Metal could appear in your museum collections in any number of forms. Anything from medieval weapons to technological innovations could be made predominantly from metal.
The group of metals contains any material that:
- Has a metallic lustre
- Is ductile
- Has a high density
- Conducts heat and electricity well
Whether your collections are numismatic, military or scientific, you probably have metal in your collection. It is essential to care appropriately for metals in your collections to protect them degradation and damage.
Metals may be more durable than textiles or other museum items, but displaying and storing them nevertheless comes with complications. Even the hardest metals can be physically damaged by inappropriate treatment.
Metal can corrode when exposed to unfavourable conditions. How the item undergoes this deterioration depends on the type of metal or alloy or if any protective measures are in place.
Most common metals develop a coating of metal oxide called the patina. This starts as a transparent layer and can become more visible as it thickens. Don't remove this regularly as it protects the metal from corrosion. It can even increase the value of a metal depending on its appearance.
Wear gloves when handling metals as finger marks can affect the shape and coverage of the patina.
Causes of corrosion
Corrosion processes are electrochemical. That means that electrical currents flowing through the metal cause chemical processes that cause deterioration. Prevent these processes by stymieing currents through metal items in your collections.
Moistures and salts stimulate corrosion, so keep metals in clean, dry conditions. Ideally, relative humidity would be less than 65% and items would be stored or displayed in dust-free environments.
Archaeological metals require extra levels of care as many arrive in collections having already experienced extensive corrosion. Burial in the soil exposes metals to salts and water over several centuries, causing significant deterioration.
Ensure very dry conditions for untreated archaeological items, with a relative humidity between 10 and 15%.
Humidity and pollutants are the two main threats to displaying metal - plan ahead to stop them from ever being an issue. Control the environments of metal displays and choose your materials carefully to avoid corrosion and deterioration.
Keep relative humidity low to prevent the build up moisture and, thus, corrosion in displays of metal items. Either control the overall environment in a display room or use sealed display cases to create tightly controlled microclimates.
It is often easier and more effective to control a small quantity of air such as the contents of a display case, especially if the case is sealed effectively. Use moisture-absorbing or buffering material such as silica gel or Art Sorb ™. Display cases also prevent unnecessary touching.
Reduce the amount of pollutants that could affect your metals by choosing display materials carefully. Every part of your display, including carpets, paints, text labels and backing fabrics could be the source of polluting particles or gases. These are particularly dangerous when they come into contact with lead.
Materials that are safe to use for display, which you can source from conservation suppliers, include:
- Metal, either uncoated, or with an enamel or painted finish
- Plastics such as polythene, polyester and Perspex (acrylic)
- Neoprene rubber seals
- Museum-quality, acid-free silicone sealants
- Inert foam materials such as Plastazote™
- Acid-free paper products
- Unbleached, undyed cotton and polyester fabrics, provided they have been scoured before use by putting them through a machine wash at 60ºC without detergent
- Materials that have passed the standard tests for museum use
Avoid displaying metals with materials that give off harmful vapours or can damage metals through contact, such as:
- Wood and wood products, unless sealed by barrier foil
- Acidic paper products
- Textiles, especially wool
- Products containing non-inert polymer foams, including carpets
- Most paints and varnishes, especially when freshly applied
Key preservation points
Many fabrics contain dyes or finishes that could contain pollutants. Test any fabrics intended for long-term use before incorporating them into your displays or buy pre-tested fabrics for use in museums. The British Museum offers a commercial testing service.
Avoid paints if possible, as they routinely fail museum tests. When paint is necessary, leave a long time for harmful vapours to dissipate.
Do not place organic and metal objects in the same display cases as deteriorating organic material can give off vapours that can damage metals. If items have to be displayed together, use absorbent material such as Charcoal Cloth™ or sulphide-absorbing textile to counteract any pollutants.
Some metals such as silver can be protected with lacquers or vapour-phase inhibitors to reduce corrosion. This is not suitable for all silver objects, so consult a conservator before applying such materials.
