S. R. Garcia, D. Gilroy and I. D. MacLeod
The significance of metals to human society is so great that two of the major eras of prehistory are described in terms of the Bronze Age and the subsequent Iron Age. The earliest metallic artefacts were beaten from naturally occurring copper metal and date back more than 6000 years.
The development of western and eastern societies has been closely linked with the working of metals. These materials have been used, either in pure form or when combined with other metals, to produce an enormous variety of objects including weapons, tools and decorative art objects. Consequently, metal objects make up a large part of many collections kept in museums, historical societies and private households.
Properties of metals, such as appearance, strength, malleability and chemical reactivity can be altered by alloying with other metals or by the presence of other impurities. Iron for example, can be alloyed with carbon to form cast iron or with chromium and nickel to form stainless steel. Copper is often combined with zinc to form brass and with tin to form bronze.
Distinctive colours were often prized and as a result the compositions of metals were manipulated to obtain the desired effect. Chinese bronzes and Japanese sword guards are examples of objects where colour is an integral part of the object.
The patina is a film of metal corrosion products which forms on the surface of an object, either as a result of exposure to the elements or because of deliberate artistic intervention (Figure 1).
Before any decision is made to remove the patina from an object the nature of the object and its history must be considered very carefully. For instance, under no circumstances should the patina be removed from an ancient bronze sculpture. On the other hand, as Victorian era silver candelabra were usually kept in a highly polished state, accumulated tarnish should be removed.
Do not confuse the patina with a lumpy, pustular or uneven corrosion mound that may lie above the original surface of an object. If in any doubt, avoid any treatments that will remove these minerals until professional advice has been obtained.
The tarnishing or corrosion of metals is the major issue of concern confronting those responsible for the care of metal items.
Gold, silver, platinum, mercury and copper are the only metals stable enough to be found in their natural metallic state, with only small amounts of the latter four elements being found. All other metals are more stable when combined with other elements to form oxides, sulphides, chlorides, carbonates and other compounds. There is therefore, a natural tendency for most refined metals to return to their more stable ‘corroded’ states. As a consequence, metal objects need to be protected from environmental conditions and pollutants which encourage corrosion.
In some cases as the metal corrodes the oxide film that forms acts as an ‘insulating’ barrier, slowing the rate of corrosion to an acceptable level. Copper and aluminium are two such metals in which oxide coatings form protective, passivating layers. When iron corrodes however, it does not form a protective film. The corrosion of iron will continue until no metal is left unless action is taken to protect it from the elements.
The main causes of deterioration of metals include:
- exposure to conditions of high relative humidity;
- the presence of oxygen;
- exposure to salty conditions;
- reaction with pollutants;
- contact with other metals;
- stresses induced during manufacturing processes; and
- careless handling.
Moisture and oxygen are required for corrosion (oxidation) of metals to occur. Chloride ions, present in common salt (sodium chloride), may originate from human or animal contact (perspiration) or from airborne sea salts. They can speed up the corrosion rate and are capable of penetrating protective oxide layers on metals such as copper and aluminium to form metal chlorides. These latter materials do not form a coherent surface layer, effectively removing the corrosion resistance of the metal. Oils and sweat from people’s hands are potential agents of decay, with the organic acids and chloride ions transferred upon contact between skin and metal, capable of causing significant attack on metallic surfaces.
Any agent that affects protective films on metals may enhance corrosion of the underlying metal. Physical damage to painted or coated surfaces, or the uneven application of oil coatings for example, will alter the access of oxygen to the metal surfaces. This usually causes one part of the object to corrode at the expense of another.
Normally unreactive metals such as copper and silver can suffer significant corrosion if sulphide-containing species are in the same environment as the metal. Sulphide pollutants are usually associated with the breakdown of plant matter and the decomposition of sulphur-containing proteins such as wool. Common pollutants include hydrogen sulphide and carbonyl sulphide.
Inorganic acids such as hydrochloric acid derived from the decay of plastics such as polyvinyl chloride and nitric and sulphuric acids formed from air pollutants and moisture, will attack metals that are in the same storage environment or in the open air. Outdoor monuments and sculptures are particularly vulnerable to this type of attack.
Corrosion will also occur if dissimilar alloys and metals come into contact with each other. This type of corrosion is known as galvanic corrosion. In this situation the more reactive metal or alloy corrodes while the less reactive metal is protected. If for example iron and copper were in direct physical contact, then in the presence of moisture and oxygen, the iron would corrode preferentially while simultaneously protecting the copper from these agents of corrosion. A galvanic series, which lists metals in increasing order of reactivity is provided elsewhere (see Appendix 8). Many problems associated with galvanic corrosion can be overcome by avoiding direct contact between dissimilar metals.
