Steel versus iron
To make iron hardenable it needs to contain c. 0.5–2% carbon. Such carbon enriched iron is called steel and has become somewhat less malleable. If this steel is heated until red-hot and then quickly cooled by quenching, it becomes hard as glass.
When the re-smelting method is employed, the difference between producing steel and refining the iron appears to be determined by two main factors: the depth of the furnace below the tuyere, and the volume of air supplied. If your objective is only to refine the iron, making it forgeable, the furnace below the tuyere must be shallow and the air supply moderate. In order to produce steel during the re-smelting, the furnace must be deeper and the air supply stronger. Giving the iron a high temperature and rich access to carbon gases promotes a transition from iron to steel.
High heat
The introduction of more air increases the amount of charcoal burnt, and therefore the temperature. In Evenstad’s textbook, he points out that the forge / furnace must be used multiple times the same day before steel production can start, to make sure it retains a high temperature.
The high temperature required for the metal to melt and “drip” to the bottom of the furnace produces considerable reactivity and drastically increases the metal’s ability to absorb carbon, compared to the more moderate temperatures of a carburising pack, c. 1000°C. The melting point for iron is in the range of 1500°C.
The depth of the furnace below the tuyere helps to a create a reducing atmosphere which is rich in carbon gases where the pool of molten metal ends up. A reducing atmosphere means that the metal cannot oxidise, i.e. will not be totally incinerated. High volumes of carbon gases assist the carburising process and therefore the formation of hardenable steel.
We noticed that the furnace temperature had been extremely high as several centimetre thick layers of the clay wall had melted away and turned into slag during the burning process.
Lessons learnt
Our attempts at producing steel by re-smelting iron resulted in a certain carburising effect, but it was not sufficient to manufacture good, hardenable steel. Nevertheless, the actual smelting process was highly interesting in the sense that we produced a homogenous and purified bloom – which confirmed the processing method described by Evenstad. Besides, the re-smelting process was incredibly easy to carry out with only a small bag of charcoal and a burning period of 30 minutes. Lee Sauder's article Making Steel in the "Aristotle Furnace" (2010) provides a good practical introduction to this method.
Written sources
It is hard to establish with certainty whether archaeological finds show use of the re-smelting method. The best accounts of refining iron and producing steel by re-smelting, stem from Ole Evenstad’s textbook from 1782 about the extraction of iron and steel from bog ore. Sadly, this source is of far more recent origin than the Viking Age. An earlier written source, which appears to provide an eye-witness account of the method, may therefore be important for us to shed light on the use of this technique as far back as the pre-roman Iron Age.
Aristotle’s commentary
The earlier source available to us is a passage in Aristotle’s work Meterologica, volume IV, chapter 6, sections 383 a-b (Lee 1952:323):
Wrought iron indeed will melt and grow soft, and then solidify again And this is the way in which steel is made. For the dross sinks to the bottom and is removed from below, and by repeated subjection to this treatment the metal is purified and steel produced. They do not repeat the process often, however, because of the great wastage and loss of weight in the iron that is purified But the better the quality of the iron the smaller the amount of impurity.
Theophilus, c. 1120
Notice also the wording of Theophilus in his textbook De diversis artibus (On Diverse Arts, chap. 91) where he specifies that after the iron has been produced by smelting it is to be melted in a forge and hammered to make it suitable for use.
IRON is engendered in the earth in the form of stones. When it has been dug out, it is broken up in the same way as copper above and smelted down into lumps. Then it is melted on an iron-worker's forge and hammered, so that it becomes suitable for any kind of work. Steel is named after the mountain Chalybs where the greatest use of it is found. It is prepared in the same way as iron so that it becomes suitable for work.
FERRUM nascitur in terra in modum lapidum, quod, effossum eodem modo quo cuprum superius frangitur et in massas confunditur, deinde in fornace ferrarii liquatur, et percutitur ut aptum fiat unicuique operi. Calibs dicitur a monte Calibe, in quo ejus usus plurimus invenitur; qui simili modo preparatur ut operi aptus fiat.
Ole Evenstad - the textbook
It is worth noting that when Evenstad describes the production of steel by re-smelting, he uses re-smelted iron, that is already purified iron as his starting point. In other words, Evenstad first describes the re-smelting of bloom iron into fine iron (refined iron), and then continues the process by re-smelting the fine iron to produce steel.
The ninth chapter. About the re-smelting of fine iron to make steel.
When the forge has been used to re-smelt iron for almost a whole day, so that it is thoroughly heated through, it should be cleaned, dug up and pounded the same way as for iron-smelting, but the furnace must be 2 inches deep below the pipe (the tuyère) so that the air do not touch the steel once the smelting process has caused it to fall into the forge [...]
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Literature
- Evenstad, Ole (1790): Afhandling om Jern-Malm, som findes i Myrer og Moradser i Norge, og Omgangsmaaden med at forvandle den til Jern og Staal. Det Kgl. Danske Landhusholdningsselskab, København.
- Hawthorne, J.G. og C.S. Smith (1979) Theophilus: On Divers Arts. University of Chicago Press, 1963; reprinted New York: Dover Publications 1979.
- Hendrie, R. (1847) An essay UPON VARIOUS ARTS in three books, by Theohilus. J. Murray, London.
- Jylov, Mads Rohde (2009): Fra malm til stål. Jernudvindingsteknologi i perioderne vikingetid og tidlig middelalder belyst ved eksperimentelarkæologiske forsøg og metallurgiske analyser. Afdelingen for Middelalder- og Rænæssancearkæologi, Aarhus Universitet Moesgård.
- Sauder, Lee (2010): Making Steel in the "Aristotle Furnace" (20.04.2016).
- Lee, H.D.P. (1952) Aristotle. Meteorologica. Loeb Classical Library 397. Cambridge, MA: Harvard University Press.
- Webster, E.W. (1931) Meteorologica. The works of Aristotle. Volume III. Ed. J. A. Smith and W. D. Ross. Clarendon press, Oxford.
