Metals

Metals have played a significant part in human and technological development. This is reflected in the fact that two major periods in human history have been named after metals: the Bronze Age and the Iron Age.

Metals have been used—either in pure form or combined with other metals—to produce an enormous variety of objects, including weapons, tools, machinery, decorative art objects and jewellery. Because of the huge range of uses for metals, objects containing metal, or fully made from metals, are all around us. They make up a large part of many collections housed in museums, galleries and even some libraries.

But despite their apparent solidity, metals do corrode and can be vulnerable to physical damage. It is important that those responsible for the care of metal and metal-containing objects are able to recognise problems when they see them, and take steps to halt them.

The atoms which make up metals are bonded in a way that is peculiar to metals. A metal becomes a solid at a certain temperature; and the atoms of the metal settle into a characteristic, well ordered configuration. In this configuration the atoms are fixed rigidly in relation to each other. The configuration is called a crystal lattice or a crystal.

Some of the basic qualities of metals include the following:

• they are good conductors of heat and electricity;
• high reflectivity when their surfaces are smooth;
• usually good ductility, that is, they are capable of being drawn out into wire or threads;
• good malleability, that is, they can be extended or shaped by hammering or by pressure with rollers; and
• mechanical strength.

Alloys

For centuries, the properties of metals—such as their appearance, strength, malleability and chemical reactivity—have been altered by combining them. These combinations are called alloys. For example, iron combined with carbon produces alloys such as cast iron and steel; whereas the alloying of chromium and nickel with iron forms stainless steel.

Similarly, copper can be combined with zinc, to form brass; and with tin, to form bronze.

Patina

The patina is a film of metal corrosion products which forms on the surface of an object as a result of:

Corrosion also occurs if dissimilar alloys and metals come into contact with each other. This type of corrosion is called galvanic corrosion.

Under these circumstances, the more reactive metal or alloy corrodes while the less reactive metal is protected. For example, if iron and copper were in direct physical contact in the presence of moisture and oxygen, then the iron would selectively corrode while simultaneously protecting the copper.

This problem can be overcome by avoiding direct contact between dissimilar metals. This will not be simple if the metals are components of the same object.

The oxide film which forms on particular metals acts as an insulating barrier or passivating layer barrier, slowing the rate of corrosion to an acceptable level. This type of passivation occurs with copper and aluminium.

When looking at the way in which artefacts have corroded and when deciding on management strategies, it is important to ask these basic questions: is it necessary to treat an object? or can the causes of deterioration be controlled?

For more information

For more information on the effects of humidity, dust and pollutants, please refer to Damage and Decay.

When you are handling metal objects, particularly ones with polished surfaces, always wear clean cotton gloves or surgical gloves. This prevents the transfer of sweat and fats from the skin to the metal object, and helps reduce the risk of corrosion.

Always give your objects adequate support, and remember that metals can weaken over time.

Don’t lift metal objects by their handles. The joint between the handle and the object could be weak.

When displaying a hinged object open, take care to support it—so that the hinges are not carrying the weight of a part or all of the object.

Because some of the major contributors to the deterioration of metals are oxygen, water and air- borne pollutants, it is important to provide an environment which offers protection against these factors. This action can prolong the life of your metal objects.

Simple steps can greatly improve the longevity of objects. Steps such as:

• wrapping objects in unbuffered, acid-free tissue;

• placing them in acid-free boxes; and

• storing them on painted—preferably baked enamel—metal shelving.

CAUTION:

Avoid chipboard or wood cabinets. These materials give off formaldehyde and organic acid vapours, which can accelerate corrosion.

Silver’s lustrous appearance and relatively low natural abundance, its corrosion resistance and ability to be easily worked have made it a prized metal. Silver is often used for coins, jewellery and cutlery.

Most silver items found in Australia will be either sterling silver or plated silver.

Sterling silver

Sterling silver is the standard alloy used in jewellery and cutlery. It is made up of 92.5% silver and 7.5% copper. The addition of copper to silver increases hardness of the alloy, without any significant loss of lustre or colour.

Plated silver

The two common forms of plated silver are Sheffield plate and silver plate/electroplate.

Sheffield plate is made by fusion-bonding— sweating sterling silver to both sides of a copper or brass sheet; it is then worked to produce the desired object.

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 for electroplated nickel or silver, or EPBM for electroplated Britannia metal.

