In this study, copper slabs - ingots, from both Gdańsk and Krakow were examined. Besides metallographic examinations, attention was focused on analyses of corrosion products. The following techniques were applied: scanning electron microscopy with fluorescent X-ray microanalysis and X-ray diffraction. The conducted investigations enabled determination of the causes of corrosion in the old copper slabs, due mainly to the mediaeval alloying techniques and copper processing technologies

A. Garbacz-Klempka

R. Klempka

S. Rzadkosz

W. Ossowski

English

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217METALURGIJA 54 (2015) 1, 217-220A. GARBACZ-KLEMPKA, S. RZADKOSZ, R. KLEMPKA, W. OSSOWSKIMETALLOGRAPHIC AND CORROSION RESEARCH OF COPPER FROM ARCHAEOLOGICAL SITESReceived – Prispjelo: 2014-04-29Accepted – Prihvaćeno: 2014-09-10Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 54(1) 217-220 (2015)UDC – UDK 669.3:620.193:669:620.18=111A. Garbacz-Klempka, S. Rzadkosz, AGH - University of Science And Technology, Faculty of Foundry Engineering, Historical Layers Re-search Centre, Krakow, PolandR. Klempka, AGH - University of Science and Technology, Faculty of Electrical Engineering, Automatics, Computer Science and Biomedical Engineering, Krakow, PolandW. Ossowski, University of Gdansk, Institute of Archaeology and Eth-nology, Gdańsk, PolandIn this study, copper slabs - ingots, from both Gdańsk and Kraków were examined. Besides metallographic examina-tions, attention was focused on analyses of corrosion products. The following techniques were applied: scanning electron microscopy with fluorescent X-ray microanalysis and X-ray diffraction. The conducted investigations ena-bled determination of the causes of corrosion in the old copper slabs, due mainly to the mediaeval alloying tech-niques and copper processing technologies.Keywords: copper, corrosion, archaeometallurgy, microstructureINTRODUCTIONAs raw materials, copper, lead and tin are found at many archaeological sites, among which the most inter-esting are wrecks of the sunken merchant ships, such as the Copper Ship (Miedziowiec) from Gdańsk (Poland), Kopparvraket near Trelleborg, Skafto wreck (Sweden), Helgoland (Germany) or Stavanger wreck (Norway). Copper as raw material was also found at the Main Mar-ket Square in Krakow (Poland), which proves that this town participated in European copper trade.The biggest medieval raw materials find in Poland, two tons of copper in the form of 213 ingots of different size and weight, comes from the bottom of the Bay of Gdańsk. The wreck of a sunken ship called Copper Ship (Miedziowiec) is a unique object in the Baltic and the North Sea, where such a well-preserved and varied cargo was found.Copper played an important role in the Euro-pean trade as early as in the Middle Ages, and economi-cal progress was significantly connected with ore exca-vation and trading. In the Middle Ages, the city of Kra-kow played a very important part in the far-reaching metal trade. Krakow obtained a monopoly on trade and transit via Gdańsk to Flanders and England. As the con-sequence, Krakow became a member of the Hansa – the most powerful medieval trade organisation – where it was named the Copper House. In Krakow, copper trade was conducted on the Main Market Square, in the build-ing of the Great Weigh together with the smelting houses. The remnants of the old activity of the Scales, i.e. of weighing and partitioning of the large lumps of copper, are, among others, copper–saturated archaeological lay-ers and large amounts of copper chips coated with corro-sion products. There are also larger fragments of copper – a quarter of a slab, or an irregular copper slab. One must think here of an analogy between these copper slabs and the slabs recovered from the ship that sunk in the Bay of Gdańsk probably in 1408 and, on account of its cargo, was called the Copper Ship (Miedziowiec) [1 - 3].In this