Analysis of four slag samples from Ahvenkoski, Viirankoski, Koirankallio and Myllykylä. ArchaeoMaterials Report 1

Transcription

1 Analysis of four slag samples from Ahvenkoski, Viirankoski, Koirankallio and Myllykylä. ArchaeoMaterials Report 1 Eleanor Blakelock ArchaeoMaterials 21 Malsis Road Keighley West Yorkshire BD21 1EY UK Report Compiled for Jouni Jäppinen March 2014

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3 Introduction Analysis was undertaken on four slag samples, each from different sites in Finland. Table 1 below details the slag, find location and the suggested date. The analysis was carried out to determine whether the samples selected were smelting, smithing or refining slags. Sample Site Date JJ No1 Ahvenkoski P: I: Radiocarbon date 980 AD JJ No3 Viirankoski P: I: Radiocarbon date BC JJ No4 Koirankallio P: I AD JJ No5 Myllykylä P I: BC-200AD Table 1. List of samples with find location and date. Slag Background In the indirect process when the iron melts, both FeO and P 2 O 5 are almost completely reduced into the metal, with MnO and SiO 2 partially reduced depending on the conditions in the furnace. In the bloomery process only FeO and P 2 O 5 may be reduced. Other compounds are not reduced during this process, or are totally re-oxidised at the end of the process: in bloomery smelts these include MgO, Si 2 O, Al 2 O 3, K2O and CaO, which are known as nonreduced compounds. Therefore if these compounds are present in the system, either in the ore, furnace lining or fuel, they will be present in both the slag and the slag inclusions. McDonnell (1987) has argued that slag containing enhanced levels of manganese oxide (MnO) are more likely to be smelting slag as any MnO present in the ore will enter the slag. This does not apply to ores with low levels of MnO as slag produced from smelting these will not display enhanced MnO levels. Iron slag typically consists of four phases: fayalite, wüstite and metallic iron in a glassy matrix. On cooling from the liquid state, the silicate and any free iron oxide mineral phases in the slag crystallise. The iron silicate present in slag is fayalite (Fe 2 SiO 4 ) which often appears as laths although crystal development depends primarily on the cooling rate. Wüstite (FeO) is a mineral form of insufficiently reduced iron oxide and has a dendritic appearance. Magnetite, also an iron oxide (Fe 3 O 4 ), is also often found in iron slag, especially those with a high iron content or those exposed to more oxygen, e.g. tap or smithing slag (Bachmann 1982). Methodology The samples were mounted using an epoxy resin and prepared by grinding on successively finer grinding paper before being polished to a 1-micron finish. These were chemically analysed using the Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (SEM-EDX). The SEM used was a Hitachi S-3700N Variable Pressure SEM, set at an accelerating voltage of 20 kv and at an acquisition time of 150 seconds. Images were recorded in the Backscatter Electron (BSE) mode. The EDX compositional data were obtained using an Oxford Instrument INCA EDX microanalysis system with an INCAx-act Silicon Drift Detector (SDD). The SEM-EDX was calibrated using cobalt prior to any analytical work. An average of approximately three analyses were taken in three different areas of each slag to account for the general heterogeneous nature of slag. Results and discussion

4 The average results from the analysis of the four slag samples are detailed in table 1, with all the data provided in the appendix. Sample Na 2 O MgO Al 2 O 3 SiO 2 P 2 O 5 SO 3 K 2 O CaO TiO 2 MnO FeO JJ No1 Bulk n.d. n.d n.d. n.d Fayalite n.d. n.d n.d. n.d JJ No n.d n.d JJ No n.d JJ No n.d. n.d n.d Table 2. Average results from the chemical composition of the four slag samples (all the results are in table 3 in the appendix). n.d. indicates where the concentrations were below the detection limits of the SEM-EDX. Sample 1 from Ahvenkoski The majority of sample 1 comprised of fayalite in a glassy matrix, the fayalite laths were large and fragmented. Throughout the sample there were very fine dendrites of wüstite present (Figure 1). One area of the slag consisted of mainly fayalite (Figure 2). The results from the analysis reflected this difference with the 6 areas analysed in the bulk of the slag having higher Na 2 O, Al 2 O 3, K 2 O and CaO than the fayalite rich region (Table 2), due to the larger proportion of glassy matrix to fayalite. No MnO was detected in this sample which could suggest that it is a smithing slag. Even so given that the local iron ore is relatively free of MnO smelting slag cannot be discounted. The dense nature of the slag sample sent possibly supports the suggestion that this may be a smelting furnace slag. If it is a smelting slag from the results above (Table 2) it difficult to determine whether the local Ahvenkoski village ore or the Timmer swamp ore was being used. Figure 1. SEM-EDX backscatter image of sample 1 showing the laths of fayalite and the very fine dendrites of wüstite.

