Slag inclusions in iron objects and the quest for provenance: an experiment and a case study

Slag inclusions are found within most archaeological bloomery iron artefacts and are remainders of slag created during the smelting and smithing processes. Although they are widely believed to provide data with the potential for provenancing iron artefacts, previous slag inclusion studies have mostly proven inconclusive. The main aim of the work reported here is to analyse experimental smelting and smithing assemblages (including ore, furnace lining, fuel and slag), to compare these to slag inclusions in the resulting bloom and worked objects, and then explore the relationships between ore, slag and slag inclusions. This study has revealed that the composition of slag inclusions most closely relates to the smelting slag produced, whereas provenance to a specific ore would be difficult due to the chemical variability derived from furnace lining, fuel and any fluxes used. Some compounds in the slag inclusions are particularly affected during smithing of the artefact, i.e. those present in the sand flux and fuel used. However, trends are observed in the K₂O/MgO, MnO/SiO₂, Al₂O₃/SiO₂, Al₂O₃/MgO, Al₂O₃/K₂O and Al₂O₃/CaO ratios that allow comparison between slag inclusions and smelting slag in these experiments, and may therefore be used during other provenancing attempts. The knowledge gained from the experimental assemblages was subsequently applied to an archaeological case study, examining objects from the 900 Cal BC smithing site of Tel Beth-Shemesh, Israel and the 930 Cal BC smelting site of Tell Hammeh, Jordan. The analyses suggest that none of the artefacts examined derived from the Hammeh smelting system.     

Does it come from the Pays de Bray? Examination of an origin hypothesis for the ferrous reinforcements used in French medieval churches using major and trace element analyses

A new methodology based on major and trace element analyses of slag inclusions is proposed to determine (or exclude) the provenance of iron artefacts. It is applied to verify if the Pays de Bray, a French area between Rouen and Beauvais, could have been an important supplier for the ferrous reinforcements used in the Middle Ages for the building of churches and cathedrals in these two towns. To this purpose, the behaviour of trace elements during both direct and indirect operating chains is studied combining experimental smelting and different analytical methods, such as SEM–EDS, ICP–MS, LA–ICP–MS and INAA, performed on archaeological samples. The chemical signature of the Pays de Bray iron ore and slag is determined considering MnO and P₂O₅ contents as a first rough filter and seven couples of trace elements. Then, the major and trace elements are analysed using the same methods in the slag inclusions of 32 artefacts from the Beauvais and Rouen churches, made by the bloomery process. The trace element signature of the inclusions from each artefact is compared with the ore from the Pays de Bray area. The iron used in the Rouen and Beauvais churches seems not to come mainly from the Pays de Bray.     

Explaining the evolution of ironmaking recipes – An example from northwest Wales

Building on insights from previous Darwinian studies of technology, this paper explores the potential of evolutionary models to explain diversity and change in bloomery ironmaking recipes. Bloomery, or direct process ironmaking, involves the solid state reduction of iron oxide to metal and was the predominant means of producing iron in the pre-industrial world. The most archaeologically accessible record of bloomery practice is slag, an essential by-product of the smelting process. Ironmaking recipes can be characterized by their slag chemistry using a combination of multivariate statistics, ternary phase diagrams, and oxide ratios. Models derived from evolutionary theory are used to explain the shape, structure, and trajectories of ironmaking lineages identified from patterns of slag chemistry in terms of invention, selection, and socially mediated constraint processes. The utility of this approach is demonstrated by its application to slag excavated at Llwyn Du, a late medieval bloomery in northwest Wales.     

Large-scale 2nd to 3rd century AD bloomery iron smelting in Korea

Iron production in Korea has traditionally been seen in the shadow of developments in cast iron technology in China, with limited indication for a northern influence via Russia’s Maritime Province. The possibility of the existence of bloomery iron production in ancient Korea has been little explored, and relevant discussion is fraught with speculations based primarily on the early use of cast iron. The recent excavation of a site in South Korea recovered substantial amounts of slag providing direct evidence of bloomery smelting. The accelerator mass spectrometric dating of burnt wood from inside one of the slag pieces showed that the site was in use in the early 3rd century AD or earlier, which is in agreement with the assessment based on ceramic typology. The traits of a bloomery process evident in the slags’ microstructure, shape, composition and excavation context are discussed along with the implications for historical iron technology in Korea, where cast iron and the influence from China have been overly emphasised.     

