Copper artefact

Single metals and alloys such as wrought iron, bronze and lead will corrode even though they are not joined to a different metal or alloy. In reality, anodes and cathodes are set up on the surface of the metal. This will be due to the different phases present in the alloy, e.g. iron (ferrite) will be the anode and graphite the cathode in a cast iron, impurities such as sulfides present in wrought iron and variation in the copper/zinc ratio in adjacent grains in a brass alloy. The reactions taking place on the anode and cathode will be the same except the anode may be a different metal if the object was not iron. With a copper artefact, for example, the anode reaction will be ... 

Figure 5.9. Examination of segregation bands of arsenic in copper in archaeological artefacts...

Ancient artisans were able to confer special colourings to their artefacts by applying particular techniques and treatments, which were lost in later centuries. They were also able to give copper based alloys the appearance of precious metals. Some of these special methods have been discovered and identified on ancient objects. The most famous of these alloys in Roman times was certainly Corinthian bronze, a copper alloy containing small amounts of precious metals, which acquired a purple-black or blue-black patination... 

Pei, J., Tercier-Waeber, M.-L. and Buffle, J. (2000). Simultaneous determination and speciation of zinc, cadmium, lead and copper in natural waters with minimum handling and artefacts by voltammetry on gel-integrated microelectrode arrays, Anal. Chem., 72, 161-171. 

Ponting, M. and Segal, I. (1998). Inductively coupled plasma-atomic emission spectroscopy analyses of Roman military copper alloy artefacts from the excavations at Masada, Israel. Archaeometry 40 109-122. 

Walker, C., Hancock, R.G.V., Aufreiter, S., Latta, M.A. and Garrad. C. (1999). Chronological markers Chemical analysis of copper-based trade metal artefacts from Petun sites in southern Ontario, Canada. In Metals in Antiquity, ed. Young, S.M.M., Pollard, A.M., Budd, P. and Ixer, R.A., BAR International Series 792, Archaeopress, Oxford, pp. 317-325. 

The second of these assumptions has been the subject of some debate (Budd et al., 1995a), and is discussed further below. Despite these possible complications, the method of lead isotope provenancing was applied enthusiastically to copper alloy artefacts, especially those from the Late Bronze Age of the Aegean (e.g., Gale and Stos-Gale, 1992, and references therein) up until the late 1990s, when this activity virtually ceased, in part because of the contradictory interpretations which were being proposed. 

Glascock, M.D., Spalding, T. G., Biers, J. C., and Corman, M.F. (1984). Analysis of copper-based metallic artefacts by prompt gamma-ray neutron activation analysis. Archaeometry 26 96-103. 

Mauk, J. L. and Hancock, R. G. Y. (1998). Trace element geochemistry of native copper from the White Pine mine, Michigan (USA) implications for sourcing artefacts. Archaeometry 40 97-107. 

Thickett, D. and Odlyha, M. (2000) Note on the identification on an unusual pale blue corrosion product form Egyptian copper alloy artefacts. Studies in Conservation, 45, 63-7. 

Figure 5.9 Example of a geochemical image with unlevelled data (copper in stream sediments from Wales). The horizontal E-W line is shown just above the southern extent of the 1988 field campaign boundary. A subde but noticeable linear feature can be seen coincident with the boundary between samples collected in different years. The results in the south had to be reduced by levelling in order to remove this analytical artefact.

Hancock, R.G.V., L.A. Pavlish, R.M. Farquhar and W.D. Finlayson. 1995. Analysis of copper-based metals from archaeological sites at Crawford Lake, south-central Ontario, Canada. In Trade and Discovery The Scientific Study of Artefacts from Post-medieval Europe and Beyond. edited by Duncan R. Hook and David R.M. Gaimster British Museum Occasional Paper 109. pp. 283-297... 

A relatively simple method is to dissolve out the chloride by immersion in distilled or de-mineralised water. Most of the common metals and alloys will continue to corrode in this solution although at a slower rate due to the lower conductivity of these solutions compared to seawater. This can be serious if the artefact is to be immersed for periods up to 5 years, but has been employed if the immersion times are relatively short. A small copper alloy sheave recovered from the Mary Rose was soaked in running demineralised water for 27 days and the chloride levels dropped from 106 to 10 ppm over this period. 

By using boiled water, the dissolved oxygen is expelled and hence, there should be no corrosion as the cathode reactant has been eliminated from the electrolyte. Unless the boiled water is kept in sealed containers, air (oxygen) will slowly dissolve into the water and corrosion of the metal or alloy will re-commence. As an alternative, using hot demineralised or distilled water will reduce the concentration of dissolved oxygen and hence corrosion, but this must be counter-balanced by the rise in reaction rates with temperature. In open conservation tanks, a temperature of 70°C is required to notice a significant reduction in rates of corrosion of metals. Small copper alloy artefacts from the Mary Rose were treated in this way using water at 80°C for 30 days. At the end of this period, the chloride levels in the water dropped to below 1 ppm. 

Thermoplastics in the form of a lacquer have poorer mechanical strength than the thermosetting resins but are more easily removed, should this prove necessary. Polyvinyl acetate and polyurethane are good examples of this class of consolidants, particularly on wrought iron artefacts. Many of the copper and copper-based alloys, such as bronzes and brasses recovered from the Mary Rose, were consolidated with a solution of acrylic resin dissolved in toluene (Incralac). 

Dussubieux, L., Deraisme, A., Frot, G., Stevenson, C., Creech, A., Bienvenu, Y. (2008) LA-ICP-MS, SEM-EDS and EPMA analysis of Eastern North American copper-based artefacts impact of corrosion and heterogeneity on the reliability of the LA-ICP-MS compositional results. Archaeometry, 50,643-657. 

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