As commercial electroplating was developed in the 1840s, it is likely that a lot of the materials in the collections of local museums in Australia will be made of silver plate.

Signs of corrosion on silver

Silver artefacts tarnish if they are not kept polished. The silver surface changes through a faint purplish hue to a deep black. The tarnish is a layer of silver sulphide.

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.

CAUTION:

Do not use hydrochloric acid and phosphoric acid, because they will attack the underlying metal.

Iron may be plated with zinc, as in galvanised iron or tin, copper, chromium or nickel. These coatings protect the base iron sheet from corroding, and also provide a bright surface finish.

Corrosion of plated iron

Corrosion usually occurs after the breakdown of the surface plate. This exposes the iron, which then starts to rust.

Treatment of plated iron

To remove the rust, a citric acid solution containing an inhibitor can be used. The inhibitor is included to prevent any attack on the plating metal.

The solution is 10g of thiourea and 50g of citric acid in 1 litre of water. Thiourea is the inhibitor.

Test the solution on an inconspicuous area of the object, or on a scrap piece of the same material, before proceeding with the treatment.

Following removal of the rust, dry the object by dipping it in three successive acetone baths. At this stage, or earlier if there are no rust problems, the plate can be cleaned with industrial methylated spirits. Corroded areas can be removed with a mild abrasive such as pumice powder in methylated spirits.

CAUTION:

Check your chemical safety data sheets, and take the appropriate precautions.

Coating plated iron

If a bright surface finish is required after cleaning, a proprietary metal cleaner can be used as a once- only polish. The artefact can then be coated with a clear, acrylic lacquer.

Lead is a soft, grey metal, used mainly in combination with tin to form pewter. Because of toxicity problems associated with the use of pewter food vessels, the lead in pewter was replaced in the 19th century with antimony and some copper. Modern leadless pewters are usually alloyed tin and Britannia metal.

Corrosion of lead and pewter

The main corrosion product on lead and pewter is white-grey basic lead carbonate. This provides a deep, protective patina to the metal surface, which should not be removed.

If the pewter and lead have been in a low-oxygen environment and exposed to sulphide compounds, a rich, lustrous, grey-black patina of metal sulphides remains on the surface. These minerals are stable and should also not be removed.

Lead and pewter are particularly susceptible to attack by acids given out by wood. Acetic acid combines with lead and pewter to form lead acetates.

Both tin and lead are very soft and are susceptible to denting and scratching.

Cleaning lead and pewter

The stable patinas which form on lead and its alloys, the white-grey lead carbonate and the dark lead sulphide, should not be removed because they form protective layers which prevent further corrosion. Other corrosion products may require treatment.

Because it is difficult to obtain very mild abrasives, it is generally recommended that abrasives not be used to clean these soft metals. If, however, the white bloom on the surface of these metals is thin, the deposit can be removed from the surface using a one-micron-grade alumina powder mixed into a slurry polish.

If the white, bloom—an acetate layer—is pustular or thick, it is best removed by chemical or electrochemical reduction. These techniques are best left to conservators.

Thin layers of corrosion products can be removed by soaking the objects in a solution of 50g of disodium ethylenediamine tetraacetic acid in 1 litre of water. Avoid prolonged soaking because the dissolved oxygen can cause increased corrosion.

General cleaning can be carried out with warm water and a pure soap. Then rinse the object with fresh water, wipe it with methylated spirits, and polish it with a soft cloth.

Tin is a soft, white metal which is found in essentially pure form in some objects such as plates; but it is more commonly seen alloyed with lead in the form of pewter.

In the 19th and 20th centuries, tin was combined with a number of other elements to produce a range of alloys which were used principally for utensils and ornamental ware.

Typical examples are:

• Britannia metal which is 93% tin, 5% antimony, 2% copper. It 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, because of its lead content, was undesirable as a food container. Although the new alloy was brighter and stronger, it eventually lost favour as a metal for the production of household utensils; and

• leadless pewter which is alloyed tin.

Corrosion of tin

Although it is normally quite stable, tin reacts slowly with the atmosphere to form grey, stannous oxide and finally stable, white to grey-black stannic oxide.

Many museum objects made of tin or its alloys are covered with a dull, grey coating of corrosion products. These form a protective patina. Unless very pronounced or unsightly, this patina should be retained.

Cleaning tin

Although tin objects are quite strong, careless handling will still damage their surfaces.

