Caring for metal collections

Metal appears in museum collections in many forms. Used for military, domestic, artistic, and industrial objects, as decoration, handles, structural elements, and fixings, metal objects include:

• Household items, ornaments, frames, locks, and handles
• Lighting, candlesticks, chandeliers, and leaded glass
• Sculpture, advertising signage, decorative gates, and furniture
• Machinery, dynamic objects, and scientific instruments
• Arms and armour, weapons, and tools
• Coins, jewellery, buckles, buttons, and medals
• Transport

Understand the composition of each metal object in your collection — what metal was used, whether gold, silver, copper, or its alloys — and any surface treatments applied. Check whether items have been lacquered, gilded, japanned, or enamelled, or whether they have been conserved or repaired during their life.

Metals share these characteristics:

• Metallic lustre (shiny), unless oxidised or corroding
• Ductile
• High density
• Good conductors of heat and electricity
• Solid at room temperature, with the exception of mercury

Care for metals appropriately to protect them from degradation and damage such as rusting or pitting of surfaces. Some corrosion is acceptable, however, and is known as patination.

Metals may be more durable than textiles or other museum items, but storing and displaying them comes with complications. Even the hardest metals suffer physical damage from inappropriate treatment or environments.

Metal corrodes when exposed to unfavourable conditions, especially at high relative humidity (RH) and in the presence of pollutants and chemicals. Depending on the type of metal, its method of manufacture, the materials it contacts, past treatments, and physical condition, deterioration can present in different ways.

What causes corrosion?

Corrosion processes are electrochemical. Electrical currents flowing through metal cause chemical processes that lead to deterioration. Prevent these processes by inhibiting currents through metal items in your collections, for example through moisture.

Moisture and salts stimulate corrosion. Keep metals in clean, dry conditions. Relative humidity in your store should be below 65% for mixed collections, though metals benefit from lower RH levels. Store and display objects in dust-free environments and make sure there is no risk from condensation. The MGS guide to temperature and humidity has guidance on controlling conditions in your museum.

Patina protection

Most common metals develop a coating of metal oxide called the patina. This starts as a transparent layer and becomes more visible as it thickens. Removing this layer may devalue the item. In a stable condition, the patina protects the metal from corrosion.

Always wear gloves when handling metals, as finger marks can affect the shape and coverage of the patina.

Archaeological metals

Archaeological metals need extra care. Many arrive in collections having already experienced extensive corrosion and remain unstable and reactive. Burial in soil exposes metals to salts and water over centuries, causing significant deterioration.

Items may have undergone remedial conservation or cleaning treatments in the past. Check with a conservator if you are unsure, to protect yourself and other collections. The MGS guide to hazards in collections covers this in detail.

Keep untreated archaeological items in very dry conditions, with relative humidity between 10 and 15%. The Institute of Conservation has produced collections care guidance for archaeological materials.

Humidity and chemical pollutants are the two main threats when displaying metals. Plan ahead and maintain dust-free spaces. Some metals are more reactive than others. When multiple metals come into direct contact in the presence of water, the less noble metal corrodes faster. This is known as bimetallic corrosion.

Control display environments and choose mounts and display case materials carefully to avoid making corrosion, mechanical stress, and deterioration worse.

Stabilising humidity

Keep relative humidity low to prevent moisture build-up, which causes corrosion. Museums typically set RH and temperature levels for the organic components of composite metal objects, so make sure RH does not become too high. Control the overall environment in the display room or use sealed display cases to create tightly controlled micro-climates.

Controlling a small volume of air, such as the contents of a display case, is often easier and more effective than controlling an entire room, especially when the case seals well. Use moisture-absorbing or buffering material such as silica gel or Art Sorb™. Display cases also prevent unnecessary handling and offer increased security.

Avoiding pollutants

Choose display materials carefully to reduce the number of pollutants that could affect your metals. Every part of your display, including carpets, paints, text labels, and backing fabrics, can be a source of polluting particles or gases. These are particularly dangerous when they come into contact with highly reactive metals such as lead.

Safe materials

Materials safe for display, available from conservation suppliers, include:

• Metal, either uncoated or with an enamel or painted finish
• Glass
• 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 60°C without detergent

Unsafe materials

Avoid displaying metals with materials that give off harmful vapours or can damage metals through contact, including:

• 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 include pollutants. Test any fabrics intended for long-term use before incorporating them into displays, or buy pre-tested fabrics designed for museum use. The British Museum offers a commercial testing service.

Avoid paints where possible as they routinely fail museum tests. When paint is necessary, leave enough time for harmful vapours to dissipate and use low-VOC paints.

Limit organic and metal objects in the same display cases. Deteriorating organic material gives off vapours that can damage metals. When items must be displayed together, use absorbent material such as Charcoal Cloth™ or sulphide-absorbing textile to counteract 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 using this method.

Cleaning materials such as silver or brass polishes leave residues that continue to cause problems, especially in a sealed environment. These cleaners are also harmful to the environment. Use passive alternatives instead and seek training from a metals or object conservator on the options available.

Keep track of pollution risks within display and storage areas at all times.

