Archaeol Anthropol Sci (2010) 2:191–215 DOI 10.1007/s12520-010-0039-7 ORIGINAL PAPER Mineralogical and geochemical characterization of high-medieval lead–silver smelting slags from Wiesloch near Heidelberg (Germany)—an approach to process reconstruction Florian Ströbele & Thomas Wenzel & Andreas Kronz & Ludwig H. Hildebrandt & Gregor Markl Received: 18 November 2009 /Accepted: 26 May 2010 /Published online: 30 July 2010 # Springer-Verlag 2010 Abstract Here, we present detailed electron microprobe Keywords Lead–silver smelting slags . Smelting process . analyses and age data of high-medieval lead–silver smelting Mineralogical and geochemical characterization 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, Introduction solidus, and viscosity of the slag melt. By calculating the mass balance of the smelting process, a mass ratio of the The area of Wiesloch near Heidelberg (SW-Germany) has various compounds used in the smelting process is seen mining activities for at least 2,000 years as the oxidation determined. Through this we were able to discriminate zone of the carbonate-hosted lead–zinc deposit of the so- qualitatively between non-ferrous metal smelting slags and called Mississippi Valley-Type (MVT, Hildebrandt 1997, bloomery slags. We also report a new type and process of 1998, 2004; Pfaff et al. 2010) was exploited for lead and silver production in which argentiferous galmei (zinc silver ores since Roman times (Hildebrandt 2004). The carbonate) was used as a main silver ore together with smelting activities resulted in large slag dumps of ∼400,000 t galena. The results indicate a sophisticated high-medieval of which 360,000 t result from smelting of argentiferous smelting technology, in which a slag with a low liquidus galmei and galena, from the late tenth to the late twelfth and a low viscosity was created. century (Hildebrandt 1997, 1998). For better understanding, a list of minerals and their chemical formulae, which are used in this paper, is given in Table 1. Although the workflow of the historic lead/silver F. Ströbele (*) : T. Wenzel : G. Markl smelting process is known quite well (a good overview is Institut für Geowissenschaften, given by Goldenberg et al. 1996), there is still a gap of Eberhard Karls Universität Tübingen, knowledge concerning the composition of the furnace Wilhelmstraße 56, charge. In Wiesloch, no furnace remains have been found 72074 Tübingen, Germany e-mail: [email protected] yet. Metals related directly to the smelting process and not to the casting of artifacts are very rare. Despite this fact, A. Kronz detailed slag analyses can be used to gain knowledge on the Geowissenschaftliches Zentrum, smelting process and its specific conditions. Georg August Universität Göttingen, Goldschmidt Strasse 1, Many papers have been published on ancient iron- 37077 Göttingen, Germany smelting slags (e.g., Eggers 1999; Kronz and Eggers 2001; Kronz and Keesmann 2003;Kronz2005; Bauermeister L. H. Hildebrandt 2005; Bauermeister and Kronz 2006), while there are only Büro für Denkmalpflege und Umweltschutz, Im Köpfle 7, few studies on slags of non-ferrous metal production, 69168 Wiesloch, Germany specifically about those of lead smelting (Bachmann 192 Archaeol Anthropol Sci (2010) 2:191–215 Table 1 Formulae of minerals al. 1985). Other publications like Battle and Hager (1990)), Degterov and Pelton (1999), Ettler et al. (2001), Kongoli and Olivine Fayalite Fe2SiO4 Yazawa (2001), Muszer (2006), Nikolic et al. (2008a, b)and Forsterite Mg2SiO4 Schlesinger and Lynch (1986) focus on smelting slags dating Larnite Ca2SiO4 from the nineteenth and twentieth century. Due to the Tephroite Mn2SiO4 differences in the age of the slags and the differences in Monticellite CaMgSiO 4 the smelting processes, it is difficult to compare the Kirschsteinite CaFeSiO 4 published results to our observations from Wiesloch. (Zn-olivine Zn SiO ) 2 4 Studies on the slags of Wiesloch have been published by Spinel Magnetite Fe2+Fe 3+O 2 4 Hildebrandt (1997, 1998). A further study by Kappes Gahnite ZnAl O 2 4 (2000) deals with soil contaminations from the slag heap Ulvöspine Fe 2+TiO 2 4 but does not give bulk composition analyses of slag Hercynite FeAl O 2 4 material. This study also reports that the smelting process Spinel s.s. MgAl O 2 4 is ineffective due to the high zinc content which will be Galaxite (Mn,Fe,Mg)(Al,Fe) O 2 4 discussed later. Other slag Leucite KAlSi2O6 It is the aim of the present contribution to interpret in minerals Hyalophane (K,Ba)[Al(Si,Al)Si2O8] detail the slags of the historic mining area around Wiesloch 3+ 2+ Iscorite Fe2 Fe5 SiO10 and to reconstruct the process of their formation as an Wüstite FexO example of medieval silver smelting technology based on Hedenbergite CaFeSi2O6 unusually Zn-rich ores. Our reconstruction of the historical Speiss phases Unnamed phase Fe2As smelting process includes process temperatures, properties