If an object must be cleaned, a pure soap in warm water can be used to remove dirt and grime. This should be followed by rinsing it with fresh water, wiping it with methylated spirits and then polishing it with a soft cloth.

Ask the advice of a conservator before treating badly deteriorated objects.

Most aluminium objects found in museum collections are 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, an oxide layer forms on the surface and protects it against further corrosion. Therefore, under normal environmental conditions the metal does not corrode to any great extent.

Deterioration of aluminium

The principal function of electroplating is to make a cheaper metal look like silver. The physical properties of the materials are dominated by the underlying parent metal or alloy.

When an object is being electroplated, it becomes part of an electrolytic cell, as if it were part of a battery. The object is the negative electrode—that is—the cathode. The anode—or positive electrode—is usually made of pure silver. During the electroplating process, the object is placed in a solution containing silver salts—for example silver cyanide—and a direct, electrical current is passed through the object. As this happens, the object becomes coated with a layer of pure silver. At the anode, the silver is oxidised to produce silver ions, which replace the silver in the solution.

If an inert anode such as stainless steel or platinum is used, the bath would need regular replenishment of the silver salts, to keep the same operating conditions in the plating bath.

If woven charcoal cloth or sintered zinc oxide pellets are not available or easy to obtain, then zinc carbonate can be used as a simple and very effective treatment against the adverse effects of sulphur pollution.

Sheets of acid-free blotting paper are immersed in a bath of a soluble zinc salt such as 10g of zinc sulphate in 1 litre of water.

Once the blotter is wet, a solution of 20g of sodium carbonate in 1 litre of water is poured into the bath, producing a white, cloudy solution of zinc carbonate.

The blotter is removed from the bath, and dried under pressure—to prevent cockling.

When dry, it can be placed underneath textile coverings in the base of a display case, or rolled up and placed in a support beneath a raised display platform within the display case.

This simple treatment has prevented the tarnishing of silver objects for a period of six years in a display case which has not been opened to release pollutant build-ups.

The addition of varying amounts of zinc—Zn—to copper—Cu—produces a wide range of industrial brasses of differing physical and mechanical properties. These include:

• gilding metal. The addition of only 5% zinc produces this alloy, which is commonly used as an artificial gold in decorative uses;

• red brass: 85% copper/15% zinc. In this alloy, the underlying red colour of copper is still present;

• yellow brass: 65% copper/35% zinc. The addition of more zinc hardens the alloy and changes the colour. This alloy is used for a wide range of industrial purposes, such as hinges, taps and valves; and

• muntz metal: 60% copper/40% zinc. This alloy has a variety of uses, including sheathing for wooden sailing vessels.

In order to improve the ease of machining, varying amounts of lead are added to the hard brass alloys. Brasses containing more than 5% lead are self- lubricating, a very important factor for bearings and other similar objects.

When copper is alloyed with tin as the major additional component, different types of bronzes are formed. Bronzes can have significant differences in reactivity towards oxygen. The mechanical strength of the bronze normally increases with the addition of more tin, but the alloys become increasingly more brittle. Some of these alloys include:

• bell metal. With 20–25% tin, this is very strong, but very susceptible to cracking if struck with hard and sharp instruments;
• leaded bronzes: 80% copper/10% tin/10% lead. These are very robust and can be extensively cold-worked;
• statuary bronzes: 65–85% copper/10–30% zinc/2.5–5% tin. These are commonly used for casting; and
• china silver is an alloy of copper, tin, nickel and silver with 65% copper/20% tin/13% nickel/2% silver.

The most important alloying element for iron is carbon—because it combines to form a diverse range of alloys, including wrought iron through to steels and cast iron.

In cast iron, carbon can exist as discrete phases or minute areas of graphite, in a variety of physical forms. Because of the differences in the hardness and chemical reactivity of the various phases, steels and cast iron are subject to localised corrosion—one phase is selectively corroded while another is protected. This is a form of internal galvanic corrosion.

The addition of other metals such as nickel and chromium result in the wide range of stainless steels, which are hard and chemically durable alloys. These alloys corrode to form protective coatings of chromium oxide/nickel oxide, and transform iron into a much less reactive metal with a much wider range of uses.

Adding elements such as molybdenum further improves the corrosion-resistance properties of the stainless steel alloys in chloride solutions.

