In the custom framing industry, a mat is a thin, flat piece of paper-based material used in a picture frame, which serves as additional decoration and to perform several other, more practical functions, such as separating the art from the glass. (Wikipedia)
Decoration and Protection
The picture-framing mat is most commonly known by laymen for its use as additional decoration to enhance the look of a framed piece, sometimes in conjunction with a fillet or more rarely, liners made of wooden moulding with a cloth surface. Typically the mat or mats, if matched carefully and properly proportioned, serve to help draw the eye in towards the framed piece, or towards a particular key element of the piece.
Mats are fairly adaptable in the visual sense. Since they are typically quite thin (American-made mats are roughly 1/16th of an inch thick, for example), they are able to be cut to "stack" inside of a display, allowing for double, triple or quadruple matting, or even allowing for a fillet in between mats. Mats are available in numerous shades of every color, and can easily be found or altered to include further decorative features, such as a cloth covering (most commonly linen or silk, though mats with leather coverings or various other types of cloth covering are also available from some companies) or other decorative coverings or coatings (such as metallic coatings, or textured and patterned coatings that can include rice paper).
Since mats are made of paper-based material, they tend to take well to minor surface additions, including ink and paint; cloth-covered mats can also have objects such as pins, flags or cloth patches pinned or sewn to them, a technique frequently used in shadowboxing to avoid having to glue items to the backing. It is also possible to affix a small metal plate to the surface of a mat. Such plates are typically made of brass, and can also be affixed to the picture frame itself if the frame is made of wood.
In archival or conservation picture framing, mats have several important functions. One of the most important functions is that it separates the glass from the art or document being framed; this is primarily important because any condensation that develops on the inside of the glass can be transferred to the piece if they are not separated, resulting in water damage, mold or mildew. Photos should also be separated from the glass because the surface of photographs is particularly easy to damage, and may even separate from its original paper and stick to the glass if wet; for this reason, any framed photograph of value should be framed in such a way that the glass does not directly contact the photo. Additionally, some types of art, such as pastels or chalk pictures, can smudge easily and should be separated from the glass for that reason.
Another major function of the mat in archival framing (where the mats used are made of acid-free and lignin-free paper) comes into play during the mounting process. In archival framing, paper items are not typically glued down to the backing, as it prevents anyone in the future from being able to safely and easily remove it to replace damaged frames, backing or mats, and can make restoration of a damaged document or art piece more difficult. Typically, such items are instead held in place against the backing with mylar "photo corners" (tiny triangular pockets into which the corners of the paper are put). The added (though slight) weight of mats can help hold a piece in place while also helping to hide the backing and photo corners. In archival framing, the mats are not glued to the piece or backing, but are "hinged" to the backing with tape, though if more than one mat is used, the mats are typically glued to each other.
Acidic vs. "acid-free"
There are two main types of mat material: acidic, and "acid-free" (neutral ph). Older mats are typically acidic, because acid-free paper was not widely available or marketed until recent years; however, not all newer mats are acid-free, and one should always ask his or her picture framer about the acid content of the mats. The difference is important for the protection of the piece in most cases; acidic mats can cause what is called mat burn, brown marks that creep from the outside in on the displayed piece itself. While mat burn is sometimes reversible through cleaning the piece, cleaning may not be feasible if the piece was executed in water-soluble inks or paints, such as watercolor. Thus, it is important to know if the mats used are acid-free.
To determine the pH of an older mat with a white core, look to see if the core (visible where the mat has already been cut) has turned brownish or yellowed; if so, it is acidic. If the core has not changed color, one can determine the pH by using a pH tester.