Always keep track and be aware of the risk of pollution within display or storage areas.
Storing for preservation
Storing with collections care in mind helps to preserve metal items for future generations and keeps your items in the best condition.
Take some simple steps towards storing your items well:
- Arrange items in stackable, protective boxes made from non-pollutant material.
- Large and awkwardly shaped items should be stored off the floor or covered in a museum-quality dust sheet.
- Label everything, including dust-sheets, for easy access to collections items.
- Monitor and control humidity in your store.
- Aim for well-sealed buildings to control air pollution as much as possible
- Install pollution-absorbing devices and seals
Methods for handling, treating and maintaining metal items should be adjusted according to the specifications of each metal item. One universal rule is to use a box, tray or other container wherever possible when transporting items between rooms. If necessary, complete a risk assessment and method statement to ensure that objects are transported safely and without risk to staff.
Handling and cleaning
- Always wear gloves when handling metal objects, as the contact with skin can lead to uneven finishes in metals.
- Fresh finger marks can be removed with a clean cloth and a small amount of white spirit.
- Never attempt to remove patinas but buff them with a clean, soft cloth if appropriate.
- Keep dusting to a minimum to avoid scratching the surfaces - use a clean and soft cotton cloth or a soft brush if dusting is required.
- Oil and grease on metals can lead to deterioration and soiling, as well as posing a risk when handling. Consult a conservator on removing such coatings.
- The gold plating in gilded objects is extremely vulnerable to damage and should not be polished.
It is important that metal items are securely packed in well-padded containers. In many cases, the containers used for the storage of the items will be suitable for this. Make sure that any container has appropriate handles and straps for safe carrying.
If you need to use special transport containers, ensure that items do not stay in these for long as they are less like to be made to museum conservation standards. Do not use them for storage without advice.
To protect archaeological metal items from high humidity levels during transport, airtight containers with enclosed dry silica gel should be used.
Many metal surfaces can be treated with lacquer, Renaissance™ wax or polish. Occasionally metal items with working parts will require a little oil or grease applied with a clean, soft cloth.
With all of these choices, the appropriateness of such treatment is entirely dependent on the object in discussion. Whether you want to remove a patina or make something shinier, you should always consult a conservator before any treatment is carried out.
Guide to metal types
Aluminium is highly chemically reactive, soft and silver-coloured. It is protected by a thin coast of aluminium oxide, which is hard and unreactive. Do not polish aluminium too much to remove this protective layer. It is too soft and weak for use as a pure metal and it is frequently alloyed with copper and/or magnesium, manganese, silicon and zinc, giving a wide range of properties for different uses.
Aluminium and its alloys are generally corrosion-resistant, but problems can arise in very corrosive environments. High-strength aluminium and copper alloys, usually used in aircrafts, can corrode when exposed to saltwater. This problem, which is characterised by the appearance of blue-green and white crystals, can be treated but laboratory facilities are required.
The structure of the metal could be entirely compromised by the time it reaches your museum, or the surface obscured by corrosion and soil. Store archaeological iron with silica gel and airtight display cases. Keep relative humidity stable and beneath 15%.
Display archaeological iron in similar environments, using conservation grade display cases to seal out pollutants and control the relative humidity. Use 10 to 20 kg of silica gel per m³ of enclosed space. Store the gel in drawers to make it easier to change. Consider using dehumidifying equipment for the display.
Very fragile metal objects should be supported adequately with inert materials. If necessary, a special mount that can be used for both display and storage purposes should be made for the item and kept with it at all times.
In situations where it is not possible to use display cases (perhaps because the items are too large, such as industrial machinery), it is important that other physical barriers are used to prevent the public from touching the items. In these situations, the control of the ambient environmental conditions is very important for the preservation of the objects.
Marine archaeological iron tends to contain even more salts and can corrode far more rapidly. Consult marine archaeology experts before storing and acquiring marine iron.
