Experimental Ironmaking Once More: Combining Theory and Fi Nd Material
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DISCUSSION Q Fig. 1 Smelting according to Evenstad’s manuscript from year 1782, printed in 1790. The 60 pages long manuscript covers every aspect of contemporary ironmaking. Drawing H. Ness Experimental ironmaking once more: combining theory and fi nd material In the previous paper the Th e large number of sites, the older Borre in Vestfold, however not re- excellent quality of Norwegian ones very large with several furnac- ported in the same thorough way as Roman Age and Medieval es and up to 50 tons of slag, express those fi rst mentioned. blooms was documented. The that the process was reproducible. number of sites tells us that the In this paper I will suggest that the Smelting by the Evenstad technique method was reproducible, and processes were complex, but not by following his cookbook- like ad- that the blacksmith could make complicated. vice from the year 1782 is another, fi nished items from the blooms but successful story. By 14C-dating with a high output, very Th e quality is remarkable. A basic we know that it was introduced diff erent from many reported question is how the sintering of as- around year 1400, probably less experiments. sumed primary small fragments to than 50 years aft er the Black Death. large lumps of solid, slag-free iron Pictures, redrawn from experimen- Q Arne ESPELUND was achieved. Th e carbon control tal smelting are shown in Fig. 1. (NO) can be attributed to human eff orts while the sintering must be a pure- As shown, it is a step-wise process. ly physical phenomenon. A driving Wood is at fi rst transformed into Introduction force for the sintering process is the charcoal in the conical-shaped fur- In the previous paper A new look at lowering of the specifi c surface area, nace. It should be noted that a pri- experimental ironmaking (Espelund, as is also known for snow crystals, mary over-reduction to a carburised in EuroREA 2009) I presented the growing in time when left undis- metal is followed by an oxidising re- excellent quality of Roman age and turbed. In the year 1794 the Swedish fi ning by adding roasted ore at a late Medieval Norwegian blooms, most metallurgist Carl Rinman clearly ex- stage, when only a little remains in of which are now kept and preserved pressed that iron oxide must remain the furnace. Th is is a modern inter- in Norwegian archaeological muse- in the slag in order to fl ux it and also pretation of his very practical advice. ums. Because they are, so to speak, combine förena, the ductile grains of Th e product of the smelting is fi nal- slag-free, no smithing in order to re- iron (Rinman 1794:17). He inserted ly removed with a shovel. Some of it move slag was necessary. Th eir den- a rod of iron into the tuyere of his represents a bloom, while the rest is sities are around 6, in contrast to furnace in order to test the quality of thrown onto the slag heap. Th e proc- 7.8, which indicate that they are in the slag, a measure also known from ess therefore is direct only in the a primary state and were not infl u- the Catalan process. sense that it was performed in the enced by heavy smithing. Th is view- same furnace. It consists of reduc- point is also supported by the gen- Because of their quality, the out- tion followed by refi ning. eral lack of hearths and anvil stones put upon smithing of the primary at Roman and Medieval bloomery bloom into fi nished objects is as- As an example of a misleading ex- sites in Norway. Th ey number some sumed to represent some 90 %. periment, I would like to present 500 and 10,000 respectively. Th e Accordingly, the blooms indicate the following case study: Th e Nor- lack of entrained slag is expressed a process that is diff erent from the wegian TV NRK I on January 2nd chemically by the very low contents standard experiments of modern 2009 showed smelting which was of silicon (Si) and manganese (Mn) bloomery ironmakers with their claimed to be successful. Th e tap- (representing oxides) found by con- yield of some 15 to 20%. In particu- ping of slag went well while the iron ventional analysis of the metal, and lar I refer to reported experiments produced appeared to be brittle. their low densities due to empty mi- in a medieval type of furnace in However, when asked for details I cropores. Th ey appear to be evenly Sweden and Denmark. In Norway was told that bones had been added distributed and present no problem such experiments have been carried to roasted bog