Head of Conservation
Minnesota Historical Society
The selection of storage furniture for library and archival materials requires careful investigation. Many of the currently available furniture choices contain materials that produce by-products that contribute to the deterioration of the collections they house. In addition, some construction features are damaging and also contribute to deterioration of collections. The information that follows is intended to serve as an introduction to the subject and as a guide to what to look for in selecting storage furniture.
Until recently only baked enamel furniture was recommended. Constructed of steel with a baked enamel coating, this furniture was thought to be made of chemically stable materials. Because it is readily available, competitively priced, strong, and durable, it has been a particularly attractive choice. Questions, however, have been raised about the possibility that the baked enamel coating may give off formaldehyde and other volatiles harmful to collections if it has not been properly baked (not long enough at high enough temperatures). This concern is especially serious when collections are stored on book shelves in an area that is enclosed or has poor air circulation, or are stored in closed furniture such as map cases, file cabinet drawers, and book cases with solid doors.
Because of this concern about off-gassing, baked enamel furniture is no longer widely recommended unless it has been properly baked. For us to be certain that it has, the furniture must be tested. Testing should comply with ASTM (American Society of Testing Materials) E-595. 1 This testing requires the use of sophisticated analytical equipment. Furniture can be less conclusively tested in-house with the organic solvent methyl ethyl ketone (MEK). 2 If this crude test, known as the MEK rub test, indicates that the coating may not be properly baked, the furniture should perhaps be tested by a professional testing service to determine for certain if it is off-gassing.
Steel storage furniture with various powder coatings appears to avoid the off-gassing problems associated with baked enamel. Powder coatings of finely divided, synthetic polymer materials are fused onto the steel. Testing done thus far indicates that the coatings are chemically stable, present minimal threat of volatile evocation, and so are safe for the storage of valuable materials. Nevertheless, conducting the MEK rub test in an inconspicuous area where the steel is the heaviest gauge will confirm that the coating is properly cured and that off-gassing is not a concern. 3
Anodized aluminum storage furniture is another option. This uncoated metal is extremely strong yet light in weight. The metal itself is reported to be non-reactive and, since it has no coating, off-gassing problems are eliminated. Many people consider anodized aluminum to be the best choice, especially for highly sensitive materials, but it tends to be the most expensive.
Open chrome-plated steel shelving, made of heavy-gauge, chrome-plated steel wire, is a storage choice suitable for boxed materials. The shelving is durable, and the open-wire framework is light in weight and provides good air circulation. The wires, however, can leave permanent marks on items that are not protected, so materials should be boxed or the shelves should be lined.
Storage furniture, especially shelving, made of wood has traditionally been popular for reasons of aesthetics, economy, and ease of construction. Harmful acids and other substances, however, are emitted by wood, wood composites, and some sealants and adhesives. Although the levels of emissions are highest initially, in most cases volatiles are present for the life of the materials. To avoid potential damage to collections, storage furniture made of wood or wood products should be avoided. If this is not possible and wood must be used, precautions are necessary. Certain woods and wood composites are more potentially damaging than others. For example, oak, which has been used extensively for the storage of library and archival materials, is considered the wood with the most volatile acidity and should not be used. Also, many wood composites that are advertised as formaldehyde-free may contain potentially damaging acids or other aldehydes. Current information should be obtained prior to selecting new furniture made of wood or a wood product so that the least damaging wood can be chosen. All wood and wood composites should be tested to determine their safety for use. 4
For wooden storage furniture that is already in use, safeguards should be taken. All wood should be sealed. It should be noted, though, that no coating or sealant will completely block the emission of acids and harmful volatiles for prolonged periods of time, but it can be useful for short-term exposure. Also, some sealants are better than others at blocking damaging substances. Great care must be taken in selecting a sealant to make sure that the one chosen forms the most effective barrier and does not itself emit harmful substances.
The most readily available sealant that is recommended at this time is a moisture-borne polyurethane. Many kinds of polyurethane are available. Oil-modified polyurethanes are the most common. However, oil-modified polyurethanes, oil-based paints, and other products that contain oil or alkyd resins should be avoided. Only moisture-borne polyurethanes are recommended. Unfortunately not all moisture-borne polyurethanes on the market are safe for use. Also, formulations often change without notice. For these reasons, the polyurethane selected should be tested prior to use to guarantee its acceptability. 5 Contact a preservation professional for brand names of moisture-borne polyurethanes that are currently being recommended and begin testing with these. Because these urethanes do not completely prevent the escape of volatiles, choosing low-emission wood products is of critical importance.
Paints can also be used to seal wood if the natural appearance of the wood does not have to be retained. Oil-based paints and stains should not be used because of the potentially damaging effects of the acids in the drying oils. Two-part epoxy paints form an excellent barrier, but they are difficult to use. Latex and acrylic paints form a less effective barrier but are easier to use. 6 All coatings should be tested prior to use. Contact a preservation professional for current information before making a decision. After furniture is sealed it should be allowed to air for three to four weeks. Because of the toxicity of various components of most sealants, the sealants should be used with caution and appropriate safety measures observed.