Pure copper is characterised by the pinkish colour revealed upon scratching or polishing the surface. It is more commonly seen with a brown patina caused by rapid oxidation.
- Bronze is an alloy of copper with about 10% tin. It is harder than copper, but has a lower melting point for casting purposes.
- Brass is an alloy of copper with zinc, which gives hard, corrosion-resistant alloys with a range of colours, depending on the ratio of the components.
Copper patinas emerge after extended exposure to corrosive environments, such as in tombs or on rooftops. Typical copper corrosion does not spread evenly and can combine several colours. Copper reacts with oils and fats, which creates a distinctive green and waxy compound known as copper stearate.
Archaeological copper gets damaged by chlorides, which cause 'bronze disease.' This is characterised by the rapid growth of a soft bright green copper corrosion product from one or more spots on the surface of the object.
Gold is a soft metal, famed for its colour and value. It is often combined with silver to create a more durable alloy.
Gold and gilding is generally stable, but can tarnish or discolour. Overlaying gold and silver causes the biggest risk of deterioration as it can expose gold to thin layers of tarnishing silver sulphide.
Iron is hard, durable and silver-coloured. Its high melting point meant that before the 19th century it was only ever smelted into wrought iron, a hard and ductile material. Although newly made iron items are shiny and metallic, iron oxides soon cover the surface.
Note that iron and its alloys are the metals most vulnerable to damage by corrosion.
- Steel is the combination of iron and carbon. It is harder but more brittle than iron.
- Cast iron is made from iron and significantly more carbon.
A thin, transparent layer of iron oxide usually forms a protective layer over most iron and steel items. Keep these metals away from moisture and salts to maintain this protective layer. Store iron in clean and dry places to keep it pollutant-free.
Iron is susceptible to rust, a soft, reddish-brown corrosion product that weakens metal over time. Many museum items contain a thick layer of compact, hard rust, which occurs if the corrosion takes place slowly and evenly.
However, if the iron object is in a corrosive environment, with plenty of water and salts, the rust layer grows so quickly that it detaches from the surface, revealing bare metal that then corrodes again. Wrought iron is particularly vulnerable to this because it contains small amounts of impurities that can act as channels through the metal. If flakes of rust are continuously detaching, cracking and weeping, the object is probably unstable.
Follow these essential guidelines when storing historic iron and steel:
- Keep relative humidity stable and below 65% RH.
- Avoid contact with materials that naturally retain moisture, acids or salts, such as textiles and leather.
- Because of this, it is sometimes necessary to store swords out of sheaths, pistols out of holsters and medals separate from boxes.
- Use inert materials such as enamelled metal shelving, acid-free paper and boxes or polythene-based packaging.
- Contact conservators if your iron and steel items have already undergone serious corrosion before handling them.
- Iron objects already subject to corrosion will continue to deteriorate even in normal museum environments. Place them in storage with a relative humidity below 10-15% and use silica gel to absorb moisture until treatment can take place.
Lead is a very soft, bright silver-coloured metal that rapidly forms a dull grey surface layer of lead oxide in normal atmospheres. Lead is an important component of leaded bronze and is added to reduce the melting point and increase the pourability of the molten metal.
While metallic lead is safe to handle, lead corrosion products are toxic and should be handled and disposed of with care. Lead is stable in dry and unpolluted conditions, but is extremely vulnerable to the presence of volatile organic acids, especially acetic and formic acid. Do not store lead in wood or wood products, which often give off these pollutants.
Once started, active lead corrosion is progressive and can lead to the total destruction of the object, unless treatment takes place.
Lead should be isolated from any materials that might release acidic pollutants.
- All types of wood
- Wood products such as ply or medium-density fibreboard (MDF)
- Most paper products.
Instead, use inert plastics such as polythene or polypropylene. Acid-free tissue and cardboard are also unlikely to affect lead objects.
