Metals

Iron and its alloys

Iron is the most useful and abundant of metals and it is probably the most common metal found in the collections of local museums and historical societies. It has been known from prehistoric times and in its various forms—such as cast iron, wrought iron and various steels—it is the element upon which our present industrialised civilisation has been built.

Deterioration of iron and steel

Iron and steel, with the exception of stainless steel and other similar alloys, are readily attacked by oxygen when in the presence of moisture, forming rust.

Rust is a term used to describe non-specific corrosion products which form on the surface of degraded iron.

Unlike copper, the surface layers of iron corrosion products are not protective. They tend to accelerate corrosion of the metal by forming localised corrosion cells.

When an iron object is acquired, it should first be examined to determine the extent of deterioration and whether the corrosion is still active. If the surface is covered with yellow to brown droplets of moisture, it is a sign of severe corrosion activity, and indicates the presence of chloride salts. This necessitates a specialised conservation treatment to remove chloride ions. Please consult a conservator for more information about this treatment.

Many objects are covered with thick scales of rust—but there is often sound metal underneath.

Treatment of iron objects

Its future role, as either an object on display or in storage, will have a major impact on the treatment method.

To display an object in its working order, it may be that nothing needs to be done other than keeping it in a dry environment or coating it with an appropriate protective layer.

As with every metal type, there is a range of treatment options available; and the final decision will depend on the balance between aesthetics, economics and the function of the artefact.

Cleaning iron objects

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 or mechanical techniques.

Chemical cleaning techniques include:

• using soapy solutions to dissolve grease and remove surface dirt; and

• immersing the object in an aqueous alkaline solution—caustic soda—to remove grease and paint. Concentrations in the range of 20–40g of sodium hydroxide per litre of water are normally used.

• stripping corrosion products by immersing the objects in a solution of 50g of citric acid in water. While citric acid is relatively safe on most objects, care should be taken to ensure that cast iron, cast steel or spring steel, or combinations of these, are not left unattended for long periods, because these metals will actively corrode. Prolonged gas evolution—which you will see as bubbling—indicates that the iron surface is corroding. With harder alloys, this can also cause hydrogen embrittlement in which the hydrogen is generated within the metal and the stress of the gas pressure cracks the metal. Gas evolution can also result in pitted, weakened or destroyed objects.

Japanese .32 calibre pistol before treatment. Photograph courtesy of the Western Australian Museum

Pistol after treatment with citric acid/heat, fish oil/white spirit solution. Photograph courtesy of the Western Australian Museum

Because some mechanical cleaning techniques can be quite severe, they should be used very carefully—especially with small or fragile objects.

Techniques which can be used with care include:

• simple wire brushing. This is often very effective in removing loose or flaking rust. Wire brushes are available in a range of bristle materials, for example, steel and brass, and grades, for example, coarse to fine, so take care to select one appropriate for your object;

• sandblasting. This technique uses a high-speed jet of sand, and is usually applied to large iron or steel objects, such as agricultural implements. It is quick and provides an excellent surface for long-life coatings. You need approval from local authorities to sandblast because of the associated dust problem. 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. Sandblasting should be carried out by commercial operators, with conservators close at hand to provide any necessary advice; and

• flame cleaning. A blow torch or an oxyacetylene flame is used to quickly and effectively remove paint and rust.

The techniques described above can be combined. For example, flame cleaning and wire brushing could be used to remove large areas of rust and paint. This could then be followed by immersion in citric acid for the removal of residual rust, and neutralisation of the citric acid by immersion in a caustic bath.

If in doubt regarding the type of iron or the duration of acid treatment, more time should be spent 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 forms of corrosion leave spots on the object, which cannot be removed by citric acid. These can usually be picked off mechanically.

It is important to note that an object which has very little metal remaining should not be cleaned, but should be stored in a plastic bag containing self-indicating silica gel to keep it dry. Prior to storage, it is desirable to totally dry out the object in an oven at 110C, to remove moisture from fissures deep within the metal. This improves the effectiveness of the silica gel.

Large steel objects

When treating large objects, it can be difficult to find containers or tubs large enough to immerse the objects for caustic or citric treatments. If sandblasting is inappropriate, the acid or alkaline stripping solution can be applied to the surface by using a bentonite paste.

A note of caution about treating composite materials

As many objects are made up of a combination of different metals and/or wood, different treatments must be applied. For example:

• if an object is made up of iron and aluminium, cleaning in caustic should not be carried out, because the caustic will react adversely with the aluminium; and

• similarly, a composite of iron and brass in a citric solution will result in the copper from the brass corrosion products plating out on the iron, causing the iron to corrode more quickly.

