Storage and display materials come into very close contact with collection items. With this in mind, it helps to have a good idea of what effect these materials can have on different items in a heritage collection. This will help you choose the best material for the job.
Packing and display materials are chosen from a wide variety of plastics, paper, cardboards, metals, woods, boards, fabrics and so on. Some of these are safe to use, but some varieties contain acids and other nasties that can damage items when placed in contact with them. When choosing materials remember that many items will remain in contact with these materials for extensive periods of time, so think about the long-term effects. Products designed for use with heritage material are often more expensive, but should have superior results and last longer. On the other hand there are some standard commercial materials that are also highly effective for our purposes. In general, you get what you pay for, so try to obtain the best quality products you can afford.
The following information about commonly used materials is aimed at helping you choose the most suitable products, balancing your budget with your collection needs.
Papers, tissues and cardboards
There are several high-quality paper products available for use in storage and display. These are particularly recommended for significant paper and textile items. The highest quality paper products are manufactured from 100% cotton, often referred to as 100% rag.
Acid-free and archival are two terms used to describe papers, cardboards and tissues of higher quality suitable for long term use in your collection. These products should be pH-neutral (that is, pH 7) or alkaline (pH of 7 or above). They should also be free of lignin, which is a component of most woods and over time will break down and produce acid, and are usually un-dyed, that is, neutral coloured, as dyes can also cause acidity. Be aware that acid-free can be a misleading term, as some manufacturers use it to describe materials that are acid-free at the point of sale, but which may deteriorate and become acidic in the future.
Some products will be described as either alkaline-buffered or unbuffered. Alkaline-buffered products have a pH of over 7, in order to protect against becoming acidic over time especially when storing acidic items. This makes them a good choice for storage, particularly with paper items. They should not be used, however, with coloured photographs or silks. Some museums and collectors find it easier to use pH-neutral unbuffered boards only, to avoid mix-ups.
The following table gives details of some of the commercial paper-based products available. You may also find information in catalogues from suppliers of library and archive materials.
Plastics used in storage and display should be chemically inert and coating free. Some kinds of plastic are unsuitable because they undergo chemical degradation processes that will be damaging to many collection materials. In particular, avoid polyvinylchloride (PVC) as it can break down over time, and with moisture from the air can produce damaging hydrochloric acid. PVC can be recognised by its distinct 'plasticy' smell; think of the aroma of kids' 'floaties' or inflatable pool toys, or shiny plastic raincoats.
Suitable plastics include acrylics, polyester, polypropylene and polyethylene. Perspex™ is a tradename for rigid clear acrylic sheeting, which is often used when framing paintings and works of art on paper, as it doesn’t shatter in an accident, and can protect the work from ultraviolet radiation. It can build up static electricity though, and therefore should not be used on works with fragile media, such as pastel, charcoal, chalk.
Mylar™ and Melinex™ are trade names for polyester films that are very handy for museum usage, being clear and available in a range of sizes and thickness. Due to their rigidity and transparency, ready made Mylar™ enclosures are useful for storing such items as documents, photographs.
If using polyethylene or polypropylene products, check for the presence of a greasy film that is sometimes present from the manufacturing process. If purchasing storage tubs or other containers, choose 'food quality' polypropylene. Also, when ordering polyethylene plastic sheeting, make sure it is 'virgin' material, meaning that there is no recycled product in it.
Chipboard, compressed board (MDF etc) and plywood
These types of products should generally be avoided, as they are usually prepared with formaldehyde, which will produce acids. The presence of formaldehyde is readily detected by the strong smell that is obvious when cabinets made from chipboard or MDF are opened. However, these materials are in very common use for shelving, cupboards and display cases, and can be difficult to avoid completely. If you must use these, understand that they are aggressive to a range of materials in your collection, especially metals, textiles, paper, photographs. Where they are used they should be sealed. This is, however, not 100% effective.
Sealants, paints, coatings, adhesives
It is often suggested that using a varnish or an acrylic sealer will prevent materials, such as those discussed in the last section, from giving off (or off-gassing) acidic vapours. Unfortunately for those of us working with collections, this is not true. No sealant is truly impermeable. Their use will slow down but are unlikely to reduce the total amount of off-gassing that occurs.
Polyvinyl acetate and polyvinyl alcohol (PVA & PVOH) based paints, coatings and adhesives are generally considered safe to use. Several coats are required, and they must be allowed to fully dry. A minimum of 48 hours should be allowed before the item is used in a collection application. The longer the drying period the better.