The Question of Meteoritic versus Smelted Nickel-Rich Iron: Archaeological Evidence and Experimental Results Author(s): E. Photos Source: World Archaeology, Vol. 20, No. 3, Archaeometallurgy (Feb., 1989), pp. 403-421 Published by: Taylor & Francis, Ltd. Stable URL: http://www.jstor.org/stable/124562 Accessed: 23/08/2009 02:54 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=taylorfrancis. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected]. Taylor & Francis, Ltd. is collaborating with JSTOR to digitize, preserve and extend access to World Archaeology. http://www.jstor.org
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The Question of Meteoritic versus Smelted Nickel-Rich Iron: Archaeological Evidence andExperimental ResultsAuthor(s): E. PhotosSource: World Archaeology, Vol. 20, No. 3, Archaeometallurgy (Feb., 1989), pp. 403-421Published by: Taylor & Francis, Ltd.Stable URL: http://www.jstor.org/stable/124562Accessed: 23/08/2009 02:54
Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unlessyou have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and youmay use content in the JSTOR archive only for your personal, non-commercial use.
Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained athttp://www.jstor.org/action/showPublisher?publisherCode=taylorfrancis.
Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printedpage of such transmission.
JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with thescholarly community to preserve their work and the materials they rely upon, and to build a common research platform thatpromotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected].
Taylor & Francis, Ltd. is collaborating with JSTOR to digitize, preserve and extend access to WorldArchaeology.
smelted nickel-rich iron: archaeological evidence and experimental results
E. Photos
Introduction
The question of nickel-rich iron originating either from iron meteorites or smelted ores as the source of some of the earliest iron artefacts in continental Europe and the east Mediterranean has intrigued archaeologists and metallurgists alike throughout the
present century. Starting from Dorpfeld's (G6tze 1902) report in 1902 of nickel-rich iron in a macehead at Troy to Varoufakis' (1981) study of Late Bronze Age iron rings from Mycenae, the question is far from being resolved. At the same time, however, research into this subject has for several reasons given rise to some unintended confusion in the archaeological and archaeometallurgical literature.
One reason has been the variety of methods of analysis of these artefacts dictated by their scarcity, their consequent relative inaccessibility for detailed chemical and
metallographic investigation, as well as their variable state of preservation. Thus, corrosion products of an often wholly mineralised sample were occasionally investigated, usually by bulk chemical methods (Varoufakis 1981; Desch 1928 and 1936; Gotze 1902), or, at other times, metallic sections were examined by the
application of the electron probe microanalyses (EPMA) (Hansson and Modin 1973), the two sets of data not being directly comparable.
Secondly, the main criterion for meteoritic origin, particularly for the early analysts (Desch 1928, 1936), was an elevated nickel content comparable with that in actual meteorites. Recently, there has been invaluable work carried out towards establishing additional criteria for distinguishing artefacts produced from meteoritic as opposed to telluric (native) iron, emphasis being placed on the distribution of nickel in the metallic or corroded areas, and the mineralogy of inclusions (Li Chung 1979; Buchwald and Mosdal 1985; Gettens et al. 1971).
Thirdly the documentary evidence in the cuneiform tablets and Hittite texts relating to 'iron from heaven' (Bjorkman 1973; Muhly et al. 1985) and the Egyptian references
relating to funerary rites (such as the 'opening of the mouth of the dead' ceremony) in which 'thunderbolt' iron was used (Wainright 1932) had helped shape the hypothesis of iron meteorites as the more plausible source of early nickel-rich iron (Bjorkman 1973:124). This approach, favoured particularly among scholars at the turn of the
In the light of these sources of confusion, this paper attempts to remove misconceptions which have arisen in the literature by compiling, in tabular form, some of the more often quoted data on artefacts of meteoritic iron and of smelted nickeliferous ore. In addition, it outlines the main criteria for establishing the meteoritic origin of an iron artefact based on published work.