METALLOGRAPHIC STUDY OF IRON ARTEFACTS FROM THE EASTERN TRANSVAAL,SOUTH AFRICA

The microstructures of two adzes, two hoes and a spear point from Iron Age settlement sites in the Kruger National Park have been examined. Electron microprobe analyses of the slag inclusions were also made. Some of the objects are made of high carbon steel while others have a highly variable carbon content. All appear to have been forged at relatively low temperature and then annealed at a low temperature near 700°C. None have been hardened by quenching and tempering. Some of the objects contain only traces of included slag while others have large slag inclusions. The slag compositions are quite different from those of medieval bloomery slags and are representative of African iron smelting practice in their high content of CaO, K₂ O and Na₂O. On the basis of the titanium content of the included slag, two of the objects are identified as made of the ore from Rooiwater and one from Phalaborwa ore.     

REGIONAL VARIATIONS IN BLOOMERY SMELTING SLAG OF THE IRON AGE AND ROMANO‐BRITISH PERIODS*

This study highlights regional variation in the composition of iron‐smelting slag produced in England prior to the medieval period and attempts to link slag composition to the type of ore smelted. For many sites, the slag compositions were consistent with the use of limonite ore, but there is evidence that siderite ore was smelted at sites in Sussex in the late Iron Age/Romano‐British periods. A compositional comparison of smelting slags and slag inclusions in Iron Age currency bars, using data from Hedges and Salter (1979 ), illustrates the potential of smelting slag compositional data in provenance studies of early iron objects.     

Indigenous African Furnace Types and Slag Composition—Is there a Correlation?

Within variation, three major iron smelting furnaces were used in the Iron Age of sub‐Saharan Africa, ranging from the natural draught driven tall shaft to the forced draught powered low shaft and bowl furnaces. These furnace types are, however, mostly known from the ethnographic context. Often, archaeologists are confronted with remnants from the smelting process, forcing them to speculate on the anatomy of the extant furnaces. The presence of multiple fused tuyeres has been used to identify natural draught furnaces in the archaeological record. However, working back from smelting remains such as slag to the furnace type using physical and chemical evidence has generally proved to be ‘undoable’. Thus, when randomly selected, the chances are high that one cannot separate bowl furnace slags from those that formed in their tall or low shaft counterparts. This observation is hardly unexpected; analogous thermodynamics and thermo‐chemical reactions governed bloomery smelting irrespective of furnace type. Rehren et al. (2007) have labelled this phenomenon the ‘tyranny of system driven constraints’. In this study, we argue that the hierarchical use of statistical methods may add another layer of evidence which, when coupled to archaeological indicators, may be useful in correlating slag chemistry to furnace types used in antiquity.     

Mineralogical and geochemical characterization of high-medieval lead–silver smelting slags from Wiesloch near Heidelberg (Germany)—an approach to process reconstruction

Here, we present detailed electron microprobe analyses and age data of high-medieval lead–silver smelting slags. The mineral composition data provide a database of all silicate and oxide phases in the slag. Bulk chemistry as well as mineral composition is used to reconstruct liquidus, solidus, and viscosity of the slag melt. By calculating the mass balance of the smelting process, a mass ratio of the various compounds used in the smelting process is determined. Through this we were able to discriminate qualitatively between non-ferrous metal smelting slags and bloomery slags. We also report a new type and process of silver production in which argentiferous galmei (zinc carbonate) was used as a main silver ore together with galena. The results indicate a sophisticated high-medieval smelting technology, in which a slag with a low liquidus and a low viscosity was created.     

古代钢铁制品中的浮凸组织初步研究

为研究钢铁制品中浮凸组织产生的原因,对14件取自不同地方、属于不同时期的具有浮凸组织的样品利用金相组织鉴定、扫描电镜能谱分析、化学分析和显微硬度测定等方法进行了研究。结果表明：金相组织观察发现浮凸组织与夹杂物和铁素体晶粒间界有一定的关系,并存在有不同的形貌,其显微硬度均比附近基体高;成分分析表明有浮凸组织的样品磷含量要比没有浮凸组织的样品磷含量高一个数量级。有浮凸组织样品内部磷含量的分布也不均匀。这些结果说明,浮凸组织的产生主要原因是矿石中的磷在冶炼过程中进入铁中,而在随后的脱碳退火、炒炼和冷、热加工过程中均不能有效地脱去;同时,固溶在铁中的磷与铁形成薄膜阻止晶粒间界长大、或在埋葬条件下长期时效引起的晶界移动而产生了浮凸组织。     

