The contribution of geochemical and mineralogical characterization of iron slags in provenance studies in the Podravina region, NE Croatia
Abstract: Archaeological excavations in the Podravina region led to discovery of sites with traces of bloomery iron production during Late Antiquity and the Early Middle Ages. Mineralogical analysis of the slags recognized fayalite as the main mineral phase, while geochemical analysis confirmed high Fe contents, typical for bloomery iron smelting. Based on the previously established occurrences of bog iron ores in the study area, provenance studies were carried out using trace and rare earth elements to create a geochem… Show more
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Previous archaeological investigations in present-day Gornja Podravina have created prerequisites for the study of the wider context of iron production in the period of Late Antiquity and the Early Middle Ages. The relevant factors of the current state of research are 1) a series of known positions defined based on field surveys with traces of various activities related to iron production, 2) several positions where archaeological excavations were carried out with a pronounced character of iron production - workshops and spatially close and simultaneous settlement sites, 3) high intensity of occurrence of indicative sites in a geomorphologically unique area. During several research campaigns, from 2008 to 2019, led by Ph.D. Tajana Sekelj Ivančan (Institute of Archaeology), sites that can be linked to iron production were investigated and dated by the radiocarbon method to the 4th/5th century (Virje – Sušine), over the 5th/6th century (Virje – Volarski breg) and the end of the 6th and 7th century (Hlebine – Velike Hlebine, Hlebine – Dedanovice, Virje Sušine), end of the 7th and 8 th centuries (Virje Sušine) and until into the 8th and the beginning of the 9th century (Virje – Volarski breg, Kalinovac – Hrastova greda). At these locations, several tons of waste were collected as a result of the direct reduction process (iron bloom production) and post-reduction processing (primary and/or secondary smithing). More intensive, interdisciplinary research has followed since 2017, through the project "Iron production along the Drava River in antiquity and the Middle Ages: creation and transfer of knowledge, technology and goods" (IP–06– 2016–5047), led by Ph.D. Tajana Sekelj Ivančan, funded by Croatian Science Foundation. As part of the project, an extensive field survey and reambulation of the area of Gornja Podravina identified locations (about 160) where metallurgical waste was collected on the surface (Valent et al. 2018; 2019; 2022) resulting from the production and/or processing of bloomery iron. Surface–collected waste testifies to the high intensity and existence of different, interconnected metallurgical activities in this region. Previous publications of the investigated sites (Sekelj Ivančan 2009; 2010; 2011; 2013; 2014a; 2014b; 2016; 2018; 2019; Sekelj Ivančan and Valent 2017; Sekelj Ivančan and Tkalčec 2018) emphasized the workshop and production character. All sites in the observed area are located within a geomorphologically relatively homogeneous area, the lowland area of the Drava River upper basin, today's Podravina region, NW Croatia. The lowland area of the wider region connected to the river Drava (Podravina in Croatia, Somogy district, Republic of Hungary) is suitable for the formation of bog iron ores (Sekelj Ivančan and Marković 2017; Brenko et al. 2019; 2020; 2022; Kercsmár and Thiele 2015), the basic natural resource needed for the production iron. Samples of bog iron ore were found in the archaeological context at the mentioned sites (Karavidović 2020), but also at medieval sites dating from the 7th to the 9th century in the territory of today's Hungary (Gömori 2000a; 2000b), and they testify to the exploitation and use of local natural resources in the Podravina region (Brenko et al. 2021; 2022) as well as within the wider geographical area in the period of Late Antiquity and the Early Middle Ages. The term iron production in the context of the doctoral thesis refers to the entire production process, from the raw material to the final product, and includes research on exploitation strategies of natural resources (primarily ore as well as wood and clay), processing of raw materials (ore refining, charcoal production), smelting process (direct smelting of iron ore) and primary and/or secondary smithing. The technological aspects of iron production are defined based on an interdisciplinary approach that includes several levels of data collection, processing, and analysis: macroscopic analysis of relevant archaeological material (slag, ore), analysis of the chemical composition and mineral phases of selected samples, targeted design, performance and analysis of the results of archaeological experiments. The archaeological finds that form the backbone of the research come from research conducted at the sites Virje – Volarski breg (S 1, 2, 3), Virje – Sušine (S 5, 7, 8), Hlebine–Velike Hlebine (S 1 and 2), Hlebine – Dedanovice (S 1 – 4). The finds are kept in the Koprivnica City Museum. It is a total amount of several tons, under several basic groups of finds: technical ceramics (clay furnace walls and tuyeres), technological waste generated during the production of iron blooms and further processing procedures (smelting and primary smithing, welding), (semi)products (shards iron blooms) and resources (bog iron ore). In addition to the basic categorization, based on macroscopic analysis, the findings are further divided based on specific diagnostic characteristics depending on the type of finds to observe changes, similarities or differences in production technology, and source of raw materials from 4/5 – 8/9 centuries. The categorization is used in intra-site spatial analysis of distribution, to analyze the spatial organization of the closed archaeological contexts, workshops for iron production. For a better understanding of all iron processing procedures, experiments were conducted by scientific standards (Kettleborn 1987: 11–12), which include distinctly set goals and research questions, measurability, repeatability, and a satisfactory level of