The absence of a uniform means of presenting analytical data would not have been so critical in drawing conclusions had there been criteria for establishing a nickel-rich artefact as being unequivocally smelted from nickeliferous ores. The lack of these criteria has been due to the complete absence, until now, of archaeological evidence for the smelting of such ores. In addition, it has not been feasible to associate the findplace of most nickel-rich artefacts with an evident nickeliferous deposit (Blomgren 1980) with the possible exception of the early Mycenaean rings from Greece (Varoufakis 1981).
Recently the first, albeit limited, evidence of smelting of nickel-rich iron laterites came to light in a Hellenistic (second century BC) settlement in north Greece, in the metallic prills trapped in iron smithing slags and in a fragment of a nickel-rich iron bloom (Photos et al. 1988). The evidence cannot be considered abundant, and furthermore was not corroborated by the chemical and metallographic examination of a small number of iron artefacts from the site: the objects contained no nickel. Nevertheless, the presence of this metallurgical waste from Petres constitutes the first archaeological evidence of intentional or accidental smelting of nickel-rich iron ores in the east Mediterranean. Because of the importance of these finds a set of experimental smelts was undertaken, aimed at formulating some basic criteria for the identification of nickel-rich iron as the product of a smelted ore.
Consequently, this paper proceeds to review the archaeological evidence, discusses the experimental results, and attempts to clarify questions raised in the literature in reference to bloomery nickel-rich iron.
Meteoritic, telluric and smelted nickel-rich iron- sources
There are three sources of nickel-rich iron in artefacts dating from antiquity to the
present era in continental Europe, the east Mediterranean and South-East Asia: iron meteorites, telluric iron (native iron-bearing basalts) and nickeliferous ores.
Iron meteorites have been the subject of extensive research primarily by astrophysicists and other scientists in the field of meteorite studies. The composition and structure of most meteorites have been compiled (Buchwald 1975) Hey 1966), and most museums with relevant material have produced catalogues of their own collections (Buchwald and Munck 1965).
For the purposes of the present discussion, it suffices to say that the nickel content of the majority of iron meteorites lies in the range 5-12 per cent, although it can reach as much as 60 per cent (Mason 1962). Iron meteorites are mainly classified as hexahedrites consisting primarily of kamacite (alpha-iron, ferrite, 5-7 per cent nickel) and octahedrites consisting of lamellae of kamacite and taenite (gamma-iron, austenite, 30-50 per cent nickel). The latter display the characteristic Widmanstatten structure, namely interlocking bands of taenite and kamacite, formed during the slow cooling of
The question of meteoric versus smelted nickel-rich iron 405
the meteorite. Meteorites showing this pattern can be identified relatively easily by electron probe microanalysis (EPMA) even when the object made of meteoritic iron has corroded.
The metallurgy of iron meteorites, that of an iron-nickel alloy, has been thoroughly investigated (Ogilvie 1965; Uhlig 1954), their chemical composition in minor and trace elements particularly Ga and Ge accurately determined (Moore et al. 1969; Lovering et al. 1957). Trace elements like cobalt, copper, phosphorus or carbon do not exceed a total 2 per cent in the metal (King 1976). Phosphide (schreibersite (Fe, Ni)3P), sulphide (troilite, FeS), carbide (cohenite (Fe, Ni)3C) and silicate inclusions are quite common in meteorites, but the last of these is usually different in composition from that encountered in wrought or cast iron. The presence of characteristic features like Neumann bands, twinned thin lamellae caused by shock related mechanical deformation, have been explained in detail (Uhlig 1955). All these features have to be taken into consideration when attempting to establish the nature of a nickel-rich artefact as meteoritic in origin.