Westerveldite (Fe,Ni,Co)As of the slag, ratio of fluxes, and fuel as well as the Löllingite FeAs2 technology, e.g., the type of furnace. It is based on Troilite FeS mineralogical, petrological, and geochemical analyses of 2+ 3+ Other phases Rhönite Ca2(Mg,Fe Fe Ti)6[O2(Si, Wiesloch slags to determine the chemical properties of the Al)6O18] 2+ slag and the physical characteristics of the slag melt. Melilite (Ca,Na)2(Al,Mg,Fe )[(Al,Si) SiO7] Geological and historical background Litharge PbO Ores Galena PbS The MVT deposit of Wiesloch (Hildebrandt 1997, 1998;Pfaff Sphalerite ZnS et al. 2010) is related to the extentional tectonic setting of the Hydrozincite Zn [(OH) |CO ] 5 3 3 2 Upper Rhinegraben. The mineralized area can be found Smithsomite ZnCO 3 between the Rhinegraben main fault (W) and the villages of Jordanite Pb (As,Sb)6S 14 23 Leimen (N), Mauer (NE), Sinsheim (E) and Bruchsal (S). Geocronite Pb (Sb,As)6S 14 23 The mineralization is concentrated in the carbonates of the Fluxes Calcite CaCO 3 Middle Triassic, namely the Trochitenkalk (mo1) and the Dolomite CaMg(CO ) 3 2 Schaumkalk (mu2s). The mineralization consists predomi- Baryte BaSO4 nantly of crystalline sphalerite and a second variety of Quartz SiO2 sphalerite (Schalenblende), which is amorphous and banded Hematite Fe2O3 (also called colloform), galena and pyrite (Seeliger 1963; α Goethite -FeO(OH) Hildebrandt 1997, 1998; Pfaff et al. 2010). Mining activities have been verified at least since the Roman times (Mone 1859). The most intense historic mining activities took place between 950 and 1080 (Hildebrandt 1997, 1998). Since then, no mine trying to 1993b; Hauptmann et al. 1988; Pernicka 1981, 1983; exploit galena or galmei profitably survived longer than Pernicka et al. 1985). The first detailed publication on the 30 years. The deposit experienced a last boom between mineralogical analysis of lead smelting slags (Faber 1936) 1845 and 1880, when thousands of tons of galmei backfill gives an overview on slag mineralogy and chemistry but is were found in high-medieval or even Roman galleries. The mainly descriptive. More recent publications focus on exploitation of the deposit lasted until 1954, when the last ancient slags from the Cyclades, predominantly, where mining facilities were abandoned. For a detailed description galena was smelted, no fluxes were used, and the smelting of all mining periods, see Hildebrandt (1997, 1998, 2004, was done only to produce silver but not lead (Pernicka et 2005). Archaeol Anthropol Sci (2010) 2:191–215 193 Sample material modern organic carbon, only charcoal pieces that were embedded completely in the slag were picked for acceler- Because of the anthropogenic influence and weathering ated mass spectrometry (AMS) dating. The sample pro- conditions, sampling campaigns in slag heaps need some cessing was performed at the radiocarbon laboratory at extra care: Samples for the determination of the bulk Heidelberg by Dr. Kromer. The analyses were conducted at composition should not contain several pieces of slag to the radiocarbon laboratory of the Department of Physics avoid mixing analyses of slags which result from different at Lund University (Sweden) according to the procedure processes or which were brought to the dump later. All bulk described in Skog (2007). analyses listed below refer to one single piece of slag. Slag samples derive from the construction pit of the “Palatin Determination of the wood species Congress Centre” (built 1992, the spoil dump known as Schafbuckelhalde was sampled (Fig. 1)). Stratified samples It is important for the reconstruction of the smelting process from slag dumps of the surrounding villages came from to know the type of wood used for the production of the collections. To be able to sample the slag heap in situ, a pit charcoal. To get information on the charcoal ash composi- with a depth of 3.5 m on the plots 156 and 157 (Amalienstraße, tion, several inclusions of charcoal were picked from the D-69168 Wiesloch) was excavated. All samples are in situ and slag and analysed at a dendrochronological laboratory in were taken from the wall of the pit. Hemmenhofen, Germany, for determination of the wood It will be shown later that galmei is an important ore in species by microscopic analysis of oriented sections. Wiesloch. Galmei is a weathering product of sphalerite and is also called calamine. It is composed mainly of smithson- Production of the quenched beads ite, but also some Zn carbonates (hydrozincite and hemi- morphite) can occur. The fraction of a mineral in this Bulk analyses of slags are often difficult because of liquid mixture is variable. This is why no mineral name but the immiscibility phenomena and partially molten inclusions. term galmei will be used in this paper. Here, we tried to estimate the bulk composition of the silicate fraction of our slag, which was usually between ∼85 and ∼99 wt.%.