stakeholder experience. The experiments were designed to reconstruct and understand procedures related to the preparation and selection of raw materials, testing variations of conceptual reconstructions of smelting furnaces and smithing installations and variations in smelting/smithing procedures established based on macroscopic analysis, and how the resulting waste and record can be compared with archaeological findings and record. By comparing the results of macroscopic analysis and experimental testing, conclusions were drawn about the type and character of applied procedures and technological solutions (type and construction of smelting/smithing furnaces, type of procedures, course of procedures) and the interpretation of individual structures investigated at the observed sites was strengthened. During the experiments, the consumption of natural resources (raw materials-ore, charcoal), as well as the consumption of time and human resources were recorded, and a model was established to estimate the ratio of the consumption of all resources of basic raw materials and other resources against the quantity of the output, final product (iron bloom, consolidated and purified iron semi-products). Through quantitative analysis of different categories of waste related to the production process and comparison with the results of experiments, conclusions were drawn about the amount of iron produced, the volume of production, the intensity, rhythm, and level of use of the workshops. The laboratory analyses were performed for a more detailed classification of the selected samples, analysis of the characteristics of the raw materials used, generated waste, and the final product, and to determine the similarities and differences of the production process present in the context of the observed sites. They include mineralogical (XRD) and chemical analysis (ICP–MS, AES) of selected samples of raw materials (ore) and technological waste (slag) from different stages of iron production. The ore and slag samples were crushed to a powder fraction and their mineral composition was determined using X-ray powder diffraction (XRD). A Phillips vertical goniometer (type X'Pert) equipped with a copper tube and a graphite monochromator was used to determine the mineral composition. During the measurement, a voltage of 40 kV and a current of 35 mA with a step size of 0.02° 2θ were used. The analyzes were performed at the Faculty of Mining, Geology and Petroleum, University of Zagreb. Chemical analyzes were performed at MSALabs (Langley, Canada) using inductively coupled plasma atomic emission spectrometry (ICP-AES) after melting the samples with lithium borate. The proportions of less abundant elements were determined by inductively coupled plasma mass spectrometry (ICP-MS). The fractions of inorganic (TIC) and organic carbon (TOC) were determined by induction, while the loss of mass on heating (LOI) was determined at 1000 °C. The selection of samples is based on the macroscopic classification of finds (type of find) and technological interpretation (type and characteristics of the process of creation) as well as spatial (location of the site), temporal (absolute dating of the site) and contextual (archaeological record) properties of the find. The results were analyzed using chemometric methods, mainly multivariate statistical analysis of data groups. This approach enables detailed characterization and analysis of samples (Pollard et. al. 2006; Charlton et. al. 2010; 2012). The social context of iron production is analyzed through the organization of the production process (levels and types of activities related to workshop and settlement areas, areas of exploitation of raw materials, and the organization of the workspace) and the relationship between settlements and workshops (structure and location). This segment of the research involves the creation of a spatial database (using GIS software) of indicative sites (defined by field survey and archaeological research), geological, pedological, and geomorphological data of the investigated area of Podravina region as well as a database of qualitative and quantitative data on archaeological finds within sites, used for multi-level spatial analysis. The individual researched workshops are viewed as closed units and based on the spatial analysis of the distribution of finds and the archaeological record, conclusions were drawn about the organization of the workspace and the production process. By analyzing the relationship between the location of sites and defined archaeological units of different character (settlement, workshop) and temporal affiliation, as well as the relationship with the natural characteristics of the landscape (hypsometry, risk of flooding, presumed exploitation areas – bog iron ore formation areas and potential sources of other natural resources (wood, clay)) conclusions are drawn about mutual spatial conditioning and the combination of the influence of natural and socio-economic (pre)conditions for choosing the location of workshops. In a broader context, the available data on the appearance and development of local production and distribution of iron and/or semi-finished products in the wider Central European area in the time of late antiquity and the early Middle Ages, the period that follows the reduction in volume and/or the cessation of operation of large provincial production centers that operated under the Roman rule in antiquity throughout Europe and the destabilization of (socio)economic communications and social relations. In the period of Late Antiquity and the Early Middle Ages, iron was actively produced in the area of today’s Podravina region, and the entire chain of operations was present at the local level. The organization of work is structured, which testifies to the high level of organization of this economic branch, which reflects socio-economic relations and the needs of communities. The existence of production at the local level, changes and/or continuity visible in the selection of technological solutions and the location of workshops, the organization of the production process, and the intensity and volume of production over a long period, arise under the influence of cultural, socio-economic and natural (pre)conditions.