Corrosion may be increased by stresses imparted to metals during their manufacture. Changes in the microstructure of metals, produced by processes such as hammering, drawing and rolling, usually encourage corrosion and in doing so reduce the service life of affected metals.
Handle metal objects carefully as it is easy to over-estimate their strength and toughness. Metal objects can be easily dented, bent or mis-shaped if handled inappropriately, resulting in unnecessary damage that may be difficult to repair.
It is important to remember these basic issues when examining corroded artefacts or when assessing storage environments. Removing chloride salts, restoring oxide coatings, repatinating metals with aesthetically pleasing patinas, coating metals with protective materials such as lacquers and paints and modifying environmental storage and display conditions are all effective tools in the conservation battle against corrosion.
While it is generally acknowledged that relative humidity levels in storage and display areas should be kept below 45 % for most metals and below 35% for most iron objects, the presence of particular corrosion products will affect recommended levels. For example relative humidity levels of 15 % or less must be maintained to prevent corrosion of chloride infested iron and levels of 12 % or less are necessary to prevent corrosion of iron that is in contact with mixtures of the corrosion products Akageneite (β-FeOOH) and hydrated ferrous chloride (FeCl2.4H2O, Watkinson and Lewis 2004).
While high light levels are not damaging to metals, a maximum level of 300 lux is usually recommended so that other more sensitive objects in the vicinity are not adversely affected. Techniques to control the relative humidity and light levels are described earlier (see the chapter Preventive Conservation: Agents of Decay).
Storage, Display and Handling
Since the major agents of deterioration for metals are oxygen, water and airborne pollutants, any storage system which minimises their effects will extend the life of a metal. Simple steps such as storing objects wrapped in acid-free tissue, in acid-free boxes, on painted, preferably baked enamel or powder coated metal shelving will increase their longevity. If more than one metal object is stored in a box, use acid-free padding to separate objects and prevent abrasion. Steps such as these will help avoid both environmental attack and galvanic corrosion.
Never place objects directly on shelves or in drawers. Line these with polyethylene foam (e.g. Ethafoam) to minimise vibrations and abrasion. If metals are stored on open shelving, in addition to wrapping objects, use drapes made from washed cotton, polyethylene or Tyvek to further reduce exposure to dust.
Avoid storage in or near cabinets made of chipboard or wood as these materials give off formaldehyde gas and organic acids which can accelerate corrosion. If there is no option but to store metal objects in cabinets made of these materials, take the following steps:
- coat wood with a water-based polyurethane finish to seal the wood; and
- paint chipboard with a solution of urea (400 g) in water (1 L) and then seal with a water-based polyurethane finish.
Do not seal objects in plastic bags which may trap moisture and increase corrosion. Avoid polyvinyl chloride (PVC) bags in particular as they can give off hydrogen chloride, an acidic gas which will attack most metals.
When handling metal objects wear well-fitting plastic or white cotton gloves. This will prevent the transfer of sweat and fats from the skin to the metal object, minimising potential corrosion problems. Wash cotton gloves regularly to avoid the build-up of salts and organic acids. Do not handle silver objects with latex rubber gloves as there is an increased risk of tarnishing the silver.
The lustrous appearance, relatively low natural abundance, corrosion resistance and ability to be easily worked has ensured that silver has always been a prized metal, often being used for coinage and jewellery. Most silver objects in collections of former British colonial countries will be sterling silver or plated silver.
Sterling silver is the standard alloy used in jewellery and cutlery and is made up of silver (92.5 %) and copper (7.5 %). The addition of copper to silver increases the hardness of the alloy without any significant loss of lustre or colour.
There are two common forms of plated silver, Sheffield Plate and silver plate or electroplate. Silver plate or electroplate is formed when a thin layer of pure or sterling silver is deposited electrolytically on the surface of a base metal. Common base metals include copper, brass, nickel silver (an alloy of copper, zinc and nickel) and Britannia metal (a tin alloy with 5 – 10 % antimony). Electroplated materials are often stamped EPNS or EPBM for electroplated nickel silver or electroplated Britannia metal respectively. As commercial electroplating was developed in the 1840s it is likely that many artefacts in Australian collections will be made of silver plate.
Sheffield plate is a duplex alloy where sterling silver has been fusion bonded (‘sweated’) to both sides of a copper sheet. It is subsequently worked to produce the desired object.
Unless kept polished, silver artefacts will usually tarnish, forming a layer of black silver sulphide. Artefacts excavated from underground or from the sea may be coated with grey silver chloride and blue-green copper corrosion products (Figure 2).
With age and polishing, silver may be removed from the high spots of plated silver, exposing the base metal which often looks duller than the silver layer. Even if the gentlest polishing techniques are used, all electroplated silver ultimately will show pinpricks of corrosion as the plating wears thin. Once the plating has been perforated the underlying metal is prone to pitting corrosion and gradually the surface will become covered with a blotchy black and green-blue corrosion matrix.