5 Figure 2. SEM-EDX backscatter image of sample 1 showing the fayalite rich area in the slag. Sample 3 from Viirankoski Sample 3 consisted of fayalite in a small amount of glassy matrix. The wüstite present formed as globules rather than dendrites (Figure 3). The exception was an area where mostly wüstite was present. The c.1.2 wt% MnO present suggests that this is a smelting slag. Sample 4 This sample consisted of laths of fayalite in a glassy matrix and large dendrites of wüstite (Figure 3). The sample was fairly homogenous. The exception was an area where mostly wüstite was present. The c. 0.6 wt% MnO present and the homogeneity of the sample suggests that this is a smelting slag. Sample 5 The majority of sample 5 comprised of fayalite in a glassy matrix, the fayalite laths were large and fragmented. Throughout the sample there were very fine dendrites of wüstite present (Figure 5). The sample was fairly homogenous. The c.1.3 wt% MnO present suggests that this is a smelting slag rather than smithing slag. The outer surface of this slag sample has been effected by corrosion processes which has preferentially attacked the glassy matrix (bottom right of Figure 6).

6 Figure 3. SEM-EDX backscatter image of sample 3 showing the laths of fayalite and the globules of wüstite. Figure 4. SEM-EDX backscatter image of sample 4 showing the laths of fayalite and the dendrites of wüstite.

7 Figure 5. SEM-EDX backscatter image of sample 5 showing the laths of fayalite and the fine dendrites of wüstite. Figure 6. SEM-EDX backscatter image of sample 5 showing the laths of fayalite and the fine dendrites of wüstite. Also note the edge of the sample where the glassy matrix has been corroded preferentially leaving the fayaltite.

8 Conclusions The analysis of the four slag samples suggests that they are all most likely smelting slag. Sample 1 is the only sample with very low quantities of MnO so may be a smithing or refining slag, but the dense nature of the slag and the microstructure suggests that it may be a smelting slag using a local ore which is relatively free of MnO. The four slag samples have different chemical compositions and are unlikely to have used the same recipe. Viirankoski and Myllykylä are the closest in composition but there is a distinctive difference in P 2 O 5 content. There are a number of factors that can dictate the composition of a slag, including ore, furnace lining, fuel, the addition of any fluxes and the conditions in the furnace itself. All these factors can be influenced by economic reasons (such as availability of ores) but also the technological decisions made by the metalworkers themselves (Rehren et al. 2007). References Bachmann, H.-G., The Identification of Slags from Archaeological Sites, London: Institute of Archaeology; Occasional Publication 6. McDonnell, G., (1987). The study of early iron smelting residues, in The crafts of the blacksmith: essays presented to R.F. Tylecote at the 1984 symposium of the UISPP Comitâe pour la Sidâerurgie ancienne, eds. B. G. Scott, H. Cleere & R. F. Tylecote Belfast: UISPP Comitâe pour la Sidâerurgie ancienne, Rehren, T., M. Charlton, S. Chirikure, J. Humphris, A. Ige & H. A. Veldhuijzen, (2007). Decisions set in slag: the human factor in African iron smelting, in Metals and Mines: Studies in Archaeometallurgy, eds. S. La Niece, D. R. Hook & P. T. Craddock London: Archetype Publications,

9 Appendix Sample Area Na 2 O MgO Al 2 O 3 SiO 2 P 2 O 5 SO 3 K 2 O CaO TiO 2 MnO FeO JJ No n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d n.d. n.d n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d JJ No n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d JJ No n.d n.d n.d n.d. n.d n.d n.d n.d. n.d n.d n.d n.d n.d. n.d n.d n.d n.d n.d JJ No n.d. n.d n.d n.d. n.d n.d n.d. n.d n.d n.d. n.d n.d n.d. n.d n.d n.d. n.d n.d n.d. n.d n.d. n.d n.d n.d. n.d Table 3. Full table of results from the chemical composition of slag samples. n.d. indicates where the concentrations were below the detection limits of the SEM-EDX.