Introduction to storage and display materials
Choosing the right materials for the display and storage of your items is one of the most important ways you can care for your collections.
Some storage and display materials contain unstable chemicals which may react with objects. The wrong mixture of materials and objects can cause corrosion, discolouration, and general deterioration, either through harmful vapours or direct contact. High temperatures and relative humidity can accelerate this damage.
Harmful substances and dangerous objects
Harmful and volatile substances:
- Acetic acid
- Formic acid
- Sulphuric acid
- Sulphur dioxide
- Nitrogen dioxide
- Ammonia gas
- Wood products
- Acrylic resins
- Lacquers and varnishes
- Glues and adhesives
- Pesticides, insecticides and fungicides
- Furnishing fabrics
- Adhesive tape and other sticky materials such as tack
- Some electrical machinery, such as electrostatic air cleaners and photocopiers
- Burning fossil fuels
Planning for materials
Use inert materials whenever you can in a museum. This may involve additional storage and display costs, but helps to conserve your objects.
If you’re working with a limited budget, replace the unsafe materials gradually when you can afford to. Start with materials near the most vulnerable items first. If you’re in any doubt about using a material, get a small sample tested before placing it in your museum.
How museum objects react
All metals are affected by harmful vapours released by wood, although some more than others. Pay particular attention to lead, which corrodes from the acetic acid in woods such as oak. Silver and copper objects, metal embroidery threads, sequins, and silver photographic images will tarnish as a result of sulphur gases released by wool.
Cheap, acidic mountboard can discolour and deteriorate most paper products, including:
Adhesive tape will eventually come loose from paper and leave behind persistent yellow stains. Some watercolour pigments are sensitive to acidic vapours.
Photos, slides, prints, and negatives are all affected by the vapours and plasticisers used in polyvinyl chloride (PVC) and other plastic. The metal plates on daguerreotypes are tarnished by sulphur gases. Paper prints can oxidise in gaseous pollutants, resulting in a bluish metallic sheen called ‘silver mirroring’.
Textiles deteriorate more rapidly when they’re in contact with acid-releasing materials such as cheap mount board and acidic cardboard rollers. Fabrics used for display can damage historic textiles, as they’ve often been treated with dyes and fire-retardant materials that can cause fading.
Look out for pins and tacks used in framing, as they can rust when in contact with the natural moisture in fabrics and consequently damage the object.
Bone and ivory
These can deteriorate when in contact with the vapours from rubber and urethane foam, found in:
- Foam rubber
- Vinyl tiles
- Flooring adhesives
- Rubber-backed carpets
Volatile sulphur compounds from rubber will cause yellow or orange discolouration in ivory and cellulose acetate artefacts. Adding a plasticiser such as the one used in PVC can make cellulose nitrate more stable, although you should consult conservators before treating celluloid.
Magnetic tapes are found in cassettes, reel-to-reel films, and computers. These can be damaged by other magnetic items in the museum, such as the catches on doors.
Modern plastics and rubber
These items are very unstable are easily affected by the plasticisers found in wrapping materials such as PVC, sleeves, and sheeting. Ozone, which can be generated by electrical machinery, and some metals speed up the deterioration of rubber.
Feathers and ethnographic objects will often fade and discolour when they come into contact with sulphuric acid.
Many different types of wood contain formaldehyde, which reacts with other materials to create formic acid. Formic acid can corrode and deteriorate several other items in your museum collections.
Choosing safe materials - Protection and support
Materials that protect, shape, and support museum objects are the first line of defence against airborne pollutants and other harmful substances. These materials should be as chemically stable as possible as they’re often in direct contact with the object.
Packing and storing
Acid-free tissue paper
Acid-free tissue is used as interleaving tissue when rolling or storing flat items. It’s also used as a wrapping or padding material for three-dimensional objects.
Source tissue made from cotton, linen rags, or highly purified wood fibre materials, without any trace of acid-producing lignin. You’ll see such tissue listed as ‘acid-free’, ‘museum-quality’ or ‘archive-quality’, indicating the absence of lignin and its suitability for packaging items. Always check the acidity of tissue paper before using it.
Some acid-free tissues contain buffering alkaline compounds which prevent the migration of harmful acidic products onto paper objects. These can sometimes be harmful and should never be used on photographic material and textiles.
Melinex is transparent polyester sheeting that comes in sleeves of various sizes. It’s useful for storing photographic materials and paper items. The sleeves shouldn’t be used for anything with loose media, such as pastel and charcoal drawings, as static can cause the media to come loose. Melinex can also be purchased as a continuous film that can be heat-sealed into custom-sized sleeves.
Self-sealing bags are available in various sizes and are suitable for the storage of small items. They’re free from coatings and plasticisers, making them fully inert. Don’t use polyvinyl chloride or other plastics that contain chloride or nitrate as they give off harmful vapours.
High-density polyethylene fibre sheeting that prevents the passage of water in one direction, from the smooth outside to the rough inside. Crucially, it allows for the passage of air in both directions. It can be used to make dust covers for costumes, upholstered furniture, and rolled textiles.