Telluric, or terrestrial iron, found in native iron-bearing basalts in a limited number of geographical regions such as the Disko district in west Greenland, exhibits an overall lower nickel content (0.5-4 per cent in ferrite) than meteoritic iron. This explains why investigators in the last century originally confused it with meteoritic iron since the latter was also found in Greenland, Cape York. However, telluric iron can contain
varying amounts of carbon ranging from malleable nickel-iron (c. 0.2 per cent) to white nickel cast iron (1.7-4 per cent) (Buchwald and Mosdal 1985:21). It is reasonable to assume that the only workable telluric iron would be of the low carbon range. Indeed the analytical investigation of knives made by Greenland Eskimos supported this
proposition. There are a number of iron ores which upon smelting can potentially yield a high
nickel iron. These include the sulphide nickel mineral pentlandite or nickeliferous pyrrhotite ((Ni,Fe)9S8; 35 per cent nickel) associated with chalcopyrite and small amounts of cobalt and arsenic as sulphides and arsenides. However, smelting these ores would probably lead to the production of matte (a sulphide of iron, nickel and copper) due to the affinity of these elements for sulphur, thus preventing the production of a ferro-nickel alloy (Gilchrist 1980:378).
In antiquity, it is more likely that two other sources of nickel-rich ore would have been used, namely garnierite, a hydrated nickel magnesium silicate ((Ni,Mg)3- Si2O5(OH)4, (45 per cent nickel)), the weathering product of a nickel-bearing serpentine. An alternative source could have been nickel-rich iron laterites, hydrated iron oxides containing about 1-1.4 per cent nickel. They are the products of laterisation, the leaching of elements like sodium, potassium, calcium and magnesium from a basic igneous rock under wet tropical conditions (McNeil 1974).
Greece is known for its nickel-rich lateritic iron deposits (IGME 1973; Marinos 1982; Albadakis 1974, 1981). The largest of these is presently being exploited at the modern plant at Larymna in Lokris, central Greece, where it is reduced in electric furnaces to produce a ferro-nickel alloy (70 per cent iron-30 per cent nickel). Albadakis (1981) has identified a number of such deposits in Greece which are of varying extent and economic values together with some chemical data. Among the less exploited deposits
406 E. Photos
is the lateritic outcrop in north-west Greece, in the district of Edessa and Kastoria
(Garagounis 1971). Mineralogically the deposits consist of iron oxides, often associated with chromite (FeCr204). The nickel-bearing mineral is garnierite and the clay mineral
pimelite. Cobalt does not exceed 0.1 per cent.
Meteoritic and telluric iron: artefacts
Early nickel-rich iron artefacts of unquestionably meteoritic origin have not been
clearly identified in the east Mediterranean. Limitations on sampling may have been one of the reasons, coupled with usually extensive corrosion. By contrast, some researchers working on Chinese artefacts have been considerably more successful in
identifying objects of undoubtedly meteoritic origin (Li Chung 1979; Gettens et al.
1971). Furthermore, they have been able to establish criteria for differentiating meteoritic from smelted nickel-rich iron by building on the extensive literature on meteorites. Thus they sought within the corroded iron the expected nickel-cobalt ratio and the distribution of both elements in the artefact, the presence of characteristic trace
elements, and the nature of any inclusions. A point of considerable importance is the determination of the nickel content of the corroded iron matrix. To that end, and
following the example of these investigators, it would be desirable to report both the bulk, chemically determined, nickel content, as well as the microprobe analysed nickel content of the kamacite and taenite bands. Table 1 presents their results.
Metallographically, the Widmanstatten structure can be used as a safe criterion of meteoritic origin. The structure is retained if the object is cold-worked and
subsequently annealed at low temperatures (Buchwald and Mosdal 1985). However,
Table 1 Objects of meteoric iron from China and Sweden
Type Date Ni content Method of Reference analysis
China Yeuh axe Shang Dynasty 1.9-2.0% NiO XRF Li Chung (1979)
0.8% Ni (kamacite) EPMA 2.8% Ni (taenite)
Broad axe Chou Dynasty 8.53% NiO chemical Gettens et al (1971) (34.10). 7% Ni (kamacite) EPMA (Fig. 22)
Dagger axe " 0.7-0.8% NiO chemical (34.11). 6% Ni (kamacite) EPMA (Fig. 23)
Sweden Adze (662) 900-1200 AD c.7.25% Ni. 0.1% P " Buchwald & Mosdal
(1985) (Table 4)
Ulo (12004) 700-900 AD c.7.2% Ni
Knife (12605) 900-1200 AD c.7.2% Ni, 0.38% P
Ulo (13069) " 7.1% Ni, 0.18% P
The question of meteoric versus smelted nickel-rich iron 407
Table 2 Objects made of telluric iron (EPMA results, after Buchwald and Mosdal 1985: table 7).
