Choosing the right materials for displaying and storing your items is one of the most important ways you can care for your collections.
Storage and display materials contain chemicals, many of which can be unstable and react with museum objects. The wrong mixture of materials and objects can cause corrosion, discolouration and general deterioration, either through harmful vapours or direct contact. High temperature and relative humidity can speed up this item damage.
Harmful and volatile substances
- Acetic acid
- Formic acid
- Suphuric 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 come with the extra cost of paying more for storage and display, but it helps to conserve your items.
If you are working to a limited budget, replace the unsafe materials gradually when you can afford it. Start with materials near the most vulnerable items first. If you are 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 the majority of 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 daguerrotypes 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 are 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 have 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 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 defense against airborne pollutants and other harmful substances. However, as they are often in direct contact with the object they should be as chemically stable as possible. Any harmful substance in protective material can have the opposite of the desired affect.
Packing and storing
Acid-free tissue paper
Acid-free tissue is used as interleaving tissue when rolling or storing flat items and 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 yourself before using it.
Some acid-free tissues contain buffering alkaline compounds which prevent the migration of harmful acidic products onto paper objects. These can, however, be harmful and should never be used on photographic material and textiles.
Melinex is transparent polyester sheeting that comes in sleeves of various sizes, useful for storing photographic materials and paper items. They should not 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 are free from coatings and plasticisers, making them fully inert. Do not 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.
When mounting prints, drawings and other small flat objects, ensure that your mounting board is acid-free. This can also be used for storage, as it is sometimes buffered to stop the movement of acids. As with acid-free tissue, buffered board should not be used for items such as photographs and textiles.
Ethafoam and Plastazote
These foams are made from stable polyethylene and blown with nitrogen, an inert gas. They can easily 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 not only protect objects from exterior air pollution and fluctuating environmental conditions, but they should not emit any harmful substances themselves. 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 is strong, smooth, inert, non-flammable and does not emit any harmful vapours, which makes it well equipped to protect objects from all manner of threats.
To prevent rusting, metal should always be covered with a protective layer, 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. In particular, 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 is to be used, ensure it is air-dryed 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 and also release formaldehyde from the adhesive. Medium density fibreboard, or MDF, does have 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 are effective as long as the foil is in tact. Nailing, stapling and drilling holes destroys its effectiveness.
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 have regularly been tested and proved to be 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 give off harmful vapours. Avoid using them if possible.
Use cotton and linen that 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 with a piece of white cotton, first dry and then repeated with damp cotton.
Glass is an ideal material for museums. You can customise it with UV-filters or laminates, it's impermeable to gases and impervious to scratches. The only problems come with its heaviness 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.
Such materials do come with some disadvantages. Perspex can be permeable to gas, easily scratched and less rigid. 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 mentioned in the overview.
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, themselves, 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, therefore, 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 is adequate air exchange.
Pollutants in the Museum Environment (Hatchfield, P. Archetype, 2002, ISBN 1873132964) is a helpful book for in depth reading on preventing pollution. For a broader look at creating the right museum environment, use the advice guides created by Museums Galleries Scotland and the Collections Trust.
If you have any further questions, contact Museums Galleries Scotland.