Previous archaeological investigations in present-day Gornja Podravina have created prerequisites for the study of the wider context of iron production in the period of Late Antiquity and the Early Middle Ages. The relevant factors of the current state of research are 1) a series of known positions defined based on field surveys with traces of various activities related to iron production, 2) several positions where archaeological excavations were carried out with a pronounced character of iron production - workshops and spatially close and simultaneous settlement sites, 3) high intensity of occurrence of indicative sites in a geomorphologically unique area. During several research campaigns, from 2008 to 2019, led by Ph.D. Tajana Sekelj Ivančan (Institute of Archaeology), sites that can be linked to iron production were investigated and dated by the radiocarbon method to the 4th/5th century (Virje – Sušine), over the 5th/6th century (Virje – Volarski breg) and the end of the 6th and 7th century (Hlebine – Velike Hlebine, Hlebine – Dedanovice, Virje Sušine), end of the 7th and 8 th centuries (Virje Sušine) and until into the 8th and the beginning of the 9th century (Virje – Volarski breg, Kalinovac – Hrastova greda). At these locations, several tons of waste were collected as a result of the direct reduction process (iron bloom production) and post-reduction processing (primary and/or secondary smithing). More intensive, interdisciplinary research has followed since 2017, through the project "Iron production along the Drava River in antiquity and the Middle Ages: creation and transfer of knowledge, technology and goods" (IP–06– 2016–5047), led by Ph.D. Tajana Sekelj Ivančan, funded by Croatian Science Foundation. As part of the project, an extensive field survey and reambulation of the area of Gornja Podravina identified locations (about 160) where metallurgical waste was collected on the surface (Valent et al. 2018; 2019; 2022) resulting from the production and/or processing of bloomery iron. Surface–collected waste testifies to the high intensity and existence of different, interconnected metallurgical activities in this region. Previous publications of the investigated sites (Sekelj Ivančan 2009; 2010; 2011; 2013; 2014a; 2014b; 2016; 2018; 2019; Sekelj Ivančan and Valent 2017; Sekelj Ivančan and Tkalčec 2018) emphasized the workshop and production character. All sites in the observed area are located within a geomorphologically relatively homogeneous area, the lowland area of the Drava River upper basin, today's Podravina region, NW Croatia. The lowland area of the wider region connected to the river Drava (Podravina in Croatia, Somogy district, Republic of Hungary) is suitable for the formation of bog iron ores (Sekelj Ivančan and Marković 2017; Brenko et al. 2019; 2020; 2022; Kercsmár and Thiele 2015), the basic natural resource needed for the production iron. Samples of bog iron ore were found in the archaeological context at the mentioned sites (Karavidović 2020), but also at medieval sites dating from the 7th to the 9th century in the territory of today's Hungary (Gömori 2000a; 2000b), and they testify to the exploitation and use of local natural resources in the Podravina region (Brenko et al. 2021; 2022) as well as within the wider geographical area in the period of Late Antiquity and the Early Middle Ages. The term iron production in the context of the doctoral thesis refers to the entire production process, from the raw material to the final product, and includes research on exploitation strategies of natural resources (primarily ore as well as wood and clay), processing of raw materials (ore refining, charcoal production), smelting process (direct smelting of iron ore) and primary and/or secondary smithing. The technological aspects of iron production are defined based on an interdisciplinary approach that includes several levels of data collection, processing, and analysis: macroscopic analysis of relevant archaeological material (slag, ore), analysis of the chemical composition and mineral phases of selected samples, targeted design, performance and analysis of the results of archaeological experiments. The archaeological finds that form the backbone of the research come from research conducted at the sites Virje – Volarski breg (S 1, 2, 3), Virje – Sušine (S 5, 7, 8), Hlebine–Velike Hlebine (S 1 and 2), Hlebine – Dedanovice (S 1 – 4). The finds are kept in the Koprivnica City Museum. It is a total amount of several tons, under several basic groups of finds: technical ceramics (clay furnace walls and tuyeres), technological waste generated during the production of iron blooms and further processing procedures (smelting and primary smithing, welding), (semi)products (shards iron blooms) and resources (bog iron ore). In addition to the basic categorization, based on macroscopic analysis, the findings are further divided based on specific diagnostic characteristics depending on the type of finds to observe changes, similarities or differences in production technology, and source of raw materials from 4/5 – 8/9 centuries. The categorization is used in intra-site spatial analysis of distribution, to analyze the spatial organization of the closed archaeological contexts, workshops for iron production. For a better understanding of all iron processing procedures, experiments were conducted by scientific standards (Kettleborn 1987: 11–12), which include distinctly set goals and research questions, measurability, repeatability, and a satisfactory level of stakeholder experience. The experiments were designed to reconstruct and understand procedures related to the preparation and selection of raw materials, testing variations of conceptual reconstructions of smelting furnaces and smithing installations and variations in smelting/smithing procedures established based on macroscopic analysis, and how the resulting waste and record can be compared