Type (Site) %Ni
Knife (Disko) 3 Knife (Hunde) 0.23 Ni and Co Ulo 1.90
hot working at c. 1000?C can cause the loss of Widmanstatten structure resulting in the formation of martensite (Tylecote 1987:103).
Objects of telluric iron have a uniformly distributed nickel content not exceeding 3-4
per cent. Eskimos of west Greenland cold-hammered meteorites (Table 1) as well as telluric iron (Table 2), flattening pea-sized fragments into disks and using them, with a bone handle, as serrated knives or ulos (women's knives).
It is suggested that bulk chemical analysis and microprobe examination of a- and y- iron bands in metal or corroded surfaces complemented with the study of inclusions can determine effectively the meteoritic origin of a nickel-rich artefact.
Nickel-rich iron of possibly smelted origin
A number of review articles have been written on the evidence for early (3000-1200 BC) nickel-rich iron artefacts in the east Mediterranean, the earliest
reported to be of meteoritic origin (Waldbaum 1980; Piaskowski 1983). There are nickel analyses for a fair number of these (see Table 3), but some, like the objects in
King Tutankhamen's tomb, have been attributed a meteoritic origin in the absence of
any analytical data and principally on the grounds of textual evidence (Wainright 1932:7).
However, the record is slowly being clarified. Recently, an 'iron lump' of LBA data from Ayia Triada in Crete, originally reported as an iron meteorite (Iakovidis 1970), was analysed by Varoufakis (1982:317) and was shown to be corundum.
Table 3 shows some of the most well-known early nickel-rich iron artefacts together with their findspot, date, nickel content and other impurities when available, as well as their method of analysis. The data have been divided into three groups: a) objects dating from 3000-1200 BC in the east Mediterranean (Greece, Egypt, Anatolia, Iraq and Iran). b) objects dating from the first millennium BC mainly from Central Europe. c) objects dating from the present era and deriving primarily from Sweden but also from South-East Asia.
Table 3 illustrates the variation in the reporting of the results based on bulk chemical
analyses or electron microprobe data of oxide layers or metallic sections. Generally speaking, and particularly in the case of a), definite conclusions are hard to come by on the basis of chemical composition alone and in the absence of any EPMA results on
metallographic sections (see analyses by Brun 1939 and Gotze 1902: Table 3). The
Table 3
Type/Site
Bead Tepe Sialk, Iran
Bead El Gerzeh, Egypt
Tool fragment Uruk-Warka, Iraq
Tool fragment Ur, Iraq
Amulet Deir-el Bahari, Egypt
Pin Alaca Huyuk, Turkey
Plaque Alaca Huyuk, Turkey
Macehead Troy, Turkey
Axe-head Ugarit, Syria
Dagger blade Tomb of Tutankhamen, Thebes, Egypt
Headrest Tomb of Tutankhamen
16 iron implements Tomb of Tutankhamen
Date (BC)
4600-4100
3500-3100
3100-2800
3000-2000
2133-1991
2400-2200
2400-2200
2400-2200
1450-1350
1340
%age Ni
Widmanstatten structure
7.5%
10.9%
10.0%
3.44% NiO, 72.2% Fe203
3.06% NiO, 76.3% Fe203
a. 2.44% NiO, 72.94% Fe203, 1.12% CuO b. 3.91% NiO, 62.02% Fe203, 1.82% CuO
3.25%, 84.95%Fe, 0.19%S, 0.39% P, 0.41% C, 10.8% Fe oxides
The question of meteoric versus smelted nickel-rich iron 411
nickel contents of the Mycenaean rings have been measured in the presence of lead, silver and copper contents (with up to 50 per cent lead) but were calculated in the absence of these three elements (Varoufakis 1981). The presence of lead, silver, and copper in the analysis is probably due to contamination by these metals present in the bezels of some rings.
