Copper alloy

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Objects Specialty Group Conservation Wiki
Contributors: Tessa De Alarcon, Casey Oehler, Robin Ohern.
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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

History

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

Materials

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

Technology

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

Identification

The identification of metals is often general without additional analytical analysis to confirm the metal and its alloying components. Conservators generally categorize objects as copper alloy (a generic term) unless it is possible to make a more specific identification through analysis. The appearance of copper alloys is variable depending on the alloying elements, and for archaeological copper alloys, the burial environment. This can make identification based 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 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.

Deterioration

The type and stability of the corrosion is dependent on the metal, its alloying components and its environment. Stable corrosion is often referred to as a patina rather than as corrosion, and may be left in place during treatment.

Copper alloys in a good state of preservation often have formed a copper oxide corrosion layer which preserves the shape and form of the original object. The stable layer frequently consists of a cuprite layer with secondary copper oxide layer such as malachite. However, in unfavorable environments, particularly in the presence of chlorides, 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 precursor and as a result can cause lifted surfaces and structural instability in the object. This unstable corrosion process is sometimes called "bronze disease," and can result in cyclical damage. Gilt or silvered copper alloy surfaces may be particularly compromised due to active corrosion of the underlying copper alloy.

“Bronze disease is a progressive deterioration of ancient copper alloys caused by the existence of cuprous chloride (nantokite) in close proximity to whatever metallic surface may remain. Cuprous chloride may lie dormant until reaction with moisture and oxygen causes this unstable compound to expand in volume on conversion to one of the copper trihydroxychlorides. This creates physical stress within the object affected, resulting in cracking or fragmentation.”

While studies of bronze disease date back over 100 years (with early work by Rathgen and Berthelot), the reactions between copper corrosion products are thermodynamically complex and not yet completely understood.

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

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 find a conservator, please visit AIC's Find a Conservator page.

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

Documentation

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.

Conservators should always try to document and preserve surface information embedded in copper alloy corrosion products, such as pseudomorphs indicating contact with other materials (e.g. textiles or wood).

Preventive conservation

The electrochemical corrosion process requires two things: an electrolyte (usually water) and oxygen. If either of those things are removed, metals corrosion will not occur. One method for addressing active corrosion is therefore to construct a microclimate, or a climate-controlled storage container, in which either the relative humidity has been significantly reduced or oxygen has been removed. There are numerous ways to construct a microclimate. Some popular methods utilize a sealable plastic container or a bag constructed of a vapor-barrier film such as Escal, with slica gel used to engineer low-humidity microenvironments or an oxygen scavenger, either used alone or in addition to silica gel, used to create an anoxic environment .

Interventive treatments

A wide range of treatment approaches are used for dealing with copper alloy artifacts. A small, informal survey of archaeological conservators undertaken in Summer 2015 investigated current practices of professionals working in the field. The summary of results also reveals areas where additional research and perhaps training is necessary.

Historically many different treatments have been undertaken to preserve copper alloys. In recent decades, treatment of copper alloys has typically included one or more of the following steps: (1) mechanical cleaning, usually aided by (2) water or solvents, (3) desalination through soaking in deionized water, (4) treatment with corrosion inhibitors such as benzotriazole, and (5) coating with synthetic resins to provide a barrier layer to protect the object from handling. Other chemical, electrochemical and electrolytic methods have also been used to clean copper alloys, through these techniques can be quite damaging and may aggressively strip away surface patina if not performed in a controlled manner.

Reduction of corrosion products

Mechanical cleaning is currently the preferred method for removing corrosion and involves the use of scalpels, dental scalers, and skewers while the object is 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 materials. It is now less commonly used, as mechanical cleaning has become the preferred method. For electrolytic reduction, 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 plating or gilding, loss of original surface leaving areas that appear pitted or spongy, and loss of information that exists only in corrosion layers. 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 copper plated.

Many collections have objects that have been cleaned using a variety of cleaning methods that are no longer preferred. It is important to be aware of these treatment histories because they 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.

Corrosion Inhibitors

Benzotriazole

Benzotriazole (BTA) has been extensively used as a copper corrosion inhibitor but questions remain about its efficacy. It is usually utilized via immersion in an aqueous or ethanol solution in low concentrations, sometimes with the aid of a vacuum desiccator (this protocol seems to have been in use since at least the 1970s at the Institute of Archaeology, London) , though it can also be brush-applied. BTA molecules are thought to complex with CuCl preventing further bronze disease outbreaks from occurring. However, in many cases the chloride corrosion is not stabilized by one treatment of BTA, and other methods may be required to stabilize problematic objects. Other corrosion inhibitors such as cysteine and AMT (5-amino-2-mercapto-1,3,4-thiadiazole) have also been tested, but BTA remains the most commonly used (see the 2015 Copper Alloy Treatment Survey). There is currently more data and research on immersion treatments: 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.

Coatings and Adhesives

Metals may need to be reconstructed after cleaning and initial study. Metals are often coated to prevent damage from handling, and the coating additionally consolidates the surface in cases in which corrosion has caused loss of structural or surface integrity. Today, stable, non-yellowing, and reversible acrylic resins such as Paraloid B72, B44 or B48N [Rohm and Haas] are commonly chosen for this purpose, and these same resins are used in thicker concentrations as an adhesive for joining metal fragments.

====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

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Golfomitsou, S., and J. Merkel. 2007. Understanding the efficiency of combined inhibitors for the treatment of corroded copper artefacts. METAL 07 Proceedings of the Interim Meeting of the ICOM-CC Metal Working Group (5): 38-43.

Leyssens, K., A. Adriaens, C. Degrigny. 2005. Electrochemical monitoring of the storage or stabilization of archaeological copper based artifacts in sodium sesquicarbonate solutions. Proceedings of ICOM-CC 14th Triennial meeting. 301-309.

Merk, L. 1978. A study of reagents used in the stripping of bronzes. Studies in Conservation (23): 15-22.

Scott, D. 1990. Bronze Disease: A Review of Some Chemical Problems and the Role of Relative Humidity. Journal of the American Institute for Conservation (29):193-206.

Scott, D. 2002. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Los Angeles, Getty Conservation Institute.

Sharma V.C., U.S. Lal, and M.V. Nair. 1995. Zinc dust treatment-- an effective method for the control of bronze disease on excavated objects. Studies in Conservation (40): 110-119.

Weisser, T. 1987. The Use of Sodium Carbonate as a pretreatment for difficult-to-stabilise bronzes. Recent advances in the conservation and analysis of artifacts. Jubilee conservation conference, London 6-10 July 1987. University of London, Institute of Archaeology, Summer Schools Press. 105-108.

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