While any piece of glass or plastic will protect a framed picture from casual physical damage, conservators and glass manufacturers developed museum-quality glass to provide the best possible, long-term protection for museum objects.
Museum-quality glass protects art from damage such as fading or brittleness caused by ultra-violet (UV) rays. It also reduces glare when a work is viewed from different angles, minimizing the mirror-like effect of typical glass and improving the intensity of colors and details.
Museum-quality glass is a 2 to 3 millimeters thick piece of glass with a UV coating, sandwiched between two layers of anti-glare optical coating. It can be used when displaying any object that may be damaged by UV-light, including artwork, photography, documents, fabrics and crafts.
While regular picture glass blocks about 40 percent of UV rays and reflects about 8 percent of light, museum-quality glass blocks 98 to 99 percent of UV rays and reduces glare to 1 to 2 percent. Though sometimes used interchangeably, museum-quality glass is not the same as conservation glass, which will protect against UV rays or reduce glare, but not both.
Museum-quality glass costs several times more than regular picture glass. Prices vary greatly depending on size and shipping destination, growing exponentially as the dimensions increase. The higher cost must be weighed against the value of the artwork and take into consideration how much UV light it will be exposed to when displayed.
In their "Guide to Preservation Matting and Framing," the Library of Congress notes that UV coating does not protect against damage from visible light, which can also damage paper and cause color or ink to fade.