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Conservation

Selecting STORAGE SUPPLIES

From what materials should containers, supports, and mounts be constructed?

Whenever possible, containers, supports, and mounts should be made from materials that meet standard museum preservation requirements. These requirements vary depending on the material. In general the materials should be chemically and physically stable, durable, and non-damaging. Materials that do not meet these requirements can cause irreparable visual, chemical, and physical damage.

What do the terms “archival quality,” “conservation quality,” and “preservation quality” mean?

These terms have been used over the years to imply that materials meet standard museum preservation requirements. The terms have been loosely used, however, and given a variety of meanings, especially by manufacturers and suppliers of storage materials. For this reason, it is best not to rely on these terms but to use specific characteristics, as described below.

Are there standards that list specifications for storage materials?

Yes, there are national and international standards. They are produced by such organizations as the American National Standards Institute (ANSI), the National Information Standards Organization (NISO), and the International Standards Organization (ISO). These important standards specify in technical terms the characteristics that are recommended by the organization producing the standard.

For example, one standard, ISO 18902.2001, formally ANSI IT 9.2 – 1998, specifies storage materials for photographic processed films, plates, and papers and can be followed in general for the storage of most non-photographic paper-based materials as well. Although there is still not complete consensus among the organizations, they are moving toward agreement. Very few of these standards are cited in supplier catalogs at the present time. For your purposes, it is probably best to rely on the characteristics discussed below when selecting storage materials.

Why is chemical stability important?

Some of the deterioration that items in storage suffer is caused by the acids and other harmful substances in the containers, supports, and mounts that are used to protect them. These harmful substances migrate from storage materials into the items, causing such problems as discoloration, corrosion, and embrittlement. For example, discoloration caused by an acidic window mat can disfigure and hasten the deterioration of an Indian drawing on paper that has been matted and framed. Similar damage takes place when a textile item, such as a shirt, is wrapped in acidic tissue that discolors the textile and transfers acidity from the tissue to the textile, speeding its deterioration.

To avoid these problems, it is essential for all storage materials to be chemically stable—to not generate any harmful substances. If this is not possible, a chemically stable barrier can be used between the storage material and the item. For example, an acidic cardboard tube can be covered with a stable material such as polyester film, which acts as a protective barrier between the acids in the tube and the item rolled on it.

Paper-based storage materials should be acid-free.

Paper-based materials are used widely, being readily available commercially and relatively affordable.; Paper materials are made from cotton, linen, or wood fibers, with wood being used most often. Wood, however, contains highly damaging impurities that lead to the formation of harmful acids. For this reason, only paper materials made from wood pulp that has been chemically purified to remove lignin and other damaging impurities are safe to use. Paper materials made of 100 percent cotton or linen are also safe to use. Folders, envelopes, tissue, and papers for interleaving sheets should be lignin-free and made of chemically stable fibers. The board for boxes should also be lignin-free and chemically purified. The board used for matting Indian drawings on paper should be 100 percent cotton or linen rag board or an otherwise lignin-free, chemically purified conservation mounting board. Tapes for making mats, folders, and boxes should be chemically stable, non-staining, and free of damaging components if possible. Such materials commonly are described as acid-free. It is important to be aware that not all paper-based materials are acid-free. Standard museum preservation practice maintains, however, that only acid-free materials should be used.

Why is pH important?

Knowing the pH of paper-based storage materials will tell you whether they are acid-free. The acidity and alkalinity of paper and paper-based materials are expressed by pH, a measurement on a scale of zero through fourteen. Seven is the neutral point, with measurements under seven indicating increasingly acidic, and over seven indicating increasingly alkaline conditions. Although the recommendation varies for what an ideal pH for storage enclosures should be, depending on the item to be stored, a pH of 7.0 through 8.5 is a good general range.

It is advisable to measure the pH of purchased storage materials to ensure that they are acid-free (pH over 7), because sometimes materials do not meet their advertised levels. There are several methods for measuring pH. The simplest is the use of a pH detector pencil or pen, which indicates the surface pH of the material being tested (never to be used on a cultural object). This method is suitable for most situations. These pencils and pens are relatively inexpensive and readily available from conservation suppliers. A more specific pH reading can be obtained by using pH indicators strips. The most accurate readings are those provided by pH meters. These latter two methods are used primarily by museums.

What is an alkaline reserve?

Some paper-based storage materials contain a buffering agent, such as calcium carbonate, added during manufacture. This buffering agent is referred to as an alkaline reserve. The alkaline buffer neutralizes acids as they form in the storage materials and helps keep the materials acid-free long-term. Over time, however, the buffering agent may eventually be depleted.

Should buffered or unbuffered materials be used?

Buffered materials are appropriate for storing some American Indian items but not others, and you must know which to use. Museums keep supplies of both buffered and unbuffered materials and use whichever is appropriate for the item being stored. It is, however, expensive to keep both types of supplies on hand. Also, it is impossible to distinguish between them visually, so they must be clearly marked. The easier and safer approach for most people is to use acid-free unbuffered materials for everything.

What are molecular traps?

One relatively new type of storage material incorporates molecular traps to provide added protection from gaseous pollutants. Molecular traps, such as activated carbon or natural or synthetic zeolites, capture and retain pollutants. These are most suitable for storage materials that will be used in highly polluted areas or for items that are particularly sensitive to pollutants. Storage materials that contain molecular traps are available as paper or board and are sold under the trade name of MicroChamber.

Durability

Items should be stored only in containers that are sufficiently durable to protect them. If containers are not sturdy, the items they contain may become distorted or broken, or the container itself may become damaged or even fall apart. Needlessly strong storage containers may also present problems, adding unnecessary weight and bulk that can lead to handling and spatial difficulties.

Are plastics safe to use?

Plastics lend themselves well to constructing containers, supports, and mounts, but they vary greatly in chemical stability and should be used knowledgeably. Some plastics are unstable chemically and produce by-products as they deteriorate that accelerate the breakdown of many materials used in American Indian items. These should always be avoided, even though using them is tempting because they are easily obtained and inexpensive. Three types of plastic meet preservation standards. These are polypropylene, polyethylene, and polyester. These plastics come in many forms with different characteristics—planks or foam, rolls or sheets, hard or soft, thick or thin, opaque or transparent—and are sold under different trade names.

Polyvinyl chloride (PVC) should not be used in any form—not as sheeting, a photograph sleeve, or a tube—because of the damaging by-products it emits. The same is true of bubble-pack; do not use it, because of possible coatings, physical damage, or harmful by-products. Avoid polyurethane, like that commonly found in seat cushions; it turns to powder as it ages and gives off damaging by-products. Finally, do not use polystyrene, as this has a tendency to become brittle and yellow as compared to other acceptable plastics. Generally it is important to determine that the plastic materials you use for long-term storage are one of the three safe types.

From Caring for American Indian Objects: A Practical and Cultural Guide. Sherelyn Ogden, ed. St. Paul: Minnesota Historical Society Press; 2004.