Apply the guidelines for safely storing and displaying metals described in the general introduction to this chapter.
If there is no option but to place an object in a display case previously shown to be corrosive towards silver, add acid-free blotter impregnated with either zinc oxide or zinc carbonate, to the base of the case. This will minimise tarnishing by absorbing acidic and sulphurous compounds.
Zinc carbonate-impregnated blotting paper can be prepared as follows:
- in a flat tray, prepare a solution which contains a soluble zinc salt such as zinc sulphate, by dissolving 10 g of the salt in 1 L of water;
- immerse a sheet of acid-free blotting paper in the solution;
- once the blotter is wet, pour a solution of sodium carbonate (20 g in 1 L of water) into the bath, producing a white cloudy solution of zinc carbonate; and
- remove the blotter from the bath and allow it to dry under glass to prevent puckering.
When dry, place the blotter underneath textile coverings in the base of a display or storage cabinet, or rolled up and placed in a support underneath a raised platform within a display cabinet. This is a very effective means of minimising corrosion.
Alternatives to blotting paper are commercially available sintered zinc oxide pellets or sachets of multi-metal vapour phase corrosion inhibitors.
Only clean silver when absolutely necessary and not as a routine treatment, as any cleaning will remove minute amounts of silver. If a piece is in good condition, maintain it in that condition rather than continually cleaning it and wearing down the silver coating. Wiping with a silver cloth, followed by storage and display under low relative humidity conditions will provide the simplest method of protection. If cleaning is necessary, avoid abrasive cleaners as they can cause fine scratching of the surface and will remove small amounts of silver.
Take care to differentiate between tarnish and decorative treatments which are an intricate part of the object and which would be destroyed by cleaning. An example of such a decorative treatment that should not be removed is niello, a black silver sulphide deliberately used to highlight incised sections of silver jewellery and silver ornaments.
Many commercially available ‘silver dip’ solutions readily remove tarnish. These are usually made up of thiourea and acid mixtures. Only use silver dips when the object is badly disfigured and apply them with care. Never use silver dip on objects with decorative treatments such as niello or on composite objects with bronze, stainless steel or organic components. Also avoid the use of silver dips on objects that have hollowed components as there is a danger of the liquid entering any cavities. If using a silver dip, only apply or dip the solution for as long as it takes to remove the tarnish. It is safer to use a cotton bud (or similar) to apply the dip to corroded areas. Following treatment with the dip, rinse the object in hot water to remove residues and then dry it with a lint-free cloth. After drying, use a silver cloth to apply a thin layer of corrosion inhibitors to the surface of the silver.
Any kind of cleaning may expose areas of fire scale (copper oxide) that is sometimes formed during manufacture. These marks can discolour the surface and are very difficult to remove.
If Sheffield plate is in reasonable condition, just wipe it with a silver cloth and display or store it under conditions of low relative humidity. Do not replate the object as it completely devalues it by removing the technological evidence of its manufacture.
Electroplate or silverplate objects in good condition, but on which the silver has been worn away to reveal underlying metal, can be restored by using commercially available solutions which gradually deposit small amounts of silver on worn areas. This is a better way to rejuvenate the surface than standard electroplating. The latter process is a lot more costly and is not always successful.
Electroplate which shows signs of blotchy black and blue-green pitting corrosion will not usually respond to treatment with silver polishing cloths, silver dips and cleaning foams. If confronted with materials of this type, remove the gross corrosion by using a solution made up of thiourea and citric acid. This treatment will remove much of the silver corrosion since thiourea is a very strong complexing agent that also dissolves silver sulphides. To prepare and use this solution:
- dissolve thiourea (10 g) and citric acid (50 g) in water (1 L);
- immerse the object in the solution and gently brush the corrosion areas with a soft brush;
- after cleaning, place the object in a bath which contains sodium carbonate (10 g) in water (1 L). Leave the object to soak for about one hour to neutralise and remove any citric acid from beneath the electroplate; and
- remove any residual sodium carbonate by a final wash in deionised or distilled water.
If the corrosion damage is not too severe then touch up the underlying metal with one of the commercially available silver solutions that gradually redeposit small amounts of silver. Only use this treatment for electroplate or silverplate after carefully considering the history of the object. Polish with a silver cloth to complete the cleaning process.
There are commercially available lacquers for coating silver objects that slow tarnishing processes significantly. On the downside, these lacquers can be troublesome to remove if they break down. In addition, unless an even coating is applied, a patchy and blotchy tarnish may develop on the surface.
Most 19th and 20th century nickel silver objects will be found either as the unchanged copper-nickel-zinc alloy or with a thin film of electrodeposited silver on the surface. A simple spot test may be used to distinguish these materials (Appendix 9).
