It should be stiff enough to hold the contents of the frame in plane vertically without bowing. To minimize the effects of pollutants and changes in relative humidity, some conservators recommend placing a layer of impermeable material, such as Mylar or a plastic-aluminum laminate, between the backing board and the work of art or outside the backboard. Glazing Materials Glass or plastic incorporated in a framed work of art, called glazing, is used to prevent physical damage and to keep dirt and dust from soiling the surface. Other plastics may be unstable and should be avoided. The use of ultraviolet filtering glazing materials will alleviate fading of colors and some other types of damage.
Acrylic is lighter than glass and is shatterproof.
It tends to scratch, but scratch-resistant grades are available. It tends to take on static charge, so glass, not acrylic should be used over powdery materials such as pastel, charcoal, or chalk. Even though ultraviolet light is responsible for most of the damage to works of art on paper, visible light will also harm artwork. Knowledgeable collectors limit the illumination of their works of art on paper, both in the quantity of light and the length of exposure. Further Reading Balicki, Alan.