Storing with collections care in mind preserves metal items for future generations and keeps them in the best condition.

Steps to store items well:

• Arrange items in stackable, protective boxes made from non-polluting materials
• Store large and awkwardly shaped items off the floor or cover them with a museum-quality dust sheet
• Label everything, including dust sheets, for easy access to collection 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

Always carry out a visual assessment of an item before handling it. Check the construction, look for loose or missing areas, and identify anything that could cause harm such as sharp or rusty areas.

Use a box, tray, or other container to transport smaller items between rooms wherever possible. Complete a risk assessment and method statement for objects that need more complex movement, to make sure staff can transport them safely.

Handling and cleaning

• Always wear gloves when handling metal objects. Skin contact causes uneven finishes and corrosion.
• Remove fresh finger marks with a clean cloth and a small amount of white spirit. Some finishes can be removed by solvents, so test in a less obvious area first. Follow COSHH guidelines when using any solvent.
• Never attempt to remove patinas. Buff them with a clean, soft, dry cloth if appropriate. Always check for dust first.
• Keep dusting to a minimum to avoid scratching surfaces. Use a clean, soft cotton cloth or a soft brush when dusting is needed.
• Never introduce water or use spray cleaners on metal items.
• Oil and grease on metals leads to deterioration and soiling and poses a risk when handling. Consult a conservator before removing these coatings, as in some cases they are protective, for example on objects that are still used and operated.
• The gold plating on gilded objects is extremely vulnerable to damage. Do not polish or clean with any solvent unless a conservator carries out the work.

Transport

Pack metal items securely in well-padded containers or strap them to pallets to prevent movement during handling. In many cases the containers used for storage are suitable for transport if made from inert material. Make sure any container has appropriate handles for safe carrying, or use a skate or trolley to move it.

Use special transport containers only when needed, and do not keep items in them longer than necessary as they are less likely to meet museum conservation standards. Do not use them for storage without advice.

Protect archaeological metal items from high humidity during transport by using airtight containers with enclosed dry silica gel.

Many metal surfaces have been treated in the past with lacquer, Renaissance™ wax, or polish. Metal items with working parts may have had oil or grease applied.

Current conservation practice moves away from over-waxing, which attracts dust, and from using chemical cleaners, oils, and polishes on collections. These can physically alter items, increase the chance of further corrosion, and some contain carcinogens or environmental pollutants.

The right treatment depends on the object. Always consult a conservator before carrying out any treatment, for your own protection and that of the object.

Aluminium

Aluminium is highly chemically reactive, soft, and silver-coloured. A thin coat of aluminium oxide protects it. This coating is hard and unreactive. Do not attempt to polish aluminium, as polishing removes this protective layer. Aluminium is too soft and weak for use as a pure metal and is frequently alloyed with copper and/or magnesium, manganese, silicon, and zinc, giving a wide range of properties for different uses.

Deterioration risks

Aluminium and its alloys are generally corrosion-resistant, but problems arise in very corrosive environments. High-strength aluminium and copper alloys, usually used in aircraft, corrode when exposed to saltwater. This problem, shown by the appearance of blue-green and white crystals, needs laboratory treatment. These crystals are a health hazard, so take appropriate action.

Archaeological iron

The structure of archaeological iron may be entirely compromised by the time it reaches your museum, or the surface may be obscured by corrosion and soil. Limit handling and store archaeological iron with silica gel in airtight display cases. Keep relative humidity stable and below 15%.

Display archaeological iron in similar environments, using conservation-grade display cases to seal out pollutants and control 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 dehumidifying equipment for the display. Follow COSHH guidelines when using silica gel and have processes in place for reactivating it once exhausted.

Support very fragile metal objects with inert materials. Use a special mount for both display and storage, and always keep it with the object.

Where display cases are not possible — for example, for large items such as a sword or coffin — use other physical barriers to prevent the public from touching the objects. Control ambient environmental conditions carefully and limit dust settling on the item.

Marine archaeological iron contains even more salts and corrodes far more rapidly. Consult marine archaeology experts before storing or acquiring marine iron.

Copper

Pure copper has a pinkish colour revealed upon scratching or polishing the surface. It is more commonly seen with a brown patina caused by rapid oxidation.

Common alloys:

• Bronze is an alloy of copper with about 10% tin. It is harder than copper but has a lower melting point for casting.
• Brass is an alloy of copper with zinc, giving hard, corrosion-resistant alloys with a range of colours depending on the ratio of components.

Deterioration risks

Copper patinas form after extended exposure to corrosive environments, such as in tombs or on objects kept outside. Typical copper corrosion does not spread evenly and can show several colours. Copper reacts with oils and fats, creating a distinctive green and waxy compound known as copper stearate. This compound can be toxic if ingested, so always wear gloves and maintain good hygiene in workspaces.

Archaeological copper suffers damage from chlorides, which cause bronze disease. This shows as the rapid growth of soft, bright green copper corrosion, presenting as one or more spots on the surface.