context some issues regarding the process of copper manufacturing are also worth noticing. The work of medieval miners and metallurgists was studied and disseminated by an eminent scientist, Georgius Ag-ricola (1556) [2 - 3].To produce copper from ore, Agricola recommended performing a number of operations, which included mining and enrichment of ore, preparation of fuels and fluxes, metal smelting and refining, and separation of noble admixtures. Studying the history of metallurgy is also significant for the comparison of technological processes of different metal materials [2 - 3].MATERIALS AND METHODSFrom a rich variety of archaeological material, for the present studies the following metal items were se-lected: one copper slab from the Great Weigh in Kra-kow and copper slabs from Gdańsk. Analyses were made of copper slabs fragments, obtained from the Copper Ship wreck (Miedziowiec), which left Gdańsk filled with a heavy copper cargo and sank shortly after-wards [1].Studies of copper from archeological sites provided the data about microstructure, chemical composition and corrosion destruction of the old raw material [4 - 7]. The analysis provided information about the level of old copper metallurgy, as well as destruction degree of 218  METALURGIJA 54 (2015) 1, 217-220A. GARBACZ-KLEMPKA et al.: METALLOGRAPHIC AND CORROSION RESEARCH OF COPPERthe artefacts, by establishing the kind and extend of their corrosion [8 - 9]. Understanding the corrosion pro-cesses is significant for conservation of metal artefacts as well as maintenance of industrial equipment working in specific conditions. Researching oxide layers of cop-per is common nowadays, because of widespread ap-plication of copper, especially in electronics and con-struction sector [10 - 15].RESULTS AND DISCUSSIONThe investigations of copper slabs tell us that the metal was of an relatively high purity and the copper content sometimes reached even 99%. As a conse-quence of studying the Copper Ship cargo, two types of raw copper, differing in chemical content, were distin-guished (Table 1).Type I is characterised by relatively high concentra-tion of Cu 95,7 ÷ 98,3 %, with an insignificant content of impurities 0,1 ÷ 1,1 % Pb, 0,6 ÷ 1,1 % As, 0 ÷ 0,6 % Sb and trace amounts of Ag, Fe, Sn. Type II: 91,4 ÷ 93,6 % Cu, 1,7 ÷ 3,0 % Pb, 0,3 ÷ 0,7 % As, 0 ÷ 0,3 % Sb, 1,0 ÷ 1,6 % Fe, 0 ÷ 0,2 % Ag and 2,1 ÷ 2,9 % Sn, the con-tent of which changes the character of the raw material examined. The differences between chemical contents of Type I and Type II copper are presented in Figure 1.The results are similar for the slabs from Krakow and Gdańsk (type I only). The greatest differences oc-cur in the content of iron, which in the specimen from Krakow amounts to 1,29 % of Fe, 0,4 % of Pb and 0,1 % of Zn (type III).In the chart (Figure1), the concentration of the cho-sen, characteristic elements for the three researched copper slabs (Pb, As, Fe, Sn). The mean values of ele-ments concentration together with standard deviation are shown in the chart as well. A significant difference Table 1  The results of chemical analysis XRF of the archeological copper specimens examined for the presence of trace elements Specimens 1 - 8 from Gdańsk slabs, specimen 9 from Krakow slab / wt. %Type I Type II Type III1/hz/966 2/w5/100 3/hz/805 4/hz-831 5/hz-810 6/hz-69-8 7/hz-69-5 8/hz-69-3 9/Kr P1Pb 0,48 1,10 0,13 0,38 2,44 1,89 1,71 2,98 0,41As 0,89 0,91 1,06 0,64 0,54 0,67 0,28 0,61 0,1Sb 0,13 0,14 0,58 0,0 0,0 0,28 0,26 0,11 0,09Zn 0,0 0,0 0,0 0,0 0,22 0,0 0,0 0,0 0,14Fe 0,11 0,15 0,13 0,32 0,98 1,31 1,14 1,59 1,29Ni 0,0 0,17 0,12 0,12 0,04 0,03 0,03 0,03 0,0Ag 0,11 0,0 0,18 0,0 0,0 0,19 0,15 0,20 0,02Sn 0,0 0,0 0,0 0,0 2,14 2,46 2,89 2,61 0,05Cu 98,3 95,7 97,9 96,7 93,6 92,8 92,2 91,4 97,83Figure 1  The results of chemical analysis of the archaeological copper specimens examined for the presence of trace elements - types I, II, IIIFigure 3  SEM images