iron ore. Bones con- upon smithing. out at Kittilsbu in Oppland and at tain calcium oxide (CaO) and also 4 euroREA 7/2010 Experimental iron making once more DISCUSSION phosphorus( P), detrimental for the Th e removal, or exclusion, of car- quality of the iron produced. By re- bon in γ-iron can be expressed by placing iron oxide (FeO) in the slag, the equation: this (CaO) can increase the output as well as the fl uidity of the slag, C + FeO ↔ Fe + CO however, at the cost of metal quali- ty. Th e archaeologist T. Gansum has Th is is a chemical equilibrium, also added bones to his charge – in where C expresses carbon dissolved his case deliberately (Gansum & et in the metal. A high content of FeO al 2002). However, analysis of hun- corresponds to a low value for dis- dreds of ancient slag samples have solved carbon. only rarely revealed contents of CaO above 1% (Espelund 2009a). Th e idea should therefore be to main- tain iron oxide (FeO) together with In this paper I shall go into theoret- the metal produced, if possible “at ical details, show some analyses, ex- birth”, in the statum nascendi. How- press something about the ore sit- ever, at the operating temperature of Q Fig. 2 The Fe-C-diagram expressing stability ranges uation and present my ideas about a bloomery furnace – some 1100 °C for the different phases. Abscissa % C, ordinate tem- how a two-step process can be car- – FeO is solid. Its chemical function perature. A vertical line is drawn at % C = 0.2, a mate- ried out. can be taken over by FeO dissolved in rial of the best possible quality for general purposes in a liquid iron silicate, so-called fayalite the smithy. This iron must be produced as a solid in the Basics of the iron-carbon slag. Th is slag is fl uid at normal op- γ-state around 1100 °C, which is the operating tempera- erating temperatures and is therefore ture of a bloomery furnace -oxygen system separated from the solid iron. Th e re- Th e iron-carbon diagram is, from a maining question now is how we can metallurgist’s point of view, indis- secure a proper slag. Over-reduction pensable when the formation and to a carburized metal without the the quality of iron and steel are dis- benefi cial eff ect of FeO dissolved in a cussed. It is shown in Fig. 2. silicate slag seems to be the problem. Noteworthy is the line Acm, rep- From fi nds to theory resenting saturation with iron car- and back again bide Fe3C (or elemental carbon C), which reaches 2% at 1100 °C. Among archaeometallurgists in Carbon atoms are accommodated Norway the character of the slag in the face-centred crystalline cu- now is regarded as very signifi cant. bic structure of γ-Fe, also named Th e Roman type is vesicular, present austenite. A metal with 2 % carbon also as large lumps, expressing that it cannot be forged. solidifi ed in a slag pit. Organic ma- terial (wood) in the pit was imbed- Th e Baur-Glässner diagram, shown ded in liquid slag and later decayed. in Fig. 3 expresses both the relations Th e Medieval slag is mostly dense between gas mixtures, such as those and fl at with a smooth upper and a created in a bloomery furnace, and rough lower side, expressing tapping Q Fig. 3 The Baur-Glässner diagram for a combined pres- the phases α- and γ-Fe, carbon and sideways at ground level, while the sure of CO + CO2 = 0.4 atm., balance nitrogen N2. This is wustite (FeO). “Evenstad” type is porous, and that the typical gas created by combustion of charcoal in gas evolution, due to an addition of a bloomery furnace. Abscissa ratio CO/(CO + CO2), ordinate Th e lower curve shows the gas com- ore at a late stage, took place during temperature. Dotted lines in the γ-field express the carbon position in chemical equilibrium with solidifi cation. Because of its specif- content at different temperatures and gas mixtures. solid carbon. Notice that the content ic character, 14C-dating of charcoal of carbon monoxide (CO) rises dras- from a slag heap will rarely be a sur- tically between 650 and 800 °C. As prise to the experienced archaeo- seen, no reduction of FeO to metallic metallurgist. Th e three categories Fe can take place below a tempera- normally represent periods of max- ture of about 660 °C, which is a red- imum production around year 200, glow heat. Th e right curve expresses 1100 and 1600 AD, respectively. the gas ratio for coexistence of the lowest oxide FeO and the metal. Th is Wherever slag is present in large metal is practically carbon-free, well amounts, it expresses a large and suc- suited for forging, while the values cessful production.