thilo Rehren. 2014. ‘Iron production and the Kingdom of Kush: An introduction to UCL Qatar’s research in Sudan’. Mitteilungen der Sudanarchäologischen Gesellschaft zu Berlin, Sonderheft: 177–190, 424.  

Pleiner, Radomír. 2000. Iron in archaeology: The European bloomery smelters. Praha: Archeologický Ústav Avčr.  Also at: www.academia.edu/16271280.

Tylecote, R. F. 1970. ‘Iron working at Meroë, Sudan’. Bulletin of the Historical Metalurgy Group 4.2: 67–72.

———. 1977. ‘Iron working at Meroe, Sudan’. Wissenschaftlichen Arbeiten aus dem Burgenland 59: 157–171. www.zobodat.at/pdf/Wiss-Arbeiten-Burgenland_059_0157-0171.pdf 

Wagner, Donald B. 2008. Science and civilisation in China. Vol. 5, Part 11: Ferrous metallurgy. Cambridge: Cambridge University Press.

Last edited by DBW 24 February 2023