The analyses of objects in b) and c) in Table 3 are more detailed, the conclusions of the investigators being substantiated by a considerable body of data. Metallographic analyses complemented by EPMA have revealed two types of artefact. One type is produced from the forge-welding and piling of two iron sheets, one high in nickel, the other low (see Eskilstuna axe, Tandra tool, Etruscan weapons, Carpathian socketed axe), and the other type consisting of a ferritic matrix with high nickel streaks of pearlitic and/or martensitic structure (see currency bars, Sweden). The Indonesian krises, the nickel-rich iron blades of the wave-shaped daggers from the Field Museum of Natural History in Chicago, have only been surface analysed by spot analyses (Bronson 1987). However the analytical investigation of an undated Malaysian kris (Clough 1986:82) has shown that it belonged to the first group, with laminated high and low nickel areas.
The experimental work of Tholander and Blomgren (1985) has shown how the structure arising from forge-welded/piled nickel-rich iron resulting in a laminated structure can be produced. By contrast, the structure and nickel distribution in the second group has not been well understood. The experimental smelts presented in a subsequent section of this paper attempt to clarify this issue.
Archaeological evidence
Recent excavations in a strongly fortified Hellenistic settlement 1.5km north-west of the modern village of Petres and 35km south-east of Florina, north-west Greece (Adam- Veleni 1983), have brought to light extensive evidence for iron working. Seventy kilogrammes of slag were distributed in nine out of twenty four rooms, some of which were decorated with fine wall paintings. This distribution of metallurgical waste suggests that smiths may have occupied the premises after part or all of the settlement had been abandoned and rebuilt closer to the modern village of Petres, at the end of its second and last phase of destruction (Photos et al. 1988).
Most of the slag from Petres is typical smithing iron slag. Although most prills (metallic inclusions) trapped in the slag were of ferritic iron, two samples contained nickel-rich iron prills (Table 4). One prill in particular was composed of nickel with 4.5
Table 4 EPMA analysis of metallic prills in smithing slags and bloom from Petres.
boundaries; ferrite (b) at the grain boundaries; slag (c)(200x).
per cent iron. In addition, a shapeless mass of iron, a fragment of a bloom (diameter c. 5cm) found in a pithos jar amidst a number of blacksmiths' tools revealed on analysis an average nickel content of 3.2 per cent.
The microstructure of the Petres bloom fragment showed martensite in the middle of the section, with ferrite and pearlite of Widmanstatten structure at the edges (Figure 1). On the Vickers scale (100gm load), the hardness of the martensite was 492, the pearlite 290 and the ferrite 170. The carbon content was estimated by visual examination to c. 0.2 per cent (R. F. Tylecote pers. comm.). The slag consisted of a calcium-rich iron aluminosilicate matrix with dendrites of wustite (Photos et al. 1988).
The question of meteoric versus smelted nickel-rich iron 413
In view of the known laterisation in the district of Edessa (Garagounis 1971), the
presence of nickel-rich prills in smithing slags together with the bloom fragment suggests that nickel-rich iron laterites had been the source of the bloom. Whether these laterites were indeed smelted on or near the site intentionally or accidentally remains
conjectural. The fact that none of the metallic artefacts examined (fragments of two
handles, a nail, a strigil and a clamp) (Photos et al. 1988) contained any nickel indicates that nickel-rich iron may have either not been utilised for small domestic objects, or not used at all.
Experimental evidence
The Petres finds, although limited, constitute the first archaeological evidence for
smelting of terrestrial nickel-rich iron ores. In the light of these results, a set of
experimental smelts was undertaken to compare nickel-rich iron and associated slag produced under known conditions with the Petres slag and bloom. The experimental furnace conditions and results (yields of metal and slag) are discussed at length elsewhere (Photos 1987). The present account focuses only on points relevant to this discussion.