Metallurgical Characterization of A Coated Roman Iron Coin By Analytical Investigations

A coated Roman iron coin from Villa Loig in Salzburg, Austria was investigated. The coating is a copper alloy consisting of Cu, Pb, Sn, Zn and negligible amounts of Ag, a variant of gunmetal known as ‘leaded red brass’. The numismatic term for such a coin is subferratus (Latin). From an archaeometallurgical point of view, information about the chemical composition, the microstructure and the manufacturing technique are of interest. To achieve these objectives, different analytical techniques and metallographic examinations were applied. Droplet‐shaped iron inclusions were observed in the red brass coating, while at the grain boundaries, inside the iron core, copper and lead were detected (liquid metal embrittlement). The dendritic microstructure of the coating, the spheroidal‐shaped iron inclusions in the coating and the liquid metal embrittlement show that the iron coin was plated by immersing it in a molten copper alloy. The iron core is a low‐carbon steel with slag stringers, both of which are characteristic of a bloomery iron. Deformation twins (Neumann lines) were observed in the microstructure of the iron core and indicate that the coining was performed after the flan was cooled.     

Variability in single smelting episodes – a pilot study using iron slag from Uganda

Most archaeometallurgical studies of iron smelting are based on the analyses of slag fragments randomly selected from slag deposits, and assume that these samples are representative of the typical smelting conditions of the given context. However, little archaeometallurgical research has been published to explore the variability in slag composition within a single smelt, or between individual smelts at the same production site. The material used in this pilot study originates from two iron smelting sites identified in the Buganda Kingdom, Uganda, dated to the 18th and 19th centuries AD. The remains represent evidence of the industrial scale iron production that supported the growth and power of the kingdom. The slag survives in large clusters of complete blocks, in some cases weighing over 100 kg, each resulting from a single smelting episode in a pit furnace. A multi-sample analytical approach has allowed an insight into the compositional diversity within the slag from single smelting events, reflecting changing parameters in the smelting systems. The internal variation of the slag blocks is subsequently compared within and between sites, to address issues of standardisation and to differentiate two technological traditions that would appear very similar at the macroscopic level. On this basis, some sampling recommendations are made for future slag block studies.     

Provenance of Iron Age iron in southern Germany: a new approach

Ores, slag, and blooms from an Early Iron Age smelting site in the Black Forest, southern Germany, were analysed in view of their possible provenance in combination with products from a modern smelting experiment. Rather than employing lead isotope ratios, like in a previous study, osmium and strontium isotope ratios are used for the discussion of provenance. The results of the smelting experiment with iron ores from the archaeological site and their comparison with original finds show the high potential of osmium as a tracer to determine provenance. Strontium may be an additional indicator but possible contaminations need to be assessed. A Celtic iron ingot has also been analysed to check the potential of osmium for further provenance studies.     

IRON IN ANCIENT COPPER

Analysis of early copper-base artifacts invariably reveals traces of iron. Iron enters the copper during the smelting process and the level of iron in the metalwork is an indication of the smelting technology. In areas such as Western Europe where prehistoric slag heaps are absent even in the proximity of undoubted ancient mines, the iron content is low reinforcing the link between smelting technology and iron content. Very occasionally the iron content was deliberately encouraged and alloys containing between 30% and 50% of iron in copper were made, mainly for use in currency. These alloys are without modern parallel and their metallography and method of production are considered in some detail here.     

Mineralogical And Chemical Investigations Of Bloomery Slags From Prehistoric (8th Century Bc To 4th Century Ad) Iron Production Sites In Upper And Lower Lusatia,Germany

More than 400 fayalitic bloomery slags from prehistoric iron production sites in Upper and Lower Lusatia, eastern Germany, as well as bog iron ore samples and intermediary samples of the smelting process, were analysed by chemical and mineralogical techniques. While the precursor bog iron ores exploited in the two regions under investigation were very similar in composition, consisting of low‐manganese/low‐barium as well as high‐manganese/high‐barium types of ore, pronounced differences in slag composition were detected. Slags from 17 investigated sites in Upper Lusatia showed average P₂O₅ contents between 1 and 3 mass%, whereas slags from 15 investigated sites in Lower Lusatia were generally much richer in phosphorus, reaching values as high as 7 mass% P₂O₅. Since a reasonable correlation exists between calcium and phosphorus contents in the slags of the latter sites, it is conjectured that deliberate addition of CaO to the ore/charcoal charge of the bloomery furnace may have taken place in order to fix the phosphorus in the slags effectively. In many samples, this conjecture is being supported by the detection of a slag mineral Ca–Fe phosphate Ca_9?xFe_1+x(PO₄)₇ that presumably crystallized from a residual phosphorus‐rich melt and shows a cotectic relationship to both Ca‐rich fayalite and wustite, as well as to members of the solid solution series magnetite–hercynite.     

