Copper alloy

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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.


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Materials and technology[edit | edit source]

History[edit | edit source]

Brief Summary or introduction to historical context, art historic background, function, use, etc.

Materials[edit | edit source]

Raw materials; composition; definitions; parts; types; substrate(s); surface decoration; finish; chemical, optical, physical, or thermal properties; etc.

Technology[edit | edit source]

Sources, processing, tools, fabrication, manufacture, design, construction, decorative techniques, etc.

Identification[edit | edit source]

In archaeological contexts the identification of metals is often general without additional analytical analysis to confirm the metal and its alloying components. The appearance of archaeological copper alloys is variable depending on the alloying elements, and the burial environment. This can make identification on visual examination alone unreliable. For example, silver objects often include some copper and depending on the amount of copper and the chemistry of the burial environment, the copper may corrode preferentially giving the object the appearance of a copper alloy rather than a silver object. Elemental analysis is a more reliable form of identification.

Portable X-ray fluorescence (pXRF) is often used to identify metals and the alloying components but is limited as a surface analysis technique, though it can be used on a cut surface in tandem with other analytical techniques. Typically, pXRF is used for semi-quantitative results unless standards of none compositions, are used to calibrate the results. If a sample is taken for metallographic analysis, the exposed surface can also be used for analysis with a variety of elemental analysis techniques including, XRF and SEM-EDS. ICP-MS (Inductively coupled plasma mass spectrometry), which requires a sample is removed, is also a method that has been used to identify metals and their components.

Manufacturing techniques can be identified using a variety of techniques. For non-invasive analysis, X-radiography is often used to determine if an object is worked or cast as these produce different textures on the X-ray. Working marks such as hammering produces variations in thickness that are often more clearly visible on an X-ray than through visual examination, while a cast object will have a more even and uniform appearance with a slight grain from the casting porosity. An invasive technique that is also commonly used is metallographic analysis. In this case a sample is removed and polished for examination. The sample is also often etched to reveal the metal grain structure which will indicate whether the object has been cast or worked. Cast objects typically have a dendritic structure, while worked and annealed objects have a granular structure. Repeated working and annealing will also affect the grain size (Scott 1992).

Deterioration[edit | edit source]

For archaeological metals, the type and stability of the corrosion is dependent on the metal, it’s alloying components and the burial environment. Stable corrosion is often referred to as a patina, rather than as corrosion and may be left in place during treatment.

For copper alloys in cases of good preservation, the object corrodes forming copper oxides, mostly cuprite, which preserve the shape and form of the original object. Secondary corrosion products such as malachite from over and around the cuprite layer, with an outer crust of soil and mineral accretions on the exterior of the object. However, in unfavorable environments, particularly in the presence of chlorides, the original surface may be disrupted and there may be layers of nantokite (copper chloride) within the object structure or as pustules within and between corrosion layers. When exposed to moisture and oxygen, nantokite becomes unstable and reacts to form a variety of powdery corrosion products (basic copper chlorides). These are larger in volume than the nantokite pre-cursor and as a result can cause lifted surfaces and structural instability in the object. This unstable corrosion process is often called bronze disease, and post excavation can result in cyclical damage

Detail image of copper stearate corrosion

Copper Stearate Corrosion

Copper stearate corrosion is commonly seen on leather objects with copper and brass elements. The corrosion product forms when copper reacts with the fatty acids in oils used in leather dressing. The result is waxy green accretions of organometallic salts where the leather is in contact with the metal.

Conservation and care[edit | edit source]

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

  • To learn how you can care for your personal heritage, please visit AIC's Resource Center.

Documentation[edit | edit source]

For examination or documentation of issues specific to this material, including tips for accurately and meaningfully documenting specific materials, common types of previous repairs or restoration, etc. For general recommendations, please refer to the Objects wiki article on Conservation Practices or for general conservation work practices, please refer to the main AIC wiki section on Work Practices.

Preventive conservation[edit | edit source]

Dry storage is best for metal alloy objects. One method for addressing active corrosion for metal objects is storage in microclimates, either using a sealable plastic container, or sealing the object in an Escal bag. By using a microclimate, chemical reactions such as corrosion and in particular active corrosion, can be slowed or stopped by removing either oxygen, moisture, or both from the environment. Silica gel is often used create dry microenvironments (Anderson and Riccardelli 2009). The revolutionary preservation system also uses an oxygen scavenger in addition to silica gel (Paterakis and Hickey-Friedman 2011).