Nickel-rich iron laterites were made available from the Ayios Ioannis, Larymna plant. The range of composition of the charged ore after hand sorting was the
following: 2-4 per cent SiO2, 0.5-1.3 per cent NiO, 0.7-2 per cent Cr203, 65-80 per cent Fe203, 9-11 per cent A1203, 0.5-1 per cent CaO. These analyses are in
agreement with the typical composition of the Larymna ore body allowing for increased calcium and silica contents. High purity hematite used as a control, or as an addition to the lateritic ore to increase the iron content, was kindly provided by the British Steel
Corporation. The laterites were charged in an experimental bloomery shaft furnace (105cm in
height, 25cm in internal diameter) using a fuel to ore ratio of 3:1 (Photos 1987). The blooms were analysed by EPMA for their nickel content and its distribution in the iron, while the slag was analysed for its chemical and mineralogical composition. Blooms were smithed and one of them forge-welded to a piece of modern steel (1 per cent
manganese). Table 5 shows the range of nickel distribution in the experimental blooms, over a line
scan of specified distance along a single axis. The ranges listed below were confirmed
Table 5 EPMA analysis of nickel percentage range in blooms from various smelts.
Figure 2 Smelt 5 bloom. Martensite (a); ferrite at grain boundaries (b); dark area is slag (200x).
by random analyses on the same sample. The same Table also shows that blooms
produced from the smelting of nickel-rich iron laterites alone had a uniform nickel distribution along a short line scan (0.3-0.4cm). However, the absolute nickel content varied considerably from one bloom to the next (Smelts 4 and 10), suggesting that, if an entire bloom was sectioned and analysed accordingly, the total nickel range may have been wider.
On the other hand, when mixtures of laterite and hematite were added in various ratios, the blooms showed a wider nickel distribution over the same short range (Smelt 7a) in addition to a wide variation over a longer one (Smelts 5 and 7b), reflecting the
inhomogeneity in the nickel content in the original ore. In brief, nickel content in the
experimental bloom fluctuated irrespective of whether laterites were smelted on their own or in combination with other ores. Some of the high nickel contents analysed by EPMA approach those of iron meteorites. Had these blooms been analysed without
any prior knowledge of their origin and on the basis of the nickel content alone, they would almost certainly have been ascribed a meteoritic origin.
The metallographic structure of Smelt 5 bloom consisted of martensite in the middle with ferrite at the grain boundaries (Figure 2). The same type of wide variation in nickel content was detected in nickel-rich iron prills trapped within the smelting slag (Table 6). In this instance, prills were small pieces of metal of varying size, usually irretrievable, which were not consolidated in the original bloom. Their composition ranges in experimental smelting slags were 0-67 per cent nickel, 0-0.5 per cent chromium, 0.4-1.5 per cent arsenic and 0.2-0.3 per cent copper. Their structure ranged from ferrite (traces of carbon) to cast iron (with graphite flakes).
Four different mineralogical phases were evident in the experimental smelting slags. These consisted of a chromium-rich phase, wustite (ferrous oxide), a calcium-rich olivine (kirschsteinite) and a glassy matrix of melilite composition (Figure 3 and
The question of meteoric versus smelted nickel-rich iron 415
Table 6 EPMA analysis of metallic prill size and composition.
Table 7). The phase of particular interest here is the chromium-rich one consisting of
chromium, aluminium and iron. The chromium (oxide) content may be as much as 60
per cent with a corresponding decrease in the iron and aluminium values. Nickel will
partition primarily into the chromium-rich phase and occasionally into the wustite. However, nickel will partition foremost in the metallic phase rather than in the slag. This is due to the fact that the free energy of formation of the nickel oxide is quite low
compared with that of iron (c. 200kj/mole 02) at 1200?C, a temperature well within those achieved in our experimental furnace. This observation suggests that nickel oxide will reduce to metal quite readily and before iron (Richardson and Jeffes 1948).
