1 Environment | 2 Case Design | 3 Display Methods
The need to exhibit books and paper artifacts complicates the goal of preservation. The display environment is often more difficult to control than the storage environment. The materials displayed have, almost by definition, special value, and preservation has historically been secondary to exhibit design. At the very least, exhibited objects are exposed to higher light levels than they would normally experience in storage.
Valuable paper collections should never be exhibited permanently, since this can cause irreversible fading and accelerate acidic deterioration. Whenever possible, exhibit duplicates or facsimiles of photographs and other paper-based materials.
In this section, you will consider the three basic issues that must be addressed to ensure that exhibition damages paper collections as little as possible: environment (climate, light, and pollutants); case design; and display methods.
There is a standard for exhibition of paper-based collections. American National Standards Institute/National Information Standards Organization (ANSI/NISO) Z39.79-2001 Environmental Conditions for Exhibiting Library and Archival Materials (PDF) provides more detailed guidelines than those given here. Appendices provide lists of recommended materials for constructing exhibit cases or supports. Also see NEDCC Preservation Leaflet Protecting Book and Paper Collections During Exhibition for more information on exhibition.
Light levels are perhaps the most important issue for exhibition of paper-based collections. If originals must be exhibited, light levels should be no higher than 50 lux for very sensitive materials (e.g., textiles, paper, dyed leather, 19th-century photographs, and color photographs). Moderately sensitive materials (this includes only a few paper-based materials, such as carbon black inks on high-quality paper and modern black and white silver gelatin photographs on baryta paper) can be exhibited at levels no higher than 100 lux. There should be no natural light in the exhibit area, and ultraviolet (UV) light should be no higher than 75 microwatts per lumen.
Because items have different sensitivities and some may be exhibited more frequently than others, it is best to determine the length of exhibition separately for each item. Materials should never be exhibited permanently. In fact, the NISO standard for exhibition sets the maximum length for exhibition at 52 weeks, but for most items recommends a much shorter period. The standard suggests that very light-sensitive collections that are exhibited repeatedly (defined as no more than once every two years) should be on display for no longer than 12 weeks at a time.
The concept of total light exposure (TLE) can be helpful in determining light levels and length of exhibition for specific items. A yearly limit for light exposure would be set for an item, which would be calculated by multiplying the exposure time (e.g., three weeks) by the light level (e.g., 100 lux). Because of the principle of reciprocity, the same yearly TLE could be reached by exhibiting the item for one week at 300 lux. To use this principle effectively, good record keeping of exhibition times and light levels is of paramount importance.
The NISO standard recommends that relative humidity in the display environment (e.g., inside the exhibit case) be kept at a set point between 35 percent and 50 percent with a variation of +/-5 percent, while temperature should not exceed 72 degrees Fahrenheit with a variation of +/-5 degrees. In addition to monitoring the climate in the exhibition space as a whole, climate conditions inside exhibit cases should be monitored to ensure that they are not damaging. A min/max thermohygrometer will give a general indication of conditions. Exhibit cases should not contain lights, because these cause significant changes in temperature and relative humidity within the case. Fiber-optic or LED lighting is preferable because it does not produce heat.
Pollutant levels should be limited to the greatest extent possible. Materials used to construct the exhibit cases should be as chemically stable as possible and should not off-gas damaging pollutants.
2 Case Design
Exhibit cases should be built of stable, pollutant-free materials and coatings. As with storage furniture, wood and wood composites are often used for exhibit cases, but they can be very damaging to collections because they emit volatile gases. Appendices B-1 and B-2 in ANSI/NISO Z39.79-2001 Environmental Conditions for Exhibiting Library and Archival Materials (PDF) provide detailed lists of materials considered safe and unsafe to use in the construction of exhibit cases.
If your institution has poor-quality cases and they cannot be replaced, any wood should be properly sealed, and the interior of the cases should be lined to prevent off-gassing. Wood should be sealed with a moisture-borne polyurethane or two-part epoxy sealant. Acceptable barrier materials include polyester film, 4-ply 100% ragboard, polyethylene foam sheeting, and Marvelseal (an adhesive-free laminate that is flexible and impermeable to gases and moisture). MicroChamber boards that contain activated carbon may also be an option. Barrier materials should be attached to the sides and the floor of cases; 3M double-sided tape no. 415 is recommended.
Other items that must be chosen carefully include fabric lining for cases, adhesives, and the gaskets used to seal the cases. Fabrics should have no additives and should be washed before use. Undyed cotton, linen, polyester, or cotton-polyester blends can be used. Silk is acidic, and wood off-gases sulphur compounds.
3 Display Methods
Mounts, supports, and other exhibit materials should be made from inert materials like Plexiglas and polyester, or from neutral paper or 100% ragboard.
Documents in exhibit cases should be completely supported, whether they are matted or mounted (without adhesives) to a piece of ragboard or other preservation-quality material. Edge strips or some type of corner support (photo corners will work on most small items) can be used to attach the document securely to the mounting board, but adhesive should never be applied to the object itself. Documents can also be encapsulated for protection, but keep in mind that this can cause paper to deteriorate more quickly. Use acrylic or Teflon gaskets, not rubber, to seal cases.
Documents to be exhibited on walls should be matted and framed using museum-quality framing and hinging techniques. For more on these techniques, see NEDCC Preservation Leaflets Matting and Framing for Art and Artifacts on Paper.
If the book will not remain open naturally, a polyester band closed with 3M double-sided tape no. 415 can be used to hold the book open. Books can be structurally damaged by long-term exhibition in an open position, so exhibit periods must be limited. Turning the pages of a book every few days will prevent overexposure to light on one page.