Finishing techniques for iron objects

There are many varied methods available to give the object the right colour and protective coating. The type of finish chosen depends on the intended role of the finished object, with the final decision being a balance between ethical, aesthetic, practical and personal considerations. The most commonly used techniques are described below.

Tannic acid

Rust converters are commercial products which can be applied to an object which has been cleaned chemically or mechanically, or to one which still has light rusting present on its surface. A rust converter will form a thin, black layer on the surface. Subsequent applications further darken the object.

This coating system results in the formation by chemical reaction of stable iron tannates, which passivate the metal and protect it from further corrosion.

Fish oil/white spirit mixture

A mixture of 80 parts white spirit to 20 parts fish oil can be applied very effectively to freshly cleaned iron objects. Thinning the mixture with white spirit allows it to soak into the steel. It usually dries within minutes. Several coats can be applied and, when dry, it can be painted if required. This mixture does not change the metal colour, and gives good protection. It does not involve chemical reaction with the metal.

Oil quenching

Oil quenching is an old blacksmiths' method which gives good protection from rust. The end result is a deep blue-black object. This method works best on low carbon steel, because flaking occurs in small patches on high carbon or alloy steels. The method involves heating the metal, by either an oxyacetylene torch or forge, to a dull red colour, then plunging the object into old, dirty oil-the dirtier the oil, the blacker the final colour. The object is agitated for 30–60 seconds, depending on its size, removed from the oil and then wiped with a rag. Repeated applications of this method will further darken the object. Protective clothing and eye protection must be worn.

Blueing

Blueing is a method applied to many types of firearms, especially to their barrels, to produce a lustrous, dark blue finish. Although this is usually done by commercial gunsmiths, a blueing paste is available commercially, and can be applied easily.

After treatment, take care when you're handling the treated object, because acids from hands readily etch these surfaces.

Paint

There are numerous enamel and water-based paints, which protect and beautify metal surfaces. A range of primers, undercoats and topcoats are available. If these are applied correctly to properly prepared and cleaned surfaces, adequate protection should be maintained for many years.

Inorganic zinc primer

For iron objects that are displayed outdoors-particularly in an aggressive, marine environment-an inorganic zinc primer, a high-build expoxy top coat and a final clear polyurethane coating-with a UV absorbing reagent provide good protection.

Lacquer

Clear lacquers are available in spray cans, or can be applied by brush. The desired surface finish and colour must be obtained on the iron before you apply the lacquer. Follow the manufacturer's instructions on the product. This finish gives a lasting protection against oxide build-up, provided the entire object is coated. If air or moisture penetrates this layer, it will lift and the corrosion will begin again.

Microcrystalline wax

Microcrystalline wax provides both good protection and finish to an object. The object is immersed in a hot liquid-wax solution for up to several hours, depending on the size of the object. It is then withdrawn and allowed to cool. Excess wax can be scraped or wiped off. The wax can also be mixed with white spirit and applied with a brush or rag. This latter method is not as effective as immersion in the molten wax.

Flame colouring

The colour of iron and steel can be altered by the application of direct heat from a forge or an oxyacetylene flame. After cleaning and removing rust, a gentle flame is applied to the object. The colour changes from light straw through to deep blue. When the object has attained the desired colour, it should be plunged into water.

Natural patina

If an iron object is in a stable condition, with only a lightly rusted surface, it may be that this 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/or display conditions are controlled to prevent further deterioration.

Storing and displaying iron and iron alloys

Although the general handling, storage and display guidelines apply to iron and its alloys, a few additional points need to be made.

Once an object has been treated and coated, correct storage or display conditions and careful monitoring should ensure its stability.

Large objects such as machinery and horse-drawn and motor vehicles should be housed, if not in controlled environments, at least with some protection from the elements. This protection can be in the form of a shed, a verandah or even a lean-to. Unless some protection is provided, moisture and dust accumulation will soon restart deterioration processes.

If an object is displayed in the open, it should be:

• raised and supported above ground level;

• monitored regularly for signs of deterioration; and

• treated as necessary.

Metal components of firearms should be cleaned and re-oiled frequently, and if possible kept in a protective environment.

As iron is one of the most reactive of the commonly used metals, good environmental control is essential to ensure continued stability of these materials. The preferred relative humidity is less than 45%.

Highly polished metal surfaces which are not protected by a clear lacquer can be coated with a light machine oil, a periodic spray of CRC or similar water-dispersant chemicals. As long as they are stored in a dust-free environment, this is a simple and effective means of preventing deterioration.