conditioned to 50% and should not need to be recondi- tioned during the show.
Silica gel is not a magic cure and requires a good understanding of its capabilities and limitations. It is a good idea to purchase it from a supplier who specializes in environmental controls and who can help you deter- mine what will work best. For detailed information on the use of silica gel, see Appelbaum,Guide to Environ- mental Protection of Collections; and the Canadian Con- servation Institute’s Technical Bulletin 10, “Silica Gel,” by Raymond H. Lafontaine.
M O N I T O R I N G T E M P E R AT U R E A N D R E L AT I V E H U M I D I T Y I N E X H I B I T C A S E S
There is more information on monitoring equipment in
“Monitoring Environmental Conditions” in section 1.
Suppliers of the equipment are listed in appendix B.
Temperature/Humidity Cards
These cards give approximate but consistent readings, do not need to be calibrated, and are very inexpensive. Cards with both temperature and humidity indicators measure from 80% to 20% RH and have a lavender-colored border. The relative humidity will be in the range of the square that most closely matches the color of the border.
Another type of card shows the RH only, from 10% to 100%. It does not have the colored border. They are avail- able from conservation suppliers and also from the man- ufacturer, Süd-Chemie (formerly Humidial), in quanti- ties of 100 or more. They remain accurate for a long time, but to be on the safe side, replace them once a year or immediately if they become wet.
Hygrometers
Very small instruments of this type are available, e.g., the Arten, which is only 2" (5 cm) long. It has a dial for temperature and one for RH and also a row of squares along the bottom that show RH by color change from pink to blue. Since the color change strip remains accu- rate indefinitely, a marked difference between the RH dial and the strip means that the instrument needs cali- bration. A kit is sold for this purpose.
There are various other small humidity meters on the market. All hygrometers tend to overestimate humid- ity over time, sometimes by more than 10%. When selecting one, ascertain whether and how it can be recal- ibrated. (Thermometers do not need to be recalibrated.)
Data Loggers
Another way of monitoring conditions in an exhibit case or frame is to place a data logger inside. Several models are made, some quite small, and their price continues to drop as time goes by. Some data loggers can be read from a remote location. This is a very useful feature when the exhibition is in a different building from where staff members normally work. One or more loggers can be placed in selected cases and checked frequently during the workday. An unsatisfactory reading would alert the staff to go to the gallery and investigate further. Some loggers can set off an alarm at a central station or other designated location. Many institutions place a data logger in frames when lending materials, and they some- times enclose them in packing crates to monitor condi- tions during transit.
Safe Designs and Materials for Exhibit Cases
When designing a case with it own lighting system, make sure that it will be possible to change light bulbs or make other adjustments without having to remove all the ex- hibits from a case. The same applies to cases that have compartments for silica gel. If the budget will not allow the installation of fiber-optic lighting at the time that the cases are built, leave space to put it in later. (If working with an existing case that does not provide separate access to the fixtures, have a curator or conservator oversee any maintenance and move exhibits as necessary for safety.) Many materials used in the construction of exhibit cases and mounts give off volatile organic compounds for varying lengths of time. These gases can cause damage to the materials on exhibit. Since a well-sealed case per- mits very little air exchange, it is important to keep the atmosphere inside as pure as possible.
Wood and wood products, adhesives, paints, varnishes, and gasketing are commonly used in the construction of display cases because they are easy to obtain and work with. Fabrics are often used to line cases and cover sup- ports. All of these materials give off gases, especially when they are new.
M AT E R I A L S T O AV O I D F O R C A S E I N T E R I O R S
Some woods never stop off-gassing; these include oak, a traditional and attractive construction material. All ply- woods and composition boards should be avoided be- cause the adhesives and other additives in them may be harmful. Manufacturers may change the composition of
SMALL EXHIBITIONS
these boards, so that a product that was safe last year may cause damage when it is reformulated. Try to avoid hav- ing any exposed wood on the inside of the case.
Oil-based or alkyd resin paints are unsuitable for case interiors, as are contact cements and rubber cement.
Plastics that should be avoided include polyurethane foams and polyvinyl chloride (PVC). Rubber-based ad- hesives or gaskets can give off sulfur as they degrade and cause damage. All these materials continue to give off gases indefinitely.
B E T T E R C H O I C E S
I N C O N S T R U C T I O N M AT E R I A L S
Acrylic glazing (Plexiglas) and glass are very safe choices, and they can make up a very large part of the case. Select glass shelves whenever possible; they don’t need to be lined.
Anodized aluminum, stainless steel, and powder- coated metals are good, as are high-pressure plastic lam- inates (e.g., Formica).
Acrylic, Teflon, and silicone adhesives and caulks are suitable, as are hot-melt adhesives. Acrylic or latex paints and water-based polyurethane finishes can also be used.
Any material that has to dry gives off gases until it has cured. Therefore, be sure to allow at least four weeks be- fore the exhibits need to be mounted.
C A S E L I N I N G S
Linings are used as part of the overall design, to cover unattractive parts of cases, and to serve as barriers. Light- weight exhibits can be attached to linings in various ways.
All materials used should be acid-free or inert in order to maintain good air quality in an enclosed space.
Remember that light-colored case linings reflect light. This helps illuminate the exhibits with less added light. Conversely, dark linings absorb light and may make exhibits harder to see.
Fabrics
The best choices for lining cases and covering mounts are undyed, unbleached cotton, linen, or polyester fabrics.
Cotton-polyester fabrics can also be used. They should be washed in hot water without detergent to remove any surface coatings. Avoid wool, silk and any fabric that has a discernible smell after washing it in hot water.
Paper Products
Acid-free paper is perfectly safe for library and archival materials on exhibit. Many types are available in roll
form, which is economical and permits installation with fewer seams. It is often used to cover the floors of cases or to wrap mounts and supports.
Barrier Products
When a less than ideal case must be used, stable barriers can be used to minimize off-gassing into the case and avoid direct contact with the exhibits. All the unsuitable interior parts of the case must be covered. Good barrier materials include polyester film (e.g., Mylar or Mellinex), four- or eight-ply acid-free mat board (“museum board”), and polyethylene foam sheeting (Ethafoam, Volara). All can be attached to vertical surfaces with double-coated tape such as 3-M no. 415.
Aluminum laminate vapor-barrier sheeting (e.g., Marvelseal) is a useful material that is impermeable to gases and moisture and can be bent or wrapped as needed.
It has a heat-sensitive adhesive on the back and can be ironed onto many woods. It can also be attached using double-coated tape.
Suitable fabric or paper can be used to cover the bar- riers and integrate them into the overall design of the show. This is especially important if polyester film is used as a barrier because it can cause numerous reflections and make it more difficult to modulate the light. When there are vertical panels, such as on the back of a case, a neater effect can be achieved by wrapping the decorative paper or fabric around the edges of the barrier sheet and taping it down to the back of the barrier. Then the whole panel can be attached to the side or back of the case.
Scavenger products that react with pollution in air- tight cases and neutralize it are a fairly new development in barrier materials. Activated carbon and zeolites are among the active ingredients. In general, it is better to rely on good air quality in the gallery and to use safe materials in the cases and frames.
Exhibiting Other Objects Together with Library or Archival Materials
Occasionally objects made of materials that are likely to give off harmful gases, such as rubber, are included in a show. If possible, do not put them in the same cases as paper-based materials, especially original photographs.
When they must be together, perhaps the questionable object can be in a small acrylic case within the larger case.
SMALL EXHIBITIONS