In addition to sealing wood, bookshelves and drawers should be lined with an effective barrier material. Barriers that are recommended at present include an inert metallic laminate (e.g., Marvelseal 360 and 470), PCTFE (polychlorotrifluoroethylene) high-barrier films (e.g., Alclar), sheet aluminum, glass, polymethyl methacrylate sheeting (e.g., Plexiglas), or a combination of these. 7 Polymethyl methacrylate sheeting can absorb pollutants and reemit them, so this material should not be reused once it has served as a barrier. Note that printing inks found on some of these barrier materials may be corrosive. 8 Contact the manufacturer to request information on the printing inks, or request products without printing. If these barriers do not provide an appropriate surface for the storage of materials, 100% ragboard can be used in addition. Ragboard, however, should not be used by itself, because it does not provide a sufficient barrier.
Regardless of the construction material chosen, storage furniture should have a smooth, non-abrasive finish. If steel furniture is painted or coated, the finish should be resistant to chipping since chips will leave steel exposed and susceptible to rust. The furniture should be free of sharp edges and protrusions. Exposed nuts and bolts are particularly hazardous. The furniture should be strong enough that it will not bend or warp when filled. Shelving should be bolted together as well as to the floor and perhaps ceiling so it will not wobble when collections are housed on it. Shelves should be adjustable to accommodate items of various sizes, particularly oversized ones. The lowest storage area in the furniture should be at least four to six inches off the floor to protect collections from water damage in the event of a flood. Cabinets with doors are often preferred when security and protection from dust are special concerns. These are available with shelves or drawers. The use of piano hinges for the attachment of the doors is advisable if opening the doors flat will facilitate safe removal of items from the cabinet. Condensation can be a problem in closed steel cabinets when the relative humidity where the cabinets are stored fluctuates. 9 Condensation can result in rusting or mold growth in cabinets. For this reason, conditions in closed cabinets should be monitored. This is most easily accomplished by the use of dial hygrometers or paper-based humidity indicator cards. These devices do not have a high degree of accuracy, but they are sufficient to indicate problematic conditions. If possible, the use of closed steel cabinets should be avoided unless the cabinets are well ventilated or the relative humidity is closely controlled and monitored. 10
Drawers in flat files should be no more than two inches deep (less if possible). The deeper the drawer the greater the weight on the items in it and the greater the stress on items when they are removed. Drawers should have dust covers or rear hoods to prevent items from being damaged at the back of the drawer. Drawers should have stops to prevent them from coming out of cabinets. Also, they should have ball bearings rather than slide in grooves because they will open and close more smoothly, causing less vibration to items, and the risk that they will fall out of the grooves and become stuck is eliminated. Drawers can be lined with foam core for cushioning as added protection from jarring and vibration.
High-density storage systems, often referred to as compact or movable shelving, are used by many institutions with space limitations. These systems minimize the amount of space needed by compacting ranges of shelves or cabinets of drawers tightly together. The ranges slide along tracks so they can be moved apart for retrieval of items on a particular range and then moved back together again. Moving systems such as these can be damaging to items because of the vibrations to which they subject items. Also, items can be jostled off shelves causing further damage. If a high-density storage system must be used, a design should be chosen that minimizes these hazards. It is crucial with high-density storage systems that items do not extend beyond the edge of the shelves to avoid having the items on opposite shelves collide with them when the ranges are closed. When installing high-density systems, enough overall space should be allowed to insure that sufficiently wide aisles can be opened between the ranges for the safe removal of items, particularly oversized ones, from shelves and drawers. Floor loading is a serious concern and should be taken into account if many heavy items are stored in a confined space. This is quite important with compact shelving for books. Weight estimates need to include floor treatment, furniture tracks and fittings, and shelf and drawer loads as well as the furniture. A structural engineer should be consulted. Fire detection and suppression are additional concerns. A space of a few inches always should be left between the ranges so that a fire between them can be detected and suppressed. Leaving a small space will also enhance air circulation, avoiding the build-up of pockets of damp or stagnant air. Another concern is the behavior of compact shelving during floods, fires, or earthquakes, and how to gain access to materials if the shelving fails to open because of increased weight, distortion of the tracks, or failure of electricity. Consult the manufacturer about this.
The selection of suitable storage furniture and the specification or modification of wooden storage furniture are complicated tasks. Poor-quality storage greatly accelerates the deterioration of collections. Opinion on what constitutes acceptable storage furniture is changing rapidly. A preservation professional should be consulted for the most up-to-date information before decisions with far-reaching impact are made. Making the right choice will add immeasurably to the useful life of collections.
Hatchfield, Pamela. "Choosing Materials for Museum Storage." In Storage of Natural History Collections: Basic Concepts . Carolyn L. Rose and Catharine A. Hawks, eds. Pittsburgh, PA: Society for the Preservation of Natural History Collections, 1994.
Hatchfield, Pamela, and Jane Carpenter. Formaldehyde: How Great Is the Danger to Museum Collections? Cambridge, MA: Harvard University, 1987.
Miles, Catherine E. "Wood Coatings for Display and Storage Cases." Studies in Conservation 31.3 (August 1986): 114–24.
Raphael, T. Conservation Guidelines: Design and Fabrication of Exhibits . Harpers Ferry, WV: Division of Conservation, National Park Service, Harpers Ferry Center, 1991.
The author gratefully acknowledges the assistance of Pamela Hatchfield in the preparation of this preservation leaflet.