The treatment of the back of the framed artwork varies widely, from usually nothing in the case of oils, to the frequent use of foam-core boards(available in archival acid free versions both in black and white) and other backing boards to provide support, or backing paper or "dust covers" to keep dust and insects out.
The use of backing boards is common with watermedia and other art on paper.
If the paper (or other media) were to touch the glass directly, any condensation inside the glass would absorb directly into the art, having no room to evaporate. It causes art sticking to the glass, mildew or mold spore growth, and other ill effects.
A picture frame may be of any color or texture, but gilding is common, especially on older wooden frames.Some picture frames have elaborate molding which may relate to the subject matter.Picture framing glass may be treated with anti-reflective coatings to make the glass virtually invisible under certain lighting conditions.When a picture frame is expected to be exposed to direct sunlight, or harsh lighting conditions such as fluorescent lights, UV filtering may be added to slow down the photocatalytic degradation of organic materials behind picture framing glass.Usually paper dust covers will be inexpensive craft paper or heavy duty archival papers Plique-à-jour picture frames, made of enamel by Bulushoff, are among the most expensive frames in the world.
Picture frames are generally square or rectangular, though circular and oval frames are not uncommon.The frame along with its mounts protects and often complements the artwork.Art work framed well will stay in good condition for a longer period of time.One of the earliest frames was a discovery made in an Egyptian tomb dating back to 2nd century A. in which a fayum mummy portrait was discovered at Hawara still within its wooden frame.This finding suggests the mummy portraits may have been hung in the owners' homes prior to inclusion within the funerary equipment.The lip extends usually about a quarter of an inch past the edge of the rabbet.