Metals

Copper and copper alloys

Copper, a lustrous red-brown metal, is thought to be the first metal commonly used by humans. When alloyed with other elements, it provides a range of useful materials of widely different mechanical and corrosion-resistant properties.

The two main categories of alloys are:

• those created when copper combines with zinc, to form brasses; and

• those created when copper combines with tin, which are known as bronzes.

Corrosion of copper

The types of corrosion products formed on copper and its alloys depend on the environment and the metal's composition.

The most common corrosion products are copper oxides, basic copper sulphates and basic copper carbonates. These are generally stable, and protect the underlying metal from further corrosion.

The corrosion products are sometimes produced artificially—to give the attractive green-brown patina seen on outdoor bronze statues.

The passivating layers of copper corrosion products tend to break down in the presence of chlorides. Whether the chlorides are derived from the sea or from ground water, the overall impact—accelerated corrosion—is the same.

In a humid environment, the presence of chlorides in copper alloys can cause the development of the cyclic corrosion phenomenon known as bronze disease. This type of corrosion is characterised by the presence of a light blue-green, crumbly outgrowth on the surface. If this is brushed away, a pit will be evident on the surface. To determine whether an object is affected by bronze disease, consult a conservator.

It is important to note the difference between bronze disease and a natural patina. Many bronzes are formulated specifically to obtain a certain coloured patina. If you are not sure, consult a conservator before attempting any treatment. This is particularly important for Japanese and Chinese bronzes, and for bronzes from the Renaissance period onwards, as the patinas of these objects are intrinsic to the objects and should not be removed.

What causes the corrosion of copper?

Constant high humidity, pollutants such as sulphide gases, acids and careless handling causing physical damage can all cause deterioration of copper-based objects. In addition, heating and acidic cleaning solutions can etch the zinc out of brasses—leaving a copper-red discolouration on the surface.

Copper and copper alloys which have been buried or recovered from a wet site can suffer from the effects of chloride salts, resulting in the development of bronze disease.

Objects may also have special surface coatings, such as lacquers, which can be damaged easily by scratching or improper cleaning.

Items which are in good condition need only be wiped with a dry cloth.

Alcohol, preferably ethanol, may be used to remove greasy stains, but only after spot-testing to ensure there are no surface coatings that will be affected by this solvent.

If a badly tarnished copper alloy must be cleaned, immerse it in a solution of 10g of thiourea and 50g of citric acid in 1 litre of distilled water. This treatment will clean the object and remove the corrosion products.

The thiourea is an inhibitor which prevents any chemical attack on the metal itself. If thiourea is not used in the treatment solution, dissolved copper will be redeposited on the surface of the object, leaving a salmon-pink blush on the surface. This then has to be removed by polishing.

Leave the object in the solution until it is clean. This can take from several minutes to several hours, depending on the condition of the object.

You can speed up the treatment by brushing the surface, under running water, with a soft bristle brush, for example, toothbrush or soft Nylon, bristle brush. If necessary, a fine pumice powder can be used as a mild abrasive.

If this solution is not effective in removing tarnish, then the amounts of citric acid and thiourea used in solution can be increased to up to twice the strength.

The object must then be thoroughly washed to remove all traces of acid. This can be done under running water, or by immersion in baths of clean water.

If the object has been soaking for a considerable time or is porous, immerse it in a weak sodium carbonate bath—5g in 1 litre of water—in order to neutralise any remaining acid.

To ensure that washing has been effective, the pH of the surface of the metal object and that of the wash-water should be checked. The pH should be very close to 7, that is, close to neutral.

Bronze coins before treatment. Photograph courtesy of the Western Australian Museum

Bronze coins after citric acid/thiourea treatment. Photograph courtesy of the Western Australian Museum

After washing, and before applying any protective coating, it is essential to make sure that the surface is free from grease and water. Any such contamination will show up later as corrosion areas.

Wear rubber gloves when cleaning and coating the artefacts.

If oven-drying at 100⁰C is not appropriate, the object can be dried by painting it liberally with acetone. The acetone drives out water in crevices and cracks, ensuring that the metal is fully dry.

The metal is considered dry when there is no longer any smell of acetone.

It is not appropriate to use acetone on composite materials or painted surfaces. To dry composite objects and painted objects, use a hair drier on a cold setting.

Note that this cleaning procedure will not produce a bright, shiny surface finish to the metal. If this is required, a proprietary metal polish can be used. Repeated polishing should be avoided, however, because it wears the metal surface.

Coating copper and copper alloys

A protective coating can be applied, to maintain a clean, shiny surface on copper-based objects.

Lacquers and waxes are commonly used for this purpose. The patinas—excluding bronze disease—that form on bronze and copper objects are attractive and stable, and do not need a protective coating unless they are in a harsh environment.

Do not get dirt or fingerprints on the object between the cleaning and coating stages—any such contamination shows up later in the form of corrosion areas.

For coating, apply a microcrystalline-polyethylene wax preparation. An alternative coating is an acrylic lacquer containing a corrosion inhibitor. This is most satisfactory in preventing retarnishing of bronze and copper. Incralac is a commercially available lacquer which is available in a spray can or as a brush-on paint. This product can be removed with acetone.