Subsequent to its formation, the bloom from Smelt 4 was smithed by Mr T. Piperides, a blacksmith from Kavala, north-east Greece. The nickel content range in the billet
(smithed bloom) was 0.5-2.8 per cent, while the metallographic structure showed ferrite with pearlite at the grain boundaries. A second piece of the same bloom was
Figure 3 Slag from experimental smelt 4e. Chromium-rich angular grains (a). In this photograph the core consists of iron oxide (wustite, b) out of which the chromium has exsolved. Ca-rich olivine kirschsteinite (c) and glassy matrix of melilite composition in which there is fine growth of kirschsteinite (200x).
416 E. Photos
Table 7 EPMA analysis of four mineralogical phases in slag samples from Smelt 4.
Element Cr-rich phase wustite glassy matrix Ca-rich olivine
forge-welded to a piece of manganese steel (Figure 4). The nickel distribution along the nickel-rich phase was homogeneous (1.5-2 per cent) but dropped to zero in the
manganese steel zone, suggesting that there was no diffusion along the boundary. At the interface (20 microns) the nickel content rose to a high level (15 per cent nickel), as it also rose at the interface between the high-nickel metal area and the slag inclusions,
The mineralogy of the experimental smithing slags consisted primarily of wustite and an iron-aluminosilicate matrix rich in calcium. Both of these phases have been observed in smelting and smithing slags and are also present in the slag inclusions of the forge- welded objects.
Figure 4 Section of smithed bloom from smelt 4 (A) forge-welded to a piece of manganese steel (pearlite/ferrite) (B). The boundary consists of fine slag (iron oxide) inclusions (200x).
The question of meteoric versus smelted nickel-rich iron 417
Discussion
The experimental results have led to the following observations:
a) the nickel content varies considerably within a nickel-rich iron bloom reaching 60 per cent on electron microprobe analysis. The nickel content in metallic prills of differing sizes varies accordingly; b) the sections of the blooms which can be easily worked in a smithy for the formation of a billet, are those with a low nickel content. The experimental data described here indicate that this nickel content range is 0.5-3 per cent. The high nickel fractions of the bloom shattered upon hot working and were lost in the smithing hearth. These pieces were originally thought to be slag, but upon closer examination were shown to be metallic and of high nickel content (15-20 per cent); c) forge-welding of a section of high nickel iron billet to a piece of manganese steel
(without piling) showed that, first, the nickel content along the section was uniform
(1.5-2 per cent), secondly, there was no diffusion across the boundary, and thirdly, at the interface the nickel content rose substantially. It is proposed that if the nickel content in the artefact is low but sufficient to raise the question of a possible nickel-rich ore source, it would be advisable to examine closely the interface between metal and
slag inclusions; d) particular phases in the slag, when the latter is available, can be indicative of the
type of ore smelted, for example the chromium-rich phase observed in the
experimental slags and absent in the experimental smithing slags. In view of the experimental results, it is apparent that many of the inhomogeneities
detected with the electron microprobe, particularly in the Swedish currency bars
(martensite streaks, 25 per cent nickel) (Hansson and Modin 1973) can be explained in terms of inhomogeneities within the original blooms. Therefore it seems that some
investigators (ThSllin 1973) were correct in attributing the presence of high nickel areas 'to the qualities proper to the ore or to irregularities in the reducing process'. Chilton and Evans (1955) also correctly attributed nickel segregation partly to the mechanism of solid state reduction and partly to enrichment during forge-welding. Oxidation enrichment was observed during the forge-welding between nickel-rich iron and
manganese steel, since the nickel content rose substantially at the interface.
Tylecote (forthcoming) argued that the martensitic streaks in the Swedish currency bars (Hansson and Modin 1973) originated from oxidation enrichment of the surface
during welding. He argued that since the oxidation of iron takes place at the surface, nickel will increase in the iron solid solution during forging. Upon hammering, the
grains will be elongated resulting in the martensitic streaks evident in the
metallographic sections. However, the present experimental results indicate that nickel-rich (martensitic)
areas can form during smelting. The high and low nickel areas in the bloom are the
product of different rates of diffusion of nickel in a- and y-iron, since the crystal structure of the metal affects the rate of diffusion. The latter is slower in y-iron (3.2 x 10-13 cm2/sec) compared to a-iron (5.9 x 10-11 cm2/sec) for a particular temperature
418 E. Photos
910?C) (Carter 1979: Table 9.4). Hence, nickel is an austenite stabiliser, i.e. once in the presence of carbon it will take longer to diffuse out of the lattice.