One very easy way to recognise the presence of nickel, either as nickel plating or its presence in an alloy, is to look for the bright lemon-green corrosion products which characterise nickel (II) compounds. The colour is a much more lemon-yellow-green than the characteristic blue-green of copper.
In Australia only coins minted before the introduction of decimal coinage (1966) contain any significant amounts of silver. To remove tarnish from coins containing silver, follow the procedures specifically outlined for silver.
For coinage and other nickel silver objects, treat corrosion products according to the methods outlined below for copper and its alloys (brass and bronze).
To treat electroplated nickel silver, follow the guidelines for silver objects.
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.
Iron and Alloys
Iron is the most useful and abundant metal and would probably be the most common metal type found in collections. It has been known from prehistoric times and in its various forms, such as cast iron, wrought iron and various steels, is the element upon which our present industrialised civilisation has been built.
In the presence of oxygen and moisture iron and steel (except some stainless steels) corrode to form rust. Rust is a term used to describe non-specific corrosion products which form on the surface of degraded iron (Figure 4). It is a mixture of ferrous and ferric hydroxides and hydrated ferric oxide. Iron chlorides usually form in the presence of aggressive salts such as sea water.
Unlike copper, the surface layers of iron corrosion products are not protective and tend to accelerate corrosion of the metal by forming localised corrosion cells.
When an object is first acquired, examine it to determine the extent of deterioration and to ascertain whether the corrosion is still active. If the surface is covered with brown droplets, active corrosion is still occurring and the presence of salt is indicated. This necessitates a specialised conservation treatment which involves the removal of chloride ions.
Although the general guidelines outlined in the introductory section of this chapter also apply to iron and its alloys, a few additional points need to be made. As iron is one of the most reactive of the commonly used metals, effective control of the storage environment is essential to ensure objects made from iron do not continue to deteriorate. To minimise corrosion the preferred relative humidity for storage and display of iron objects is less than 35 %. As noted earlier in this chapter however, the presence of particular corrosion products will affect recommended relative humidity levels for iron objects. Relative humidity levels of 15 % or less must be maintained to prevent corrosion of chloride-containing iron and levels of 12 % or less are necessary to prevent corrosion of iron that is in contact with mixtures of the corrosion products Akageneite (β-FeOOH) and hydrated ferrous chloride (FeCl2.4H2O, Watkinson and Lewis 2004). Details of methods used to control relative humidity are described elsewhere (see the chapter Preventive Conservation: Agents of Decay).
Once an object has been treated and coated, correct storage or display conditions and careful monitoring will maintain its stability.
If it is not possible to house large objects such as machinery and vehicles (horse-drawn and motor) in controlled environments at least provide some protection from the elements. This protection may be in the form of a shed, a veranda or even a lean-to. Unless some protection is provided moisture and dust accumulation will soon initiate deterioration processes.
If an object is displayed in the open, raise and support it above ground level, monitor it regularly for signs of deterioration and treat as necessary.
Clean and re-oil metal components frequently, particularly those of firearms (see the chapter Case Studies) and keep them in a protective environment if possible.
Coat highly polished metal surfaces that are not protected by a clear lacquer with a light machine oil or spray them periodically with a commercial product like CRC or similar. As long as they are stored in a dust-free environment this is a simple and effective means of preventing deterioration.
Although many objects may be covered with thick scales of rust there is often sound metal underneath. Do not clean an object that has very little metal remaining, but instead store it in a plastic bag containing silica gel (desiccant) to keep it dry. Note the following points:
- self-indicating silica gel is orange when active, changing to either colourless, pale yellow or green when no longer acting as a desiccant (colour change depends on the type of silica gel used);
- replace the desiccant when it changes colour;
- regenerate the activity of the silica gel by heating it in an oven (105 - 110 °C) until the orange colour returns; and
- before storing it, totally dry the object. If appropriate this may be done by placing it in an oven at 110 °C for three to four hours. Alternatively a brush down or soak in acetone or methylated spirits will assist moisture removal. The effectiveness of the silica gel is improved by removing moisture from fissures deep within the metal.
The future role of the object, either display or storage, will affect the choice of treatment method. If the purpose is to display an object in its working mode perhaps no treatment may be necessary other than maintenance, to keep it in a dry environment and/or coat it with an appropriate protective layer.
As with every metal type there is a range of treatment options available, with the final decision depending on the balance between aesthetics, economics and the proposed way in which the object will be used (for example, static display or as a functioning component).
Dirt, grease and loose or flaking rust must be removed before protective coatings can be applied to iron objects. Such deposits can be removed by chemical, mechanical and/or thermal techniques.