Interventive treatments[edit | edit source]

Reduction of corrosion products

Mechanical cleaning is currently the preferred method for terrestrial sites and involves the use of scalpels and skewers while the object is cleaned under magnification. This method requires practice and skill and is time consuming. Laser cleaning has been an area or recent research; however, this may result in micro-melting that could impact potential metallographic analysis.

Electrolytic reduction was once a frequently used method for the cleaning of archaeological metals it is less commonly used on terrestrial sites where mechanical cleaning has become the preferred method. In this method the object is immersed in an electrolyte solution and a current is run through the object to reduce the corrosion back to metal.  It is faster than mechanical cleaning but can result in loss of surface leaving areas that appear pitted or spongy. If the electrolyte solution is not changed regularly, it can also result in redepositing copper on the surface, and in worse cases an object may appear plated.

Though no longer the preferred method for cleaning archaeological metals, many collections have objects that have been cleaned using a variety of cleaning methods. It is important to be aware of these treatment histories as it can impact or limit the objects use for analysis and research as the surface and metal composition may have been altered. Some methods used in the past include boiling in formic acid and cleaning with a paste made up of zinc dust and sulphuric acid.

Reduction of copper stearate corrosion

To mechanically reduce the copper stearate corrosion exhibited on an object with copper rivets incorporated in leather straps the following treatment was developed. Working from the outside edge of the copper rivets inwards the copper stearate corrosion was gently loosened and detached using a toothpick (whittled with a scalpel to achieve required thickness and shape). Immediately following each reduction, a soft bristle brush was used to remove any lose corrosion particles from the surface. This step prevents lose particles from staining the adjacent leather. It was first attempted to use a mylar barrier layer to prevent staining, however, the method was found to be undesirable. Corrosion was present between the copper rivet and leather surface preventing a thin mylar sheet from being placed between the two surfaces. Introducing the mylar sheet to this area increased the risk of pushing the corrosion down into the leather surface. It was found that the detached corrosion particles did not present a significant risk of staining the adjacent leather unless they were to be pressed or smeared into the surface and removing them with a brush was effective.

Stabilization and Corrosion Inhibitors

Corrosion inhibitors have been used and continue to be used to stabilize objects with active corrosion and are more commonly used on copper alloys than on silver allow objects. There are a variety of corrosion inhibitors that have been used, though the most utilized is benzotriazole (BTA). It can be applied by immersion or by brush as a solution in ethanol. There is more data and research on immersion treatments, though more research on brush application and localized treatments are needed (Graham 2021). BTA has also been used in conjunction with 5-Amino-2 Mercapto-1, 3, 4-Thiadiazole (AMT) with the thought that the combination of corrosion inhibitors produces a synergistic effect and is more effective at stabilizing corrosion pits where the pH may interfere with the efficacy of BTA alone (De Alarcon 2013).

====Structural treatments====
Humidification, reshaping, removal of deteriorated previous structural repairs, structural fills, joining, mending, etc.

====Aesthetic reintegration ====
Loss compensation, fills, casting, molding, re-touching, finishing, etc.

====Surface treatments====
Polishing, coatings, etc.

====Other treatments====
If appropriate: This section heading should be used if the treatment being discussed does not fit into any of the other heading categories.

References[edit | edit source]

Use this section for: works cited, bibliography, including websites, external links, etc. Please list references used within the text in alphabetical order, following the JAIC style guide.

Anderson, Gretchen, and Carolyn Riccardelli. "Microclimate storage for metals (and other humidity-sensitive collections): Practical solutions." (2009).

De Alarcon, Tessa. "A Comparative Study of Corrosion Inhibitors for the Treatment of Archaeological Copper and Copper Alloys." 2013.

Golfomitsou, S., and J. F. Merkel. "Synergistic effects of corrosion inhibitors for copper and copper alloy archaeological artefacts." Metal 2004: proceedings of the International Conference on Metals Conservation: Canberra, Australia, 4-8 October 2004. 2004.

Paterakis, Alice Boccia, and Laramie Hickey-Friedman. "Stabilization of iron artifacts from Kaman-Kalehöyük: a comparison of chemical and environmental methods." Studies in conservation 56.3 (2011): 179-190.

Scott, David A. Metallography and microstructure in ancient and historic metals. Getty publications, 1992.

Further reading[edit | edit source]

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