Copper and Alloys
Copper, a lustrous red-brown metal is regarded as the first metal commonly used by man. It is often combined with other elements to produce a range of alloys with different mechanical properties and corrosion resistance. Things made from copper and its alloys are used in almost every facet of life and human activity.
The two main categories of alloys are those created when copper (Cu) combines with zinc (Zn) to form brasses and those with tin (Sn) which are known as bronzes.
Some typical brass compositions are:
65 % Cu
35 % Zn
85 % Cu
15 % Zn
60 % Cu
40 % Zn
95 % Cu
5 % Zn
Although the primary components of bronzes are copper and tin, secondary elements such as lead (Pb), nickel (Ni), zinc and even silver (Ag) and gold (Au) have been incorporated into these materials.
Some typical bronze compositions are:
75 - 80 % Cu
20 - 25 % Sn
80 % Cu
10 % Sn
10 % Pb
65 % Cu
20 % Sn
13 % Ni
2 % Ag
65 - 85 % Cu
10 - 30 % Zn
2.5 - 5 % Sn
Spelter bronzes, popular from the 1850s to the early 1900s, are not bronze at all but are a white, zinc-based metal to which various coatings have been applied to give the effect of patinated bronze. Any attempt to chemically clean these ‘bronzes’ renders them worthless.
Constant high relative humidity, pollutants such as sulphide gases, acids and careless handling causing physical damage can all result in the deterioration of copper-based objects. Heating and acidic cleaning solutions can etch the zinc from brasses, leaving a copper-red discolouration on the surface. Objects with special surface coatings, such as lacquers, can be damaged easily by scratching or improper cleaning.
The types of corrosion products formed on copper and its alloys depend on the environment and the metal 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 provide the attractive green-brown patina seen on outdoor bronze statues.
As noted in the general discussion on metal corrosion, 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 may lead to the development of the cyclic corrosion phenomenon known as bronze disease. Copper and copper alloys which have been buried or recovered from a wet site may suffer from this type of corrosion. Bronze disease corrosion is characterised by the presence of a light blue-green, powdery and eventually crumbly outgrowth on the surface (Figure 3). If this is brushed away a pit will be evident on the surface. The tests for and treatment of metals affected by bronze disease are described later in this chapter.
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 especially important for Chinese and Japanese 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.
The preventive conservation guidelines described in the general introduction to this chapter are applicable to copper-based objects.
Use a dry cloth to wipe copper-based objects that are in good condition. Use ethanol to remove greasy stains but only after spot testing to ensure there are no surface coatings that may be affected by this solvent.
If a badly tarnished copper alloy has to be cleaned, immerse it in a solution made up of citric acid and thiourea. Thiourea is an inhibitor which prevents 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.
Wear rubber or preferably nitrile gloves when cleaning and coating copper objects so that no marks are left on the surface of the metal.
A typical treatment regime is as follows:
- prepare a solution containing citric acid (50 g) and thiourea (10 g) in water (1 L);
- place the object in the solution and leave it until it is clean. This can take from several minutes to several hours depending on the condition of the object;
- fully immerse the object in the citric solution to prevent the development of ‘tide lines’ on the object as these are difficult to remove, requiring extensive polishing;
- brush with a soft bristle brush, such as a toothbrush, to speed up the process;
- after cleaning, wash the object thoroughly to remove all traces of acid. Immerse the object in baths of clean water (preferably distilled) or under running water; and
- if the object has been treated for a prolonged period or is porous, immerse it in a bath containing sodium carbonate (5 g) in water (1 L) to neutralize acid residues and then rinse it with clean water.
Ideally the pH of the metal surface and that of the wash water should be checked to ensure that the washing has been effective. Washing can be considered to be complete when the pH values of the wash solution and the metal surface are both neutral.
Note that this cleaning procedure will not produce a bright shiny surface finish on the metal. If this is required a proprietary metal polish can be used. Avoid repeated polishing however as it tends to wear the metal surface.
If the above solution is not effective in removing tarnish then the amounts of citric acid and thiourea can be increased (up to twice the strength). A fine pumice powder can be used as a mild abrasive if necessary.
After washing and before any protective coating is applied the metal surface must be dry and free from grease and dirt. Wear gloves so that fingerprints do not get on the object between the cleaning and coating stages. Any such contamination may show up later in the form of corrosion areas.
