This will significantly reduce the value of your art. The art should never be allowed to touch the glass. A double window mat is recommended to provide sufficient air-space, thereby reducing the potential for the growth of mold. However, if a single window mat is used the art can be held away from the glass with a small spacer tucked in the edge of the frame. The glass and mounted art can be sealed around the edges with gummed linen tape to prevent pollution from penetrating the frame.
Light is potentially the most damaging element affecting antique art.
This damage is serious and often irreversible. Therefore, conservation glass which is manufactured with a UV filter is highly recommended. These products reduce penetration of UV light to less than three percent. Alternately, UV filtering acrylic sheeting can be used to minimize the harmful effects of UV light. However, these glazing materials scratch easily and require special cleaning techniques.