of corrosion products on copper slabs from “Miedziowiec”Figure 2  SEM images of corrosion products on a copper slab found under the Main Market Square in KrakowType IType IIType III219METALURGIJA 54 (2015) 1, 217-220A. GARBACZ-KLEMPKA et al.: METALLOGRAPHIC AND CORROSION RESEARCH OF COPPERin the content of these elements is visible (mean values) as well as a significant difference in the materials ho-mogeneity, expressed as the standard deviation value. The greatest inhomogeneity is shown by the copper form Gdańsk (type II).SEM examinations revealed changes in boundary layers that took place due to the long-lasting effect of corrosive agents (Figures 2 and 3). In the case of Kra-kow slab it was the atmosphere and wet soil; in the case of slabs from Gdańsk – seawater. The investigations carried out by SEM/EDS method identified typical products of copper corrosion. These are mainly oxides, chlorides and sulphates of aluminium, iron and copper. X-ray diffraction tests established the presence of cop-per oxide(I) Cu2O, copper oxide(II) CuO and copper chloride(I) CuCl on the surface of the sample. The cases mentioned below refer to the corrosion taking place in seawater and in soil.Moreover, the slabs from Gdańsk show serious loss-es in the form of numerous pittings, which were cer-tainly caused by falling out of the whole metal grains due to the corrosion of impurities present on the grain boundaries (Figures 4 and 5). The thickness of corro-sion layers is different in both cases and it changes ac-cording to the area examined. In Krakow slab it occa-sionally exceeds 60 μm.A SEM image (Figure 6) and a fluorescent analysis in micro-areas (Table 6) confirmed the earlier micros-copy observations. Because of its inhomogeneity, the boundary layer was in this case much more porous than the centre of the specimen and coated with corrosion products, among which oxides and chlorides are pre-sent. This proves the presence of chlorine in the corro-sive medium, which in this case was soil and seawater. The thickness of the changed boundary layer varies within the range of 25 - 60 μm.The medium of seawater had a very aggressive ef-fect on the slabs from Gdańsk, which were, however, partly protected by an insulating fossil shell composed of various matter, like pitch, wood tar, etc. While the objects collected in Krakow were subjected firstly to a destructive influence of the atmosphere and next - hu-mid soil. Without any doubt, the atmosphere increased the degradation, as it contained some impurities origi-nating from the waste gasses formed during the casting processes conducted in the nearby metal smelting plant.Figure 5  SEM image of corrosion of the copper slab from  “Miedziowiec”Figure 4  SEM image of corrosion of the copper slabs from “Miedziowiec”Analysed area Cu O Cl Fe S Pb1 40,7 29,0 19,2 5,6 3,1 0,7Figure 6  SEM image and chemical composition examinations (at. %) of corrosion of the copper slabs from the Main Market Square in KrakowAdditionally. the objects were systematically coated with solid particles. like dust. dirt and soot. making moisture and salt stick to them more easily. Corrosion in soil can proceed at a high rate. and in some cases it can assume similar values to the seawater corrosion. The determining factor in this case is the electrical re-sistance related to the presence of soluble salts and high moisture content in the soil. We know very well that both these conditions did occur in the case of the soil in the Main Market Square. though the corrosion rate was slowed down by a large distance from the surface of the soil and the resulting weak access of oxygen. Yet. it should be remembered that the most corrosive kinds of soils are. so called. ‘cultural layers’ processed for a long 220  METALURGIJA 54 (2015) 1, 217-220A. GARBACZ-KLEMPKA et al.: METALLOGRAPHIC AND CORROSION RESEARCH OF COPPERtime by man and they are mainly encountered in urban areas.CONCLUSIONSCopper