The medieval iron market in Ariège (France). Multidisciplinary analytical approach and multivariate analyses

The medieval iron market in Ariège (French Pyrenees) is characterised by its complexity leading to many questions regarding the ore used in relation to the artefacts encountered in archaeological settings. Despite the conclusive study of the treaty that regulated iron vs. coal exchanges from the 14th to the 19th c. by Verna et al., many questions remain unanswered concerning provenance and circulation of ores and products. Based on a combination of trace elemental analysis and multivariate statistical methods (discriminant analysis, hierarchical clustering analysis), we propose to follow an elemental-based signature from ore to artefacts, suited to the direct iron process, to clarify provenance hypotheses. We define the chemical signature of the Ariège area and compare it to that of slag inclusions in artefacts uncovered in several main archaeological sites from the Ariège area but also outside of this region. The results from this study are consistent with prior knowledge from historical sources. In the Ariège region, results highlight the multiple origins of products encountered at Montréal-de-Sos, a site close to known trade routes, whereas the isolated Castel-Minier site primarily contained objects from local ores and most objects retrieved from the Mirabat castle (Couserans) are coming from outside Ariège, possibly an indirect consequence from the exchange treaty. The presence of pins made of Ariegean ores is also demonstrated in the far more distant Capestang collegiate (near Narbonne, outside Ariège). This opens the way to finer studies in the politico-economical control of the iron supply for the building of medieval monuments in Southern France.     

THE PROVENANCE OF IRON ARTEFACTS FROM MANCHING: A MULTI‐TECHNIQUE APPROACH*

Iron finds from the Celtic oppidum of Manching in southern Bavaria (Germany) are analysed in view of their possible provenance. The exceptional size and the location of Manching are usually attributed to the presence of abundant iron ores in its vicinity. After a review of previous approaches for source determination of iron artefacts, we introduce lead isotope analysis as a new approach. However, only by combining the trace element patterns of slag inclusions and iron metal with lead isotope ratios in the metal is it possible to distinguish various iron ore formations near Manching. As a result, it turns out that, indeed, the most obvious ones—namely, bog ores near the Danube—constituted the main resources for iron production at Manching. It was even possible to select one occurrence as the most likely ore source.     

Slag as evidence for early iron production in arctic Norway

Physical, structural and chemical analyses were made on slag remains obtained from three sites in Iron Age arctic Norway. Scanning electron microscopy and x‐ray microanalysis were employed to confirm that the slag can only be a result of iron production. Although a distinction between slag produced by smithing and smelting proved difficult, consideration of the processes and the resources available support the assumption that both were practiced at the sites. The results provide firm evidence of iron production in the region of Norway north of the 69th parallel, by at least the sixth century AD.     

微量元素示踪古代青铜器铜矿料来源的可行性

目前,微量元素示踪法在青铜器矿料来源研究中的应用不尽人意。为此,模拟熔铸青铜器,对它们的微量元素作多元统计分析,旨在探讨利用微量元素探索古代青铜器铜矿料来源的可行性。分析表明：若舍弃冶炼后富集亍炉渣中的亲石亲铁元素,而仅选取富集于铜料中的亲铜元素及一些凡具有亲铜性,又具有亲铁性元素的成分数据,则通过多元统计分析,能较好地区别不同产地的铜矿料。     

A hierarchical taxonomic procedure for provenance determination: A case study of Chalcolithic ceramics from the central Zagros

This study develops a data analysis methodology for the exploration of the variance in chemical composition of ceramic artifacts due to their typological, location and temporal characteristics. This methodology then allows for the characterization of ceramics based on these characteristics and the sequential assignment of unclassified ceramics to a region, subregion or site, with further assignment to specific ware and chronological period. The procedure uses INNA data on the elemental composition of ceramics in order to derive a series of classification functions for each category within each level of the taxonomic hierarchy. The developed procedure is used to investigate a provenance problem involving Chalcolithic ceramics from the Zagros region of western Iran. The results show that the elemental composition of ceramics, when coupled with multivariate statistical techniques, represents a powerful means for distinguishing ceramic provenance.