Ensure that your mounting board is acid-free when mounting prints, drawings, and other objects. Acid-free board can also be used for storage, as it’s sometimes buffered to stop the movement of acids. Buffered board shouldn’t be used for items such as photographs and textiles, same as acid-free tissue.
Ethafoam and Plastazote
These foams are made from stable polyethylene and blown with nitrogen, an inert gas. They can be cut into supporting mounts for three-dimensional shapes. Although they come in a range of colours, only the black and white versions are safe for use as the colourants used in others may stain or damage objects.
Perspex, or polymethylmethacrylate, can be moulded into stands for small glass, ceramic, and other solid objects in good condition. It can be difficult to process and shouldn’t be used with adhesives that contain harmful vapours.
Choosing safe materials – Containers
The correct materials in museum containers protect objects from exterior air pollution and fluctuating environmental conditions. Containers should also be free from any harmful substances. Harmful substances can quickly build up in the micro-climates of display cases, so ensure that the materials are inert and stable.
Metal is the preferred material for display cases and storage shelving or cupboards. It’s strong, smooth, inert, non-flammable, and doesn’t emit any harmful vapours. This makes it well equipped to protect objects from a variety of threats.
Metal should always be covered with a protective layer to prevent rusting. Preferably baked enamel rather than paint.
Wood produces harmful vapours such as formic and acetic acids and peroxides, although some types are worse than others. Wood that is freshly cut and unseasoned can cause the most damage.
Metal can be corroded by:
- Sweet chestnut
- Western red cedar
- Douglas fir
Iron and steel are more susceptible to the corrosive influence of wood than other metals, although teak has been known to damage even rust-resistant alloys.
If wood must be used, ensure it is air-dried and choose one of the following:
- Pinewood, especially yellow pine
- Spruce, except sitka spruce
Plywood, chipboard, particle board, and other composites can cause the same problems as wood. They also release formaldehyde from the adhesive. Medium density fibreboard (MDF) has lower levels of formaldehyde.
The release of acids by wood products is a normal chemical process that cannot be prevented entirely. Use barrier foil to minimise the emission of harmful gases. The metal foil, sandwiched between two layers of inert plastic, can be heat-sealed onto wood and is effective as long as the foil is intact. Nailing, stapling, or drilling holes in the foil renders it ineffective.
The edges of the board emit vapours at a higher rate than other surfaces so should be sealed well. Aluminium foil is a cheaper but much more vulnerable alternative.
Paints, lacquers and varnishes
These are ineffective as a barrier against vapours. They may even cause their own problems if the chemicals in the paints are unstable or damaging. If it must be used for aesthetic reasons, avoid using casein, alkyd, polyurethane, and oil-modified paints or varnishes.
Acrylic latex emulsions and epoxy resins are considered safe, but allow paint to dry thoroughly before use in a store or display. Check drying times with the manufacturer, as some paints appear dry while still emitting moisture.
Rigid mounting boards
Many lightweight mounting boards consist of an inner material such as polystyrene and a skin of acid-free paper, plastic, or aluminium. The inner material may release harmful gases that were used to create the lightness of the foam, as can the adhesives used to attach the exterior. Cover such boards in barrier foil before use.
Wool, fire-retardant fabrics, and foam or adhesive-backed fabrics have all been found to emit harmful vapours. Avoid using them if possible.
Use cotton and linen which is both undyed and unbleached, but wash them thoroughly first. Dyed fabrics and synthetic blends should be tested before use. The fastness of dyes can be checked by rubbing the fabric first with a dry piece of white cotton, and then with a piece of damp cotton.
Glass is an ideal material for museums. You can customise it with UV-filters or laminates, it’s impermeable to gases, and is scratch resistant. The only problems with glass are its weight, and the possibility that it may cause condensation and mould growth which is dangerous for organic materials.
Clean glass well. Don’t use vinegar-based agents or anything with harmful substances.
Perspex and polycarbonate sheets
Perspex and polycarbonate sheets are lighter than glass and sometimes come with built-in UV-absorbing properties, making them useful for light-sensitive items. These materials also have good impact resistance, cracking or bending instead of shattering.
These materials do come with some disadvantages. Perspex can be permeable to gas, easily scratched, and less rigid than other materials. It can create static forces during cleaning which can attract loose items within the container.
Adhesives and seals
Always check the quality of any adhesive, seal, gasket, or label. Many of them contain harmful substances.
Materials that are good to use include:
- Hot-melt glues (ethylene/vinyl acetate copolymer types)
- Dense polyurethane
- Cross-linked polyolefin gaskets
- Acid-free paper labels
Materials of museum objects
Sometimes museum objects can contain substances and materials that are harmful or detrimental to the rest of the museum.
Modern organic materials are likely to release harmful gases. Badly processed photographs can emit acidic or sulphidic gases that lead to discolouration and weakening of the paper. Celluloid objects release vapours that speed up their own deterioration.
These processes particularly occur when the objects are kept in a confined space, such as display cases or plastic boxes, as harmful vapours build up. Such objects require display and storage that allows air-exchange. Wrap them in acid-free tissue to absorb emitted gases and store them in a separate part of the storage area where there’s adequate air exchange.