We recommend using:
- Stewart Sealfresh™ boxes
- Polythene bags
- Polystyrene "crystal" boxes
- Plastazote™ polythene foam
- Jiffy™ polythene foam
The metal also known as 'quicksilver' is the only one found in a liquid state. The bright silver colour is very dense and receptive to changes in pressure, which is why it's used in thermometers. In the past it was used in fire-gilding and mirror making.
Mercury is toxic by inhalation and should be handled with care. If equipment containing mercury breaks, people nearby risk inhaling its fumes. Ensure that such spillages are dealt with according to health and safety regulations.
Mercury does not wet non-metallic objects and small spills can be swept up with a paintbrush or a synthetic sponge, and placed in a non-metal (glass or plastic) container. It must then be disposed of carefully, preferably to a certified waste contractor or possibly via the local pharmacist.
If large spills of mercury occur, health and safety advice should be obtained from an appropriate professional, and the use of a mercury spill kit, available from chemical suppliers, may be advisable.
Pewter is an alloy of tin and lead with harder properties than either metal. Pewter was historically used for vessels and flatware, when it contained about 10-20% lead. Modern pewter combines tin with antimony and copper, instead.
Silver is again a valuable metal often used in jewellery. It is a soft, bright and 'white' metal, which oxidises into a greyish-white colour. It is often combined with copper to make harder alloy.
Silver tarnishes easily in the presence of sulphide pollutant gases, though it is fairly resistant to more extensive corrosion. The thin black layer of sulphide that covers silver can actually protect the metal beneath it. Watch out for silver chloride, or horn silver, in metals that have been buried.
Silver items can be stored with materials such as Tarnprufe™ or Corrosion Intercept™, which stop tarnishing through sulphide-absorbing materials. Acid-free tissue and cardboard can also provide protection.
This soft, bright, silver-coloured metal has a very low melting point. It's brightness, due to a thin protective layer of tin oxide, makes it resistant to corrosion over extended periods of time. As such, tin is often used as a coating for other metals.
Zinc has a low melting point and is very resistant to corrosion because of the rapid formation of protective zinc oxide. The corrosion resistance of zinc has led to the wide use of galvanising processes in which it is coated onto iron and steel.
Museums Galleries Scotland has a series of advice guides on all elements of collections care. You can also contact us with specialist questions.
The Collections Trust has a range of collections care advice and guidance available
For further reading, consult The National Trust Manual of Housekeeping (Butterworth-Heinemann; The National Trust 2006 (Revised 2011), ISBN 978-1907892189).
Specialist information can be found in Metals and Corrosion: A Handbook for the Conservation Professional (Selwyn, L. CCI (Canadian Conservation Institute), 2004, ISBN 0662379845)
Alloy - A material made of two or more metals mixed together.
Barrier foil - A special composite of polymer-coated aluminium foil that prevents movement of water vapour and pollutant gases through the material to which it is attached. Useful for protecting objects in wooden cases.
Corrosion - An electrochemical process that changes and eats away the surfaces of metals. It is stimulated by the presence of moisture and salts; a destructive result of oxidation.
Corrosion intercept™ - A polymer that reacts with pollutants, preventing damage to museum objects. It can be used to bag or otherwise protect metal objects.
Ductile - Able to be hammered into sheets or drawn out into wires without breaking. A characteristic of many metals.
Micro-climate - The environmental conditions found in an enclosed area, which differ from the conditions in the surroundings.
Oxidation - A chemical reaction with oxygen or other oxidising agents, causing the formation of (metal) oxide, or other compounds such as sulphide, chloride, carbonate and so forth on the surface of a metal.
Patina - A film of oxide formed on the surface of a metal or the sheen on a surface that is caused by long handling; a protective result of oxidation.
Smelting - Extraction of metal from ore by chemical reduction, in most cases by the use of heat and charcoal.
Tarnish - Loss of metallic lustre and formation of darker surface layers, caused by surface oxidation. Also, the dark patina formed on silver.