Chemical cleaning techniques include:
- using detergent solutions to dissolve grease and remove surface dirt;
- immersing the object in an aqueous alkaline solution (caustic soda) to remove grease and paint. Concentrations in the range of 20 - 40 g sodium hydroxide per litre of water are normally used for this purpose; and
- stripping corrosion products by immersion in a solution of citric acid (50 g) in water (1 L). Do not use this solution on spring steel however. Do not use acids such as hydrochloric and phosphoric acids as they attack the underlying metal.
As the effect of some mechanical and thermal cleaning techniques can be severe, be careful when using this option, especially with small or fragile objects. Techniques which may be applied include:
- wire brushing – either manual or rotary power driven, depending on the condition of the object;
- abrasives or sandblasting; and
- flame cleaning.
Wire brushing is often very effective in removing loose or flaking rust. As wire brushes are available in a range of bristle materials (steel or brass) and grades (coarse to fine), select the one appropriate for the condition of a particular object. Never use a brass brush on an iron object however as a thin layer of brass can build up on the surface being brushed and this, along with any entrapped shedding bristles, will increase the risk of galvanic corrosion.
Abrasives or sandblasting, which uses a high speed jet of particles, may be applied to either small or large iron and steel objects. It is especially suited to large objects, such as agricultural machinery. This method is quick and produces an excellent surface for long-life coatings. Owing to the associated airborne dust problem sandblasting usually requires approval from local authorities. The use of an enclosed abrasive blasting cabinet for smaller objects will avoid this issue.
An alternative form of sandblasting, wet sandblasting, uses a suspension of sand in water combined with a corrosion inhibitor. This method causes less pollution and is more acceptable to local authorities. In both cases the work should be done by commercial operators with conservators close at hand to monitor the process and ensure damage is avoided. On drying and to prevent flash rusting, apply a protective coating immediately following wet sandblasting.
Flame cleaning involves the use of a blowtorch or an oxyacetylene flame to remove paint and rust quickly and effectively. Apply this technique very carefully as thin section metal is likely to distort rapidly and spring steel will lose its ‘temper’ and ‘spring’ if overheated. Wear eye protection as there is a risk of injury as rust particles fly off rapidly with this method.
All of the techniques described above can be used in conjunction with each other.
A typical cleaning scenario involving a steel object may proceed as follows:
- use a steel wire brush to remove as much loose rust or flaking paint as possible;
- immerse the object in a caustic soda solution to remove grease, old paint and dirt. This may require immersion for some hours. Occasionally remove the object, wash it with water, scrub it and replace it in the caustic solution;
- rinse the object under running water;
- transfer the object to a tub containing citric acid. Dissolution of rust may take several hours. Periodically remove, inspect and brush 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, often requiring extensive polishing;
- when the rust has dissolved, place the object in a caustic soda solution for a few minutes. This neutralises excess citric acid which would otherwise cause further rusting;
- thoroughly rinse the object with water and then immediately rinse it with acetone, methylated spirits or dewatering fluid to prevent further rusting. Brush these liquids onto the surface and allow the object to dry; and
- apply a protective surface coat to the object.
While citric acid is relatively safe to use on most objects, do not leave cast iron, cast steel and spring steel, or combinations of these metals unattended for long periods in a citric acid bath as they will corrode. Bubbles rising to the surface of the bath indicate that the surfaces of these iron materials are corroding. In the case of harder alloys, this can lead to hydrogen embrittlement and result in pitted, weakened or destroyed objects. Iron objects that are sprung can spontaneously break as they weaken during this corrosion process.
If in doubt regarding the type of iron or steel or the duration of acid treatment required, spend more time removing corrosion products by mechanical means. Once the worst deposits are removed, a short treatment in citric acid should clean the object with reduced risk of damage. Some spots of rust may remain on the object despite citric acid treatment. These can be picked off mechanically.
Large Steel Objects
In many cases it is impossible to find containers or tubs large enough to immerse an object for caustic or citric treatment. In such situations the acid or alkaline stripping solution can be applied to the surface by using a bentonite paste. Higher concentrations of chemicals are needed when a paste is used.
Bentonite paste is prepared and applied in the following way:
- prepare solutions of caustic soda (80 g/L of water) and citric acid (100 g/L of water);
- sprinkle and mix enough bentonite powder into each of the prepared solutions to make spreadable pastes;
- apply the pastes directly to the area to be treated in the order described immediately above;
- if the surface is not smooth, place a water-dampened tissue over the treatment area first and then apply the paste. This will minimise ‘clogging’ of surface indentations;
- if possible cover the paste layer with plastic film to prevent it drying out. Repeated applications of the paste may be required;
- remove the paste by hosing the surface with water and scrubbing with a bristle brush; and
- dry the object as described previously.
Avoid breathing caustic vapours and wear eye and skin protection when preparing and using caustic soda solutions. If large quantities of caustic solutions are used they must be disposed of according to government regulations.