with archaeological findings and record. By comparing the results of macroscopic analysis and experimental testing, conclusions were drawn about the type and character of applied procedures and technological solutions (type and construction of smelting/smithing furnaces, type of procedures, course of procedures) and the interpretation of individual structures investigated at the observed sites was strengthened. During the experiments, the consumption of natural resources (raw materials-ore, charcoal), as well as the consumption of time and human resources were recorded, and a model was established to estimate the ratio of the consumption of all resources of basic raw materials and other resources against the quantity of the output, final product (iron bloom, consolidated and purified iron semi-products). Through quantitative analysis of different categories of waste related to the production process and comparison with the results of experiments, conclusions were drawn about the amount of iron produced, the volume of production, the intensity, rhythm, and level of use of the workshops. The laboratory analyses were performed for a more detailed classification of the selected samples, analysis of the characteristics of the raw materials used, generated waste, and the final product, and to determine the similarities and differences of the production process present in the context of the observed sites. They include mineralogical (XRD) and chemical analysis (ICP–MS, AES) of selected samples of raw materials (ore) and technological waste (slag) from different stages of iron production. The ore and slag samples were crushed to a powder fraction and their mineral composition was determined using X-ray powder diffraction (XRD). A Phillips vertical goniometer (type X'Pert) equipped with a copper tube and a graphite monochromator was used to determine the mineral composition. During the measurement, a voltage of 40 kV and a current of 35 mA with a step size of 0.02° 2θ were used. The analyzes were performed at the Faculty of Mining, Geology and Petroleum, University of Zagreb. Chemical analyzes were performed at MSALabs (Langley, Canada) using inductively coupled plasma atomic emission spectrometry (ICP-AES) after melting the samples with lithium borate. The proportions of less abundant elements were determined by inductively coupled plasma mass spectrometry (ICP-MS). The fractions of inorganic (TIC) and organic carbon (TOC) were determined by induction, while the loss of mass on heating (LOI) was determined at 1000 °C. The selection of samples is based on the macroscopic classification of finds (type of find) and technological interpretation (type and characteristics of the process of creation) as well as spatial (location of the site), temporal (absolute dating of the site) and contextual (archaeological record) properties of the find. The results were analyzed using chemometric methods, mainly multivariate statistical analysis of data groups. This approach enables detailed characterization and analysis of samples (Pollard et. al. 2006; Charlton et. al. 2010; 2012). The social context of iron production is analyzed through the organization of the production process (levels and types of activities related to workshop and settlement areas, areas of exploitation of raw materials, and the organization of the workspace) and the relationship between settlements and workshops (structure and location). This segment of the research involves the creation of a spatial database (using GIS software) of indicative sites (defined by field survey and archaeological research), geological, pedological, and geomorphological data of the investigated area of Podravina region as well as a database of qualitative and quantitative data on archaeological finds within sites, used for multi-level spatial analysis. The individual researched workshops are viewed as closed units and based on the spatial analysis of the distribution of finds and the archaeological record, conclusions were drawn about the organization of the workspace and the production process. By analyzing the relationship between the location of sites and defined archaeological units of different character (settlement, workshop) and temporal affiliation, as well as the relationship with the natural characteristics of the landscape (hypsometry, risk of flooding, presumed exploitation areas – bog iron ore formation areas and potential sources of other natural resources (wood, clay)) conclusions are drawn about mutual spatial conditioning and the combination of the influence of natural and socio-economic (pre)conditions for choosing the location of workshops. In a broader context, the available data on the appearance and development of local production and distribution of iron and/or semi-finished products in the wider Central European area in the time of late antiquity and the early Middle Ages, the period that follows the reduction in volume and/or the cessation of operation of large provincial production centers that operated under the Roman rule in antiquity throughout Europe and the destabilization of (socio)economic communications and social relations. In the period of Late Antiquity and the Early Middle Ages, iron was actively produced in the area of today’s Podravina region, and the entire chain of operations was present at the local level. The organization of work is structured, which testifies to the high level of organization of this economic branch, which reflects socio-economic relations and the needs of communities. The existence of production at the local level, changes and/or continuity visible in the selection of technological solutions and the location of workshops, the organization of the production process, and the intensity and volume of production over a long period, arise under the influence of cultural, socio-economic and natural (pre)conditions.
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