The Importance of Selecting A Quality Mat
Matboards or more simply "mats", sit in between your image and picture frame and the glass. They help display the image but also keep the image in the correct place and provide a space for air circulation. Mats allow moisture in the atmosphere to be absorbed and released slowly and allow paper or the object to naturally expand and contract with changes in temperature and humidity. Artwork is also kept away from direct contact with the glass, very important in the case of photo emulsion and pastels which can adhere to glass causing irreversible damage.
Firstly mats are comprised of layers of paper laminated together. The top papers and core of mats are made from either unpurified wood pulp, purified alpha cellulose wood pulp or cotton linter pulp (from the husk of a cotton seed).The primary constituent of most paper is cellulose fibre sourced from plant material. The longest and strongest of cellulose fibres which make for the best structure and longevity are "alpha" cellulose. If not removed in the pulping process, lignin (wood pulp), waxes and other non cellulose components break down, especially in the presence of ultra violet (UV) light and moisture, releasing those unwanted acidic by products.
There are many grades of mat. If possible, an acid free mat is the best choice. The discolouration often seen as a stained looking mat with a brown bevel is a good indication of an acidic mat and is referred to as "acid burn". Your art will also turn brown and become brittle over time.
"Acid free" can be a very loose term. Not all mats are created equal and it is useful to know what you are getting for your dollar. The mat may only have an acid free coating or be acid free at the point of manufacture! Often buffering agents such as calcium carbonate (chalk) are placed in the pulp, but when the buffering agent is saturated, the acidification process continues.
Decorator grade boards are made with lower grade wood pulp with short fibres and may only be acid free at the point of manufacture. These are often called white core boards. The dye based surface papers are often not colour fast and along with plastic frames and nylon cords, make supermarket and chain store frames a recipe for disaster. These boards are not suitable for long term framing.
Conservation grade boards use alpha-cellulose pulp and are buffered with calcium carbonate or aluminium silicates to ensure protection from the destructive effects of acid migration from the matted work (sometimes the art paper is not acid free either!) and from pollution in the environment. These boards meet the minimum requirement for long term framing without harming the artwork.
Museum grade boards with the longest and cleanest alpha cellulose fibres and cotton rag board (made from the husks of the cotton seed) are the safest board. Museums and galleries will only use 100% rag mats.
Finally, a quick note on foam core. Most foam core has an acid free coating. However, only archival foam board has an acid free core as well.