Consult a conservator for advice before treatment as composite objects made up of different metals or metal and organic components require specialised treatments. For instance, if an object is made up of iron and aluminium or iron and zinc, do not clean it in a caustic solution as that solution will react with the aluminium and the zinc.
Likewise treatment of a composite of iron and brass in a citric acid solution will result in the copper from the brass corrosion products ‘plating’ out on the iron and accelerating the iron corrosion. Suspension and partial immersion of the iron portion only in the citric bath, or the localised use of bentonite paste (see above), may allow such a problem to be overcome. The selective application of a protective layer of wax may also be used to isolate and protect the surfaces that would otherwise be damaged by the citric acid solution.
Bentonite paste treatment is recommended if solder joints or related fastenings are present as these are also attacked readily by citric acid.
Iron may be plated with other metals including zinc (galvanised iron), tin, copper, chromium or nickel. These coatings protect the base iron sheet from corroding and also provide a bright surface finish. Corrosion usually occurs when the surface plate breaks down, exposing the iron which in turn begins to rust.
To remove rust, a citric acid solution containing an inhibitor can be used. The inhibitor is included to prevent any attack on the plating metal. Before the solution described below is used, test it on an inconspicuous area of the object or on a scrap piece of the same material. Prepare and apply the solution as follows:
- dissolve citric acid (50 g) and thiourea (10 g) in water (1 L);
- immerse the object in the solution, periodically remove it from the solution and brush it gently with a soft bristled brush;
- when the rust has been removed rinse the object thoroughly with fresh water followed by immediate rinses with either acetone, methylated spirits or dewatering fluid; and
- clean the plating metal with methylated spirits or, if there are rust stains present, with a mild abrasive such as pumice powder in methylated spirits.
After a final cleaning, if a bright surface finish is required, apply a proprietary metal cleaner as a once-only polish. The artefact may then be coated with a clear lacquer.
There are many methods available to give the object the ‘right’ colour and protective coating. The type of finish chosen will depend on the intended role of the finished object, with the final decision being a balance between aesthetic, practical and personal considerations. The most commonly used techniques include:
- tannic acid-based rust converters;
- fish oil;
- oil quenching;
- clear lacquers;
- microcrystalline wax;
- flame colouring; and
- natural patina.
Tannic acid-based rust converters are commercial products which can be applied to an object that has been cleaned either chemically, mechanically or which still has light rusting present on its surface. This product initially turns the object a brown-black colour. Subsequent applications darken it further until it is totally black. This coating system results in the formation of stable iron tannates which passivate the metal and protect it from further corrosion. Do not use this treatment on iron objects which will be displayed outdoors unless an additional protective coating is applied. To maintain the black appearance, tannic acid-based rust converters can be sealed with clear acrylic lacquers (see below for details).
A mixture of 80 parts white spirit to 20 parts fish oil can be applied to freshly cleaned iron objects with great effect. Thinning of the mixture with white spirit allows it to soak into micro-fissures and any light rust on the steel. It usually dries within minutes. Several coats can be applied and when dry the object can be painted if required. This mixture does not change the metal colour and gives effective protection.
Oil quenching is an old blacksmith’s method which provides effective protection from rust. The end result is a deep blue-black object. This method works best on low carbon steel as flaking occurs in small patches on high carbon or alloy steels. Wear protective clothing and eye protection to avoid injury. Steps in this method are as follows:
- heat the iron object to a dull red colour using either an oxyacetylene torch or a forge and then plunge it into old, dirty engine oil (the dirtier the oil, the blacker the final colour);
- agitate the object for 30 - 60 seconds (depending on size), remove it from the oil and then wipe it with a rag; and
- repeated applications of this method will darken the object further.
‘Blueing’ is a method that has been applied to many types of firearms, especially their barrels, to produce a lustrous dark blue finish. Although this is usually done by commercial gunsmiths, a ‘blueing’ paste is available from gunsmiths and is easily applied. Take care when handling ‘blued’ objects as acids from skin contact etch these surfaces.
There are numerous paints available, both enamel and water-based, that protect and beautify metal surfaces. A range of primers, undercoats and topcoats may be used. If applied correctly to properly prepared and cleaned surfaces adequate protection should be maintained for many years. For iron objects displayed outdoors, particularly in aggressive marine environments, use an inorganic zinc primer, a high build epoxy top coat and a final clear polyurethane coating with a UV-absorbing agent in the paint.
Clear lacquers are available in spray cans or can be applied by brush. Obtain the desired surface finish and colour before applying the lacquer according to the instructions on the product. Provided the entire object is coated, this finish gives a lasting protection against oxide build-up. If air or moisture penetrate beneath this layer however, lifting will occur and oxidation will recommence.
Vinyl co-polymer based Senson Ferroguard FS (Full Spectrum) with vapour phase corrosion inhibitor is also an appropriate sealer for bare metals but being a less durable coating is only suitable for objects that are infrequently handled such as those in museum collections.