Gold

Gold is a soft metal, valued for its colour. It is often combined with silver to create a more durable alloy, as pure gold was too expensive and soft for many uses. Higher gold content is usually found on gilded items, though gold was also applied through ormolu, damascening, and electroplating.

Deterioration risks

Gold and gilding are generally stable but can tarnish or discolour. The biggest deterioration risk comes from overlaying gold and silver, which can expose gold to thin layers of tarnishing silver sulphide.

Iron

Iron is hard, durable, and silver-coloured. Its high melting point meant that before the 19th century it was only smelted into wrought iron, a hard and ductile material. Although newly made iron items are shiny and metallic, iron oxides soon cover the surface.

Iron and its alloys are the metals most vulnerable to damage by corrosion.

Common alloys:

• 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.

Deterioration risks

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 and can stain or abrade other objects. Many museum items contain a thick layer of compact, hard rust, which forms when corrosion takes place slowly and evenly.

When iron is in a corrosive environment with plenty of water and salts, the rust layer grows so quickly that it detaches from the surface, exposing bare metal that then corrodes again. Wrought iron is particularly vulnerable because it contains small impurities that act as channels through the metal. If flakes of rust continuously detach, crack, and weep, the object is probably unstable.

Monitor collections for any change in rust appearance and colour.

Storage

Follow these guidelines when storing historic iron and steel:

• Keep relative humidity stable and below 65% RH, preferably 45% RH for ironwork
• Avoid contact with materials that retain moisture, acids, or salts, such as textiles and leather
• Store swords out of sheaths, pistols out of holsters, and medals separate from boxes where needed. Record and accession objects accordingly.
• Use inert materials such as enamelled metal shelving, acid-free paper, and polyethylene boxes or polyethylene-based packaging such as jiffy foam
• Contact a conservator before handling iron and steel items that have already undergone serious corrosion
• Place iron objects already subject to corrosion in storage with relative humidity below 10-15%. Use silica gel to absorb moisture until treatment can take place.

Lead

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 molten metal.

Deterioration risks

Metallic lead is safe to handle, but lead corrosion products are toxic and need careful handling and disposal. Lead is stable in dry and unpolluted conditions but is extremely vulnerable to volatile organic acids, especially acetic and formic acid. Do not store lead in wood or wood products, which often give off these pollutants.

Active lead corrosion is progressive once started and can lead to the total destruction of the object unless treated.

Storage

Isolate lead from any materials that might release acidic pollutants.

Avoid using:

• All types of wood
• Wood products such as ply or medium-density fibreboard (MDF)
• Most paper products

Use inert plastics such as polythene or polypropylene instead. Acid-free tissue and cardboard are also unlikely to affect lead objects.

Recommended storage materials:

• Stewart Sealfresh™ boxes
• Polythene bags
• Polystyrene crystal boxes
• Plastazote™ polythene foam
• Jiffy™ polythene foam

Mercury

Mercury, also known as quicksilver, is the only metal found in a liquid state at room temperature. Its bright silver colour, high density, and sensitivity to changes in pressure make it useful in thermometers. In the past it was used in fire-gilding and mirror making.

Usage risks

Mercury is toxic by inhalation and needs careful handling. If equipment containing mercury breaks, people nearby risk inhaling its fumes. Deal with spillages in line with health and safety regulations. The MGS guide to hazards in collections covers this in detail.

Mercury does not wet non-metallic objects. Sweep up small spills with brushes, pipettes, wire wool, or salts in spill kits, depending on what the mercury has contacted.

Dispose of mercury carefully, preferably through a certified waste contractor or via a local pharmacist.

For large spills, seek health and safety advice from a professional and use a mercury spill kit, available from chemical suppliers. If your collection includes mercury, particularly in large amounts, keep spill kits accessible and list these items in your museum’s emergency plan.

Pewter

Pewter is an alloy of tin and lead with harder properties than either metal alone. Historically used for vessels and flatware, it contained about 10-20% lead. Modern pewter combines tin with antimony and copper.

Silver

Silver is a soft, bright, white metal that oxidises to a greyish-white colour. Often used in jewellery, it is frequently combined with copper to make a harder alloy.

Deterioration risks

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 protect the metal beneath it. Watch out for silver chloride, or horn silver, in metals that have been buried.

Storage

Store silver items with materials such as Tarnprufe™ or Corrosion Intercept™, which prevent tarnishing through sulphide-absorbing materials. Acid-free tissue and cardboard also provide protection.

Tin

Tin is a soft, bright, silver-coloured metal with a very low melting point. A thin protective layer of tin oxide makes it resistant to corrosion over extended periods. Because of this, tin is often used as a coating for other metals.

Zinc

Zinc has a low melting point and is very resistant to corrosion due to rapid formation of protective zinc oxide. This corrosion resistance has led to widespread use of galvanising processes, in which zinc is coated onto iron and steel.

For more collections care guidance, find our other advice guides on collections.

The Collections Trust provides a wide range of guidance and advice on collections care.

The Institute of Conservation has produced collections care guidance for archaeological materials.

The National Trust Manual of Housekeeping (ISBN: 978-0-7506-5529-3) covers housekeeping and conservation practice across a wide range of collection types.