Characteristic of Iron Bloom and Slag Block from Experimental Iron Smelting Process
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Wojewódzki Urząd Ochrony Zabytków w Kielcach, Świętokrzyskie Stowarzyszenie Dziedzictwa Przemysłowego, ul. Zamkowa 5, 25-009 Kielce
Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie, Wydział Inżynierii Metali i Informatyki Przemysłowej, Al. Mickiewicza 30, 30-059 Kraków
Wiadomości Archeologiczne 2016;LXVII:147–173
Iron bloom was obtained as a result of an ancient iron smelting process carried out in slag-pit furnaces, apparently in use during the Roman Period in the Central European Barbaricum, more notably in the territory settled by the Przeworsk Culture people. In the 1970s, prompted by the reflections of M. Radwan (1963) and by the insights gained from the study of archaeological traces of the iron smelting process found in the great centre of iron metallurgy in the Świętokrzyskie (Holy Cross) Mts. in central Poland, K. Bielenin developed the concept of the free solidification process (Polish acronym PSK) to describe the process of the formation of the iron bloom and slag blocks inside the slag-pit – the underground part of the bloomery furnace. Bielenin found that iron obtained in these furnaces had to contain only a minor amount of slag, non-ferritic inclusions and non-carbonized ferritic inclusions. Only then would the iron have the right degree of malleability needed for successful forging. Archaeological studies of the Holy Cross Mts. centre of iron metallurgy have yielded a very modest amount of iron bloom finds, mostly in the form of flattened lumps, the product from the working of the bloom with hammers. Obviously, the obtained iron, a highly valued and prized resource, was taken out of the production site. What remained was the debris of the bloomery furnace slag-pits, filled to a various extent with slag, and iron making residue, so-called gromps, from the process of forging and consolidating raw iron blooms. Alternately, M. Radwan has interpreted these finds as debris from the smelting process claiming that this residue had formed in the shaft of the furnace during the iron smelting process. Given that the process of smelting iron in furnaces with a slag-pit is poorly documented in the archaeological record more comprehensive data had to be obtained from experimental studies. In Poland the first of these experiments were made in the late 1950s. Furnaces with a variously designed shaft (the above-ground structure) were used in the experiments (cf. Fig. 1, 4). Unfortunately, the product obtained tended to be a slag-iron agglomerate (Fig. 2, 3) markedly different from what is available in the archaeological record. To solve this problem the experiments were modified to employ K. Bielenin’s conception of the free solidification of slag blocks. This concept would be tested in practice only in the second decade of the 21st cent. during the experimental studies of A. Wrona made with modern replicas of a furnace referred to as type Kunów with a slag-pit canal (Fig. 6). The research findings outlined here mostly draw on results of an experimental process carried out during the 1st Bloomery Seminar held in Starachowice in October 2013. Similar results had been obtained by A. Wrona in 2012 and 2013, and during experiments carried out by a specialist team in 2013–2015. Their results help to supplement the analysis presented here. During the experiment made in 2013 a block of slag was obtained (Fig. 11) and iron bloom separated from the surface of the slag-block (Fig. 12). Weighing 3.65 kg the bloom had a ferritic structure appropriate for subsequent working. The experimental smelt had used 40.6 kg of iron ore and 50 kg of charcoal. The ore was locally obtained siderite (Fig. 7) and hematite imported from Bosnia and Herzegovina (Fig. 8), at a ratio of 1:1. The process was carried out in two stages, in an artificial blast furnace, with air injected under pressure from bag bellows (Fig. 9). During the first stage waste rock was reduced to slag and the formation of the iron bloom initiated. Next, air was allowed into the slag-pit canal of the furnace draining the iron bloom from the slag (Fig. 10). The iron bloom (Fig. 12) and the block of slag (Fig. 11) were next subjected to specialist studies. Observations of the microstructure of the bloom obtained during experiments made in Starachowice in 2013 identified a solid zone (Fig. 14) and a filigree zone (Fig. 13, 16) as well as a net-zone of iron formation (Fig. 17). The presence of these zones has been confirmed in blooms deriving from the earlier experiments of A. Wrona (Fig. 18–21). Furthermore, the study of the microstructure of the bloom helped establish that in a bloomery furnace equipped with a slag-pit the metallic iron is mostly obtained through processes of secondary reduction and disproportionation within the sponge gob of slag formed earlier near the tuyeres of the furnace. Throughout the process the iron bloom is in constant contact with liquid slag, which not only prevents the bloom from undergoing a secondary oxidation caused by air injected through the tuyeres, but also has an active part in the process of the gradual accretion of the bloom. Upon examination, the microstructure of the slag (Fig. 22–24) formed during the experiment was found to be consistent with the chemical composition and structure of ancient slag discovered in the Holy Cross Mts., except for compounds formed when Bosnian ore was used; the 0.07% content of K2O (cf. Fig. 11) in this ore led to the formation of leucite K2Al2Si6O16, identified during the microstructural analysis as black dendrites (Fig. 23). Similarly as experiments carried out in 2012–2015, the Starachowice experiment confirmed the validity of the assumptions made by K. Bielenin. Furthermore, observations made during these studies prompted a series of conclusions on the organization of the operation of a slag-pit furnace cluster, the feasibility of the use of artificial blast during the process (Fig. 9, 27) and the impact of atmospheric factors on the process flow. The results presented here prove that it is highly advisable to continue the experimental work to obtain a more detailed understanding of the stages of the iron smelting process, and to carry out these tests using local iron ores only. It was found also that the technical purity of the experimentally obtained iron is sufficient to classify this stage of product to working phase. Consequently, the procedures described in the literature as a post reductive stage should not be understood as a stage aimed on the removal of impurities but rather as a phase aimed on shaping the metal obtained in the process of reduction. It is also important to note the new data possibly of use in our studies of the bloomery process furnished since 2010 by the investigation of well-preserved bloomery fields in site (wilderness) Wykus in forest inspectorate Suchedniów, Kielce County.