Selecting Exhibit Case Glazing

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Careful consideration is required in the selection of an exhibit case's glazing material, because it directly impacts several important preservation features.

What conservation function does the case glazing serve?

Exhibit cases have transparent panels through which the display objects are viewed. These transparent or glazed panels are made of rigid glass or clear plastic. This glazing affects not only the aesthetics of the display, but also the level of protection provided exhibit objects. Case glazings should be carefully selected because they can provide varying degrees of protection. For instance they can serve as a barrier:

• to harmful light radiation, pests, airborne soil and pollutants;
• to adverse environmental conditions and heat gain inside a case;
• to visitors and incidental touching, vandalism or theft;
• to maintain controlled atmospheres in cases which serve as microclimates.

Exhibit case glazing should not add to the deterioration of display objects through the release of harmful emissions or by physical damage when the glazing is damaged. Glazed panels should be inert and the surface exposed within a case should be free of unknown coatings or films.

The strength of the glazing is a major security concern; weak panels may be easily broken or entered. Glass panels made of annealed, tempered, and chemical or heat-strengthened sheets offer greater resistance to breakage. Glass laminated panels generally have resins incorporated into the layers to hold broken shards together that serve to protect object contents.

What characteristics are desirable for glass glazing?

The most successful glass panels are specialized glass types manufactured by lamination (interior incorporation) or application of special surfaces. The strength of the glass and its resistance to breakage are determined by manufacturing technique.

• Plate glass: rolled, ground or polished sheet glass.
• Safety glass: plastic sheeting laminated between sheets of glass.
• Laminated glass: sheets fused together by heating.
• Annealed glass: heated after production and then cooled slowly to increase strength.
• Tempered glass: heated and then cooled gradually to increase strength.

Additional important characteristics of a glass product are its energy transmission and clarity, and the effect of surface coatings.

• Energy transmission: Common glass transmits all visible and infrared light wavelengths, and most of the ultraviolet light wavelengths. Heat buildup due to visible and infrared radiation can cause an unacceptable rise in temperature within the case; some glazing has additives or tints to reduce thermal permeability.
• Coefficient of linear expansion: Light heats up glass, causing it to expand. This expansion and the converse contraction is the coefficient of linear expansion (CVE). The relatively low CVE of glass makes it appropriate for a variety of uses.
• Clarity: Glass clarity relates to its ability to absorb light. For a glass to be perfectly clear, the frequencies of its molecular vibrations must be in resonance with all visible light wavelength frequencies. Impurities in the glass may result in a light green color. Clear glass tends to be more expensive.
• Visible light reflection: Glare from exterior light sources interferes with viewing into the case. Glass with special matte finishes or made through special production methods address this problem.
• Specialized coatings: Optically coated glass that absorbs the most damaging wavelengths of light is available. Coatings can also control reflection from glass and increase its resistance to scratching; some of these coated glasses have a tint.

What characteristics should be considered when selecting plastic glazing?

Most plastic exhibit case glazing is made of acrylic which offers the crystal clarity of glass. Acrylic sheet is up to 17 times more break resistant than glass, yet it is half the weight. Polycarbonate glazing can also be used; it is a tough, transparent thermoplastic characterized by high-impact strength and high-softening temperature. Variables to consider when selecting a plastic glazing material are:

• Energy transmission: Thermal expansion and contraction cause dimensional change in plastic sheets. Different temperature and/or humidity conditions on the inner and outer surfaces of a plastic sheet cause it to bow towards the direction of higher temperature and/or humidity. Different thicknesses are available, however, which can influence the strength of the sheet. In its normal grade, acrylic plastic may have less UV absorbing properties than glass; acrylic products that filter all UV are available and are recommended for exhibit cases. Polycarbonate is a natural filter for UV. Compared to glass, some plastics offer equal or better reduction of noise frequencies which can cause vibration to objects in a case.
• Clarity: Traditional UV filtering acrylic glazing that absorbs some visible light has a very faint yellow tint. Colorless plastics are available; some transmit slightly more UV. Acrylic materials must be sufficiently rigid and flat to minimize optical distortions. Lower rigidity can cause the material to bend and foreshorten as a result of weight and pressure.
• Electrostatic Charge: Plastic glazing experiences electrostatic charges that attract dust from the air, or friable surface particles from objects placed too close to plastic glazing.
• Moisture Permeability: Plastics are more permeable than glass as a general rule. Glass is used most frequently in microclimate case designs, however, rigid acrylic of thicknesses ⁄ inch or greater are proven to have relatively low permeability to water vapor for display purposes.

Are there guidelines for when to use glass versus plastic glazing?

Whether to use a glass or a plastic glazing for an exhibit case depends on many factors. A few are discussed below. Perhaps the most important are initial costs, type of production facility available, and size and shape of the case.

• Production facilities: Cutting, polishing and fitting glass requires a high degree of skill and more expensive equipment than does plastic glazing materials.
• Length of display: Glass will generally last much longer than plastic; over time plastic glazing tends to become scratched and may craze or become more yellow in tint. Inappropriate cleaners used on acrylic cause crazing.
• Fire: Plastics melt at temperatures typical of even small fires. The potential of toxic fumes and melted residues created by burning plastic may drive the choice to use glass glazing.
• Static: The buildup of static electricity on plastic glazing makes it an inappropriate choice for certain types of cases such as those holding fine particulate and artwork using pastels.
• Appearance: In addition to the yellow tint of some plastic glazing products, the appearance of glass and plastic can be very different. While plastic glazing does not have the light reflective problems of glass, its clarity is usually not as good as glass, especially as time goes by and scratches accumulate on the plastic.
• Case design: Plastic glazing can be bent and formed in a variety of curved profiles. The ease of adhering plastic glazing to itself makes it the primary choice for five-sided bonnet case designs.
• Weight: Plastic panels are more lightweight than glass, another reason bonnets are usually made of plastic. Large panes of glazing are usually made of more rigid glass.
• Security: Special security glass provides optimum protection against vandalism and unlawful entry into a case.

Products, Manufacturers, and Suppliers

Mention of a product, manufacturer, or supplier by name in this publication is for information only and does not constitute an endorsement of that product or supplier by the National Park Service. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.

UV Absorbing Glass Products

• Butacite (E.L. Du Pont de Nemours & Co., Inc.); Description: Butacite is laminated glass sheet viscoelastic properties provide dampening effect on noise; maintains color and clarity. The PVB inner layer adheres broken glass eliminating danger from loose shards.
• Tru-Vue Conservation Series, Museum Glass (Tru-Vue); Description: Conservation Series is UV filtering glass, clear or reflection control versions. Museum Glass is an optically-coated, reflection-free product which increases the transmission of light through the glass.

Transparent Acrylic Sheet

• Acrylite OP-2, Acrylite OP-3 (Cyro Industries); Description: OP-2 is a cell-cast sheet with a higher molecular weight than OP-3 and thus is stronger; no glare resistance, no visible light filtration, 100% filtration of UV wavelengths below 390 nm and 98% between 390 nm. OP-3 P99 is an extremely clear extruded sheet with non-glare properties; an abrasion resistant coating; no visible light filtration, 100% filtration of UV wavelengths below 390 nm and 98% between 390 to 400 nm.
• Plexiglas Acrylic Sheet, Plexiglas UF-3 (Rohm and Haas); Description: Plexiglas UF-3 is widely used for museum glazing and exhibit cases because of its ultraviolet filtering characteristics.

Polycarbonate Sheet

• Lexan Sheet (ICI Lexan); Description: Lexan is polycarbonate which has inherently high impact strength; it filters UV wavelengths; and flammability resistant sheets are available.