It has been shown that the most rapid diffusion of nickel in iron takes place at concentrations of about 2 per cent, travelling 1.5cm in two hours at 1100?C (Carter 1979: Table 9.5). Given the inhomogeneities in the nickel content in the ore and the variation in the temperature and conditions within the furnace inherent in the bloomery process, rates of diffusion will vary considerably, resulting in large fluctuations in nickel content in the bloom.
Thus in bloomery iron smelting it would be rather difficult to produce a bloom with a uniform nickel composition. In addition, the sections of the bloom with a high nickel content (and associated carbon) would not be workable in a smith's forge due to their increased hardness. It is possibly of no surprise that most of the analysed artefacts in which a high nickel iron has been used in combination with steel, in a laminated structure, do not exceed the range of 3-5 per cent nickel. The only possible exceptions are the bracelets from Czestochwa-Rakow (12-18 per cent nickel) thought to be of smelted ore, but those may not have required the extensive working needed in the socketed axes and spearheads (Table 3).
Conclusions
This paper attempts to present in a coherent manner some of the numerous analytical results reported in the literature on meteoritic iron. It emerges that the detemination of the meteoritic origin of a nickel-rich iron does not depend on a single chemical analysis of nickel and cobalt but on a combination of chemical, electron microprobe, and
metallographic examinations. These analytical techniques can determine the structure of the metal and oxide, the possible transformations induced on hot and cold working, as well as the nature of the slag inclusions.
The criteria for the determination of smelted nickel-rich iron have been less clear. Recent work carried out by the present author has shown that smelting of lateritic iron ores results in large nickel variations in the bloom. These variations may account for the presence of martensite/pearlite streaks in some published artefacts (currency bars, Sweden, Table 3). However, they do not account for the laminated structures of
published socketed axes, krises, spearheads (Table 3) where the need for extensive
working inevitably limits the nickel content to 3-5 per cent. Due to the nickel inhomogeneity in the bloom, the smelting of lateritic ores would
have resulted in considerable waste and thus low yields. This may be one reason why this raw material was not used more extensively despite the desirable mechanical
properties of the iron-nickel alloy.
Acknowledgements
The author wishes to express her gratitude to a number of collaborators for their invaluable assistance: Mrs P. Adam-Veleni and Mr R. Adams for introducing her to
The question of meteoric versus smelted nickel-rich iron 419
the Petres material and bloomery smelting respectively. Dr N. Albadakis for providing iron laterites from the Larco plant and Mr B. Hodson, British Steel Corporation, Llanwern, South Wales for Australian hematite. Prof. R. F. Tylecote, Dr N. J. Seeley, Dr R. E. Jones and Dr. G. Varoufakis are warmly thanked for comments and discussion. Grants provided by the Institute of Archaeology, University College London, are gratefully acknowledged.
l.ix.88 Institute of Archaeology University College London
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Abstract
Photos, E.
The question of meteoritic versus smelted nickel-rich iron: archaeological evidence and
experimental results
The absence, until now, of evidence for the smelting of nickel-rich iron ores has led to much
speculation about the respective nature and properties of smelted nickel-rich iron compared to iron meteorites. The relevant literature on the latter subject is critically reviewed. Recently, archaeological evidence of smelting of nickel-rich lateritic iron ores (bloom, slag) has surfaced in a second century BC settlement in north Greece. Experimental smelting of similar iron ores has shown that there are large variations in the nickel distribution in the bloom, but only those sections thereof with a low nickel content could be worked in a smithy. The heavy losses incurred
may have been one reason why this type of ore was not used more extensively in antiquity.
