Glazing
Glazing can be either plexiglass or glass. There are numerous types of glass available today, ranging from standard eighth inch thick clear to the highest quality conservation glass, which filters damaging UV rays. Regardless of whether plexi or glass is used, it is essential that the glazing material never come into direct contact with the artwork, since plexi and glass are both prone to condensation. Matting or spacers inserted along the frame rabbet (the lip of the frame upon which the glass rests) are needed to keep the artwork and glazing separate.

Glass - Standard clear is the most commonly used glass and is sufficient for most framing jobs. However, standard clear does have a slight greenish tint and possesses no UV filtering qualities. There are colorless glasses available, although they are more costly than standard clear.
To protect artwork of value, conservation glass is recommended. This filters out light in the 200-400 wavelength range. (This is the UV or ultraviolet range. Visible light falls in the 400-700 range.) By filtering out UV rays, artwork is protected from UV-induced fading, bleaching and yellowing. Conservation glass is available that filters anywhere from 97 to 100% of UV rays and is priced commensurate with its filtering capabilities.
Non-reflective (aka non-glare) glass is also available. Although its quality has improved over the past decade, non-glare glass still has a somewhat cloudy appearance. The further it is from the artwork - for instance, on a double or triple-matted piece - the cloudier it appears. We recommend this only when a piece of artwork will be hanging in an area that gets direct, intense sunlight.

Plexiglass - The advantage of plexiglass over glass is that it is lighter and is far more difficult to break. For larger pieces (over 36" x 48") we always use plexi, for weight considerations and safety reasons. Plexi also comes in non-glare and UV filtering and is the only glazing material that should be used when a piece is being shipped.
There are several disadvantages to using plexi. Because it generates a static charge, it must be cleaned regularly with a static-resistant plexi cleaner, otherwise it becomes a magnet for dust and dirt. Never use an ammonia-based glass cleaner - it will damage the plexi. Also, plexi should never be cleaned with paper towels, since they are abrasive and will create a fine network of scratches. Use a soft cotton cloth instead.
Perhaps most importantly, plexi should never be used over a pastel, charcoal drawing or any other loose medium. The static in the plexi will pull the pigment away from the paper.