Microcrystalline wax provides both thorough protection and an attractive finish to iron objects. Its preparation and application are described below:
- mix the wax with white spirit until it forms a smooth paste;
- apply using a soft rag; and
- allow the wax to harden and then gently buff the surface to the desired finish.
Microcrystalline waxes are available commercially for a range of metals and timbers, each formulated for a particular use. Use white spirit to remove the wax at any time.
Flame colouring involves alteration of the colour of iron and steel by applying direct heat from a forge or an oxyacetylene flame. After cleaning and de-rusting, apply a gentle flame to the object. The colour will change from light straw through to deep blue. When the object has attained the desired colour, plunge it into water. Note that this method can change the molecular structure of the steel depending on the grade and its carbon content. As the temper or spring will be lost if flame coloured, do not use this method on spring steel.
If an iron object is in a stable condition, with only a lightly rusted surface, it may be that this natural patina is the type of finish you require to demonstrate the history and past use of the object. Such finishes can be maintained if the storage and display conditions are controlled to prevent further corrosion.
Lead and Pewter
Lead is a soft grey metal that easily conforms to desired shapes and surfaces. In the past it was used for water storage and transmission through pipes. In combination with tin it forms pewter. Due to toxicity problems associated with the use of pewter food vessels the lead in pewter was replaced in the 18th century with antimony and some copper. Modern ‘leadless pewters’ are usually alloyed tin (Britannia metal).
Lead and pewter are prone to attack from acetic acid and other organic acid vapours given off by poor quality papers, some fabrics and various woods (Figure 5). The main corrosion product formed on lead and pewter not affected by organic acid vapours is white-grey, basic lead carbonate. This provides a deep, protective patina to the metal surface and should not be removed.
If pewter and lead have been in a low oxygen environment and exposed to sulphide compounds, a rich lustrous grey-black patina of metal sulphides forms on the surface. These minerals are stable and should not be removed.
Both tin and lead are very soft and susceptible to denting, tearing and scratching. Handle carefully.
Adhere to the general guidelines described in the introduction to this chapter.
As lead and pewter objects are particularly susceptible to attack by organic acids emanating from certain woods, storage or display in enamelled metal cupboards is very important. If silicon sealants are used to assemble glass display cabinets for example, then ensure they are neutral cure and not acetic curing types.
Do not remove the stable patinas that form on lead and its alloys, the white-grey lead carbonate and the dark lead sulphide, as they form a protective layer that prevents further corrosion. Other corrosion products may require treatment.
Carry out general cleaning in the following way:
- wash with warm water and a pure soap;
- rinse the object with fresh water. Do not soak in distilled water however as it will corrode lead;
- wipe with methylated spirits;
- polish with a soft cloth; and
- apply a protective surface coating of microcrystalline wax if necessary.
As it is difficult to obtain very mild abrasives, their use is not generally recommended on soft metals. If however, the white bloom that forms on the surface of these metals (lead or tin acetate) is thin, and one micron grade alumina powder is available, use a slurry of this alumina in water as a polish to remove the deposit from the surface.
If the white acetate layer is pustular or thick it is best removed by either electrochemical techniques or chemical reduction. These specialised techniques require the skills of a conservator.
Thin layers of corrosion products can be removed by soaking the objects in a solution of disodium ethylenediaminetetraacetic acid (EDTA, 50 g in 1 L of water). Avoid prolonged soaking as the dissolved oxygen in the solution will accelerate corrosion.
Tin is a soft white metal found in essentially pure form in some objects such as ingots but is more commonly encountered as plating on food cans. Tin is also a constituent of pewter when alloyed with lead.
In the 19th and 20th centuries tin was combined with a number of other elements to produce a range of alloys used principally for utensils and ornamental ware. Typical examples are Britannia metal (93% tin, 5% antimony, 2% copper) and leadless pewter (alloyed tin).
Britannia metal was developed in England during the mid-1700s in response to the threat to the pewter utensil industry from cheap porcelain. Old pewter was dull and undesirable as a food container because of its lead content. Although the new alloy was brighter and stronger it eventually lost favour as a metal for the production of household utensils.
Although it is normally quite stable, tin reacts slowly with the atmosphere to form grey stannous oxide and finally, stable white stannic oxide.
Many objects made of tin or its alloys are found covered with a dull grey coating of corrosion products. These form a protective patina and unless the corrosion is very pronounced or unsightly, retain this patina where possible.
Observe the general guidelines mentioned at the beginning of this chapter.
Although tin objects are quite strong, careless handling will still damage their surfaces. If an object has to be cleaned, try the following regime:
- use a pure soap in warm water to remove dirt and grime;
- rinse with fresh water;
- wipe with methylated spirits; and
- polish with a soft cloth.
