There are two main types of mat material: acidic, and "acid-free" (neutral pH). Older mats (wood based paper) are typically acidic, because acid-free paper was not widely available or marketed until recent years. While most newer mats are acid-free, there are some papers that contain acid and one should ask the picture framer about the acid content of the mats if the desired life of the piece being framed is more than 75–100 years.
The difference is important for the long term protection of the piece because acidic mats can cause what is called mat burn, brown marks that creep in from the outside onto 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 if the piece is to be preserved for a long time.
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.
There are several categories of mat board and they are all separated by the level of protection offered the art work or artifact being framed. While some say that acidic framing materials should be avoided for all but the most temporary frames, it is not safe to say that all "acid-free" mats are recommended for long term preservation use. The hierarchy of mat board quality is as follows:
Museum Board - The highest quality material available. It is constructed of 100% cotton fiber, is Archival and will protect and preserve the contents of a frame. While it is the most