is characterised by good corrosion resist-ance. Nevertheless, staying for a long time in corrosive medium, it suffered the process of significant degrada-tion, which was of electrochemical nature, of both local and general range. Both the presence of continuous lay-ers slowing down the progress in corrosion as well as changes of more local character in the form of pittings and crevices were observed. The research confirmed that the susceptibility to corrosion of archeological ob-jects made from metal depends to a great extent on the structure of metals and alloys. The source of intensive corrosion are the grain boundaries as the places of in-creased chemical activity. The higher activity of the grain boundaries is related to technological processes, i.e., with segregation of alloying constituents and impu-rities, and with crystallographic defects.The fact that the metal finds suffered from different types of corrosion deserves some attention. It should be stressed that the type of corrosive medium is very im-portant for the run of the corrosion process in items of archeological value. In the seawater, mainly pitting and intercrystalline corrosion was observed while in the at-mosphere and soil, the corrosion was of both of general and local character, but still resembling the pitting, in-tercrystalline and deposit types. The conducted analy-ses allowed to assess the similarities and differences observed in the corrosion processes.The analysis of old metallic material with respect to its history, physical metallurgy and corrosion processes is an important contribution to broaden our knowledge of the ancient objects. Further studies are expected to bring more information about the history of old objects and the related technological processes and corrosion effects.The collected data can help us with proper classifi-cation of artefacts and the choice of best conservation means and techniques, including the removal of corro-sion products, stabilising the chemical reactions, pro-tection from the destructive effect of environment and preparation for their exhibition.REFERENCES[1] M. Badura, B. Możejko, W. Ossowski, Journal of Archae-ological Science 40 (2013), 4066-4072.[2] C. Buśko, M. Dereń, A. Garbacz-Klempka, Archaeologia Historica Polona 18 (2009), 7-20.[3] A. Garbacz-Klempka, S. Rzadkosz, Archives of Metallur-gy and Materials 54 (2009) 2, 281–288.[4] E. Figueiredo, R.J.C. Silva, M.F. Fernandes, F.M.B. Fer-nandes, Microscopy and Microanalysis 19 (2013) 5, 1248-1254.[5] I.G. Sandu, O. Mircea, V. Vasilache, I. Sandu, Microscopy Research and Technique 75 (2012) 12, 1646-1652.[6] M. Fjaestad, A.G. Nord, K. Tronner, The decay of archae-ological Coopper-Aloy artefacts in soil. Metal 95 (1997) International Conference on Metals Conservation, 32-35.[7] O. Mircea, I. Sandu, I. Sârghie, A.V. Sandu, International Journal of Conservation Science 1 (2010) 1, 27-40.[8] A. Garbacz-Klempka, S. Rzadkosz, J. Banaś, Ochrona pr-zed korozją 11s/A (2007), 27-30.[9] A. Garbacz-Klempka, A. Łukaszczyk, Ochrona przed korozją 52 (2009) 11, 492–494.[10] J. Kunze, V. Maurice, L.H. Klein, H.H. Strehblow, P. Mar-cus, J. Phys. Chem. B. 105 (2001) 4263-4269.[11] H.H. Strehblow, V. Maurice, P. Marcus, El ectrochim. Acta. 46 (2001) 3755-3766.[12] I. Diez-Perez, F. Sanz, P. Gorostiza, Curr.  Opin. Solid State Mat. Sci. 10 (2006) 144-152.[13] V.D. Jovic, B.M. Jovic, J. Electroanal. Chem. 5 41 (2003) 13-21.[14] R. Babic, M. Metikos-Hukovic, Thin Solid Films. 3 59 (2000) 88-94.[15] J.O. Zerbino, L.M. Gassa, J. Solid State Electroche m. 7 (2003) 177-182.Note:  The responsible translator for English language are D. Pedrak and A. Hardek, Krakow, Poland

Metallographic and corrosion research of copper from archaeological sites

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Metallographic and corrosion research of copper from archaeological sites

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