Bentonite paste is made by sprinkling bentonite powder into a prepared solution of the alkali or acid, and mixing it into a paste. The concentrations of acid and alkali in the paste are usually higher than if a corresponding solution was being used.

For example, an 8% solution—that is, 80g per litre—of alkali and a 10% solution—that is, 100g per litre—of citric acid are recommended. The paste can be applied directly to the area to be treated.

If the surface is not smooth, residues of the paste can become clogged, making it awkward to remove. To make removal easier, first place a piece of water-dampened tissue paper over the treatment area, and apply the paste on top. It is preferable to cover the poultice of paste with cling wrap, to prevent it drying out.

Repeated applications of the paste may be required. The paste can be removed by hosing the surface with water and scrubbing it with a bristle brush. Then dry the object fully.

Bentonite paste treatment is also recommended if solder joints or related fastenings are present, because these are also readily attacked by citric acid.

The presence of chlorides in aluminium alloys containing copper is a problem because chloride ions:

• penetrate the protective oxide coating;
• cause aluminium pitting corrosion; and
• promote localised copper corrosion from within the alloy.

As the copper corrosion products move to the surface, they interact with the aluminium corrosion products and form a blue-green, copper-stained aluminium hydroxide corrosion matrix.

The real problem occurs when—as a result of electrochemical reduction by the parent metal— copper is redeposited in metallic form on the surface of the alloy. The redeposited copper acts as a cathode in a pitting corrosion cell.

The conservation problem is to remove a relatively unreactive metal deposit from the surface of a very reactive metal, while at the same time trying to remove the chloride ions.

One simple solution to the problem is to use a solution of ammonia and ammonium sulphate to wash away the chlorides and the metallic copper from the surface. This produces complex reactions, but is effective.

When a corroded sea plane float was treated this way, it took 12 months of steady soaking, scrubbing and cleaning to stabilise the corroded metal.

Spot-tests are used to distinguish different metals which make up an alloy. Simple instructions and a list of the tests are provided, to help you identify metal objects in your collections. It is important to note that these tests are only qualitative in nature—they will not tell you the relative amounts of the different metals in an alloy.

General instructions

Brown, B.F., Burnett, H.C., Thomas Chase, W., Goodway, M., Kruger, J., Pourbaix M., eds. 1977, Corrosion and Metal Artefacts—A Dialogue between Conservators and Archaeologists and Corrosion Scientists, NBS Special Publication 479, U.S. Department of Commerce/National Bureau of Standards, Washington, D.C.

MacLeod, I. D., 1981, ‘Bronze Disease: An Electrochemical Explanation’, ICCM Bulletin, VII, ICCM Inc, Canberra, pp 16–26.

Stambolov, T., 1985, The Corrosion and Conservation of Metallic Antiquities and Works of Art, CL Publication, Central Research Laboratory for Objects of Art and Science, Amsterdam.

Scott, David A., 1991, Metallography and Microstructure of Ancient and Historic Metals, The Getty Conservation Institute, The J. Paul Getty Museum in association with Archetype Books, Marina del Rey, California.

Scott, D.A., Podany, J., Considine, B.W., eds. 1994, Ancient and Historic Metals: Conservation and Scientific Research, Proceedings of a Symposium organised by the J. Paul Getty Museum and the Getty Conservation Institute, November 1991, The Getty Conservation Institute, Marina del Rey, California.

Question 1.

Select the incorrect statement from the following:

a)  damaged Sheffield plate may be repaired by electroplating;

b)  moisture and oxygen enhance corrosion;

c)  a protective oxide layer forms on the surface of copper objects;

d)  microcrystalline wax gives good corrosion protection for iron objects.

Question 2.

Gloves should be worn when handling metal objects so that:

a)  oils, fats and sweat are not transferred to the object;

b)  the object is less likely to slip from your grip;

c)  protective lacquers are not damaged by nails;

d)  your hands are not affected by toxic corrosion inhibitors.

Question 3.

Silver cleaning should be carried out only when absolutely necessary because:

a)  silver dip solutions are very expensive;

b)  any cleaning solutions remove small amounts of silver;

c)  cleaning solutions tend to accumulate in indentations and surface cracks;

d)  evidence of historic usage may be lost.

Question 4.

Select the correct statement from the following:

a)  copper is more susceptible to bronze disease than is brass;

b)  bronze disease only occurs in objects recovered from the sea;

Answer: a).

Answer: a).

Answer: b).