For badly deteriorated objects, seek the advice of a conservator.
Most aluminium objects found in museum collections will be alloys containing copper as a minor component. The addition of only 3 % (by weight) of copper trebles the mechanical strength of the parent metal.
As aluminium corrodes the oxide layer that forms protects the surface against further corrosion. Under normal environmental conditions the metal does not corrode to any great extent.
If allowed to come into contact with metals such as copper and iron, or in the presence of chloride ions (such as found in sea water), aluminium and its alloys corrode appreciably.
Do not allow aluminium to come into contact with mercury as mercury prevents formation of the protective oxide patina, promoting rapid corrosion of the aluminium.
Apply the guidelines described in the general introduction to this chapter to aluminium and its alloys.
Clean aluminium with methylated spirits or a soft brush to remove dirt. Do not use any abrasives on aluminium as these may remove the protective oxide layer.
Never use caustic soda to remove grease or paint from aluminium objects as it reacts vigorously with aluminium. Remove heavy deposits of oil, grease and petroleum products, commonly encountered on vintage car parts, with kerosene or similar products.
If the metal is heavily stained or corroded, apply a solution of phosphoric acid (1 %). This will produce a mild uniform etch on the metal surface which, after thorough washing and drying, should be left for a day to enable the protective oxide film to reform through contact with the air.
Coat aluminium surfaces with a protective clear lacquer after cleaning. This form of protection is not usually needed unless the aluminium is likely to be affected by salt such as found at a seaside location.
Gold has been used from the earliest times. It is yellow, lustrous and the most malleable and ductile of the metals. As a rare metal, it is used extensively for jewellery and coinage.
Gold is often applied as a decorative surface coating in the form of gold leaf for manuscript illumination, as a top layer on prepared gesso and as gold amalgam for gilding copper and silver. It is also alloyed with copper and silver to improve its mechanical properties.
As gold is resistant to corrosion it will usually only require polishing with a soft cloth.
No coating is required on pure gold but if it is alloyed with copper or silver, apply a clear acrylic or nitrocellulose lacquer to protect against tarnishing.
- Identify the type of metal before attempting any cleaning.
- Only consider removing a patina after ascertaining that the surface of the object is not as originally intended and that the patina is not providing protection against further corrosion.
- Wear gloves when handling highly polished artefacts.
- Keep relative humidity below 45 % for most metals in good condition and below 35 % for iron and metals showing signs of corrosion.
- Observe maximum light levels of 300 lux for metal objects and lower levels as appropriate for composite objects with light sensitive components.
- Where possible, store metals in enamelled or powder-coated metal cupboards or acid-free boxes.
- Do not carry out any treatment if unsure. Contact a conservator for advice.
- Contact a conservator for advice regarding the treatment of composite objects.
Canadian Conservation Institute, CCI Notes, Ottawa, Canada.
- N9/1 Recognizing Active Corrosion (2007)
- N9/2 Storage of Metals (2007)
- N9/3 The Cleaning, Polishing and Protective Waxing of Brass and Copper (2007)
- N9/4 Basic Care of Coins, Medals and Medallic Art (2007)
- N9/5 Tannic Acid Coating for Rusted Iron Artifacts, formerly published under the title Tannic Acid Treatment (2013)
- N9/6 Care and Cleaning of Iron (2007) N9/7 Silver - Care and Tarnish Removal (2007)
- N9/8 Mechanical Removal of Rust from Machined Ferrous Surfaces (2007)
- N9/9 Care of Objects Made of Zinc (2007) N9/10 How to Determine Metal Density (2016)
- N9/11 How to Make and Use a Precipitated Calcium Carbonate Silver Polish (2016)
Department of Materials Conservation, Western Australian Museum, 1981, Metals, in Conservation and Restoration for Small Museums, 2nd Edition, Western Australian Museum, Perth, pp. 35-48.
Drayman-Weisser, T., 1992, Metal objects, in Caring for Your Collections, National Committee to Save America’s Cultural Collections, Arthur W. Schultz (Chairman), Harry N. Abrams Inc., New York, pp. 108-121.
Selwyn, L., 2004, Metals and Corrosion: A Handbook for the Conservation Professional, Ottawa, ON: Canadian Conservation Institute.
Turner-Walker, G., 2008, A Practical Guide to the Conservation of Metals, Wang Show-Lai, Council for Cultural Affairs, Taiwan.
Untracht, O., 1975, Metal Techniques for Craftsmen, Doubleday, Garden City, New York.
Watkinson, D. and Lewis, M., 2004, ss Great Britain iron hull: modelling corrosion to define storage relative humidity, in Metal 04: Proceedings of the International Conference on Metals Conservation, 4–8 October, Canberra, Australia. Canberra, Australia: National Museum of Australia, pp. 88–103.