12 chapter two

Chapter two

The of and the Production of Steel

For much of the Ancient World, and indeed the early Middle Ages in Europe, the only ferrous material available was “ ironˮ. This was the product of heating with in a small furnace, perhaps 1 or 2m high. To oversimplify a complex series of reactions, Iron oxide + carbon = carbon dioxide + iron FeO + C = Fe + CO2 The iron ore was reduced to iron, but never melted, since the melting- point of pure iron is 1550ºC, and therefore never entirely separated from the slag formed by non-metallic impurities. The lump (or “bloom”) of iron formed might be forged out into bars, plates, or rods but they would still be full of slag inclusions. Meteoritic iron might have been forged into very good tools or weap- ons by a competent bronzesmith because of its high nickel content, but these would have remained isolated and expensive curiosities.1 The “Iron Age” could not develop in Europe until techniques for the successful reduction of iron ores had been devised and disseminated. Sophisticated techniques had been developed for working copper and its alloys by the second millenium bce and could be transferred to iron- working. Unlike those of copper, iron ores are very widespread, but the extraction of iron is not so simple, because its melting-point is much higher (iron 1550oC; copper 1080oC). Any attempt to reduce iron ores in a simple copper-smelting furnace will give an unusable mixture of iron and slag. If the iron ore is of exceptional purity, then isolated fragments of reduced iron which cannot be combined by melting together, will be formed. But if slag forms, then the particles may be sintered together. Even when the ore contains no earthy matter (“gangue”) itself, there is generally sufficient silica (silicon dioxide, SiO2) present in the stones and

1 Panseri, C. “Damascus steel in legend and reality” Gladius, 4 (Madrid, 1965) 33–43. The Smelting of Iron and the Production of Steel 13 clay which make up the wall of the hearth to react with part of the iron ore and form a slag. The iron ore is treated here as iron oxide only. 2FeO + SiO2 = Fe2SiO4 (iron oxide + silicon oxide = iron silicate) Slags are complex glass-like mixtures of oxides and silicates; the compo- nent of lowest free-running temperature that would generally be found in an ironmaking slag would be fayalite (Fe2SiO4 or 2FeO.SiO2) with a free- running temperature around 1200oC. So, even though the iron ore might have been reduced at 700o–800oC, unless the furnace temperature reached at least 1200oC the slag would not have been liquefied and there- fore could not have been separated from the iron. The use of -iron ores might result in an iron high in phosphorus (P) which would harden the iron somewhat. Iron ores reduced under such conditions can produce iron free from most of the slag, which when it liquefies, runs away from the still solid iron, which would be left as a lump (or “bloom”), porous in form and containing very little dissolved carbon but much entrapped slag. Such furnaces are therefore known as “bloomery hearths” and their products as “bloomery iron” or “wrought ironˮ. Repeated heating and forging would necessary to expel much of the slag and consolidate the bloom. If it was skilfully forged, the slag can be distributed in long “stringers” shaped like fibres, rather than globules, with a less deleterious effect on its mechani- cal properties. remained a favoured material of civil engi- neers until late in the 19th century on account of its “toughness” (defined in this case as resistance to sudden shocks) and resistance to corrosion. Until 1971 the Aston-Byers Company of the USA marketed a “puddled wrought iron” made by mixing molten pure (Bessemer) iron with molten slag. This may seem to have been a retrograde step, but in some applica- tions (e.g railway couplings) the earlier warning of impending failure that wrought iron gave was appreciated.2 These qualities, however, were not of immediate advantage to the Ancient World. Iron smelting seems to have been first developed some- where between the Caucasus and the Fertile Crescent early in the second millenium bce.3 From about 1900 to 1400 bce the use of iron ornaments and ceremonial weapons slowly spread; for example, the boy-king of

2 Ward, H.D. “Best Yorkshire” Journal of the Iron & Steel Institute (1972) 396 3 Wertime & Muhly, 1980, passim