If oven drying at 100 °C is not appropriate or possible, dewater the object by ‘painting’ it liberally with acetone or methylated spirits. These organic solvents dissolve water in crevices and cracks, ensuring that the metal is dry. The metal is considered dry when there is no longer any smell of acetone or methylated spirits. This treatment must be carried out in a well ventilated area.
To determine if corrosion products on an object are derived from bronze disease, expose it to conditions of high relative humidity (greater than 85 %) for a period of about two weeks. For smaller objects this may be achieved by placing the object in a large clear plastic bag with about 100 ml of water. Do not let the object come into direct contact with the water. Seal the bag and inspect it after the two-week period. If droplets of green solution are observed on the surface of the metal or if new fluffy outgrowths of pustular green corrosion products are found, then bronze disease is indicated.
If the object is too large or too valuable for the above procedure, then take samples from the suspected bronze disease areas and test them (using X-ray diffraction) to determine the nature of the corrosion products. The presence of basic copper (II) chloride (atacamite or paratacamite) is indicative of bronze disease.
The main aim of the treatment is to remove the majority of the chlorides from affected objects. This is done most simply by immersing the object in a solution of sodium sesquicarbonate. The preparation of this solution and its application are as described below:
- remove any protective coating (wax, lacquer, resin) from the surface of the object before treatment;
- dissolve sodium carbonate (10 g) and sodium bicarbonate (10 g) in water (1 L);
- allow the object to soak in the solution for two to four months;
- prepare a fresh solution and immerse the object for a further four to six months;
- remove residual chemicals by immersing the object in baths of clean water;
- dewater the object using either acetone or methylated spirits; and
- soak or paint the object with benzotriazole (BTA, 3 g) dissolved in methylated spirits (100 ml). Wear gloves and avoid breathing dust from BTA as it is a potential carcinogen.
If the object originally had a bright metal surface this treatment will produce a green-brown patina. This patina is quite attractive and stable. If a clean metal surface is desired, remove the patina after treatment using the citric acid stripping process described earlier in this chapter.
This method will be effective for all cases of bronze disease but treatment times will vary greatly from object to object. The treatment times given above are sufficient for most cases. If an object subsequently shows signs of renewed bronze disease, repeat the process.
As artefacts made of essentially pure copper normally do not contain significant amounts of chloride ions, one long wash of about six months is usually sufficient to stabilise them. Brass and bronze objects however, need at least two washes and possibly a third one if they are extensively corroded.
Roseate blue crystals can form on the surface of the objects that have been previously stripped of corrosion products in a thiourea-inhibited, citric acid solution and are then treated with sesquicarbonate solution. These crystals can be removed by soaking the artefacts in an alkaline Rochelle salt solution (20 g sodium hydroxide and 50 g sodium potassium tartrate in 1 L of water).
If the desalination treatment outlined above is not practical, stabilise artefacts by maintaining the relative humidity below 25 %. This slows the corrosion rate to an acceptably low level. Other treatment options include excavation of pits and chemical treatments which involve the application of corrosion-inhibiting chemicals or waxes to seal out moisture.
Mineralised and heavily corroded copper-based objects may need to be impregnated with wax to consolidate them. Consult a conservator if this is thought necessary.
Apply a protective coating (lacquer or wax) to maintain a clean shiny surface on copper-based objects. Remember that the patinas (except bronze disease) which form on bronze and copper objects are attractive and stable and do not need any form of protective coating unless they are in a harsh environment. In the latter case, consult a conservator for advice.
Microcrystalline-polyethylene wax mixtures can be used as protective coatings. A recipe that has been found to be useful is comprised of:
|microcrystalline wax||100 g|
|polyethylene wax||25 g|
|white spirits||250 ml|
Prepare the paste as follows:
- melt the wax components together and stir well to ensure thorough mixing; and
- quickly pour the mixture into the white spirits and stir constantly while it cools to produce a smooth white paste.
The sheen of the resultant wax film can be altered by varying either the grades or the proportions of the waxes used. When it is dry the wax can be either polished for a shiny finish or left untouched for a matt finish. If subsequent bronze disease treatment is found to be necessary this wax can be removed with white spirit.
An alternative coating is an acrylic lacquer containing a corrosion inhibitor. This has proved most satisfactory in preventing re-tarnishing of bronze and copper. A commercially available lacquer (‘Incralac’) can be obtained either in a spray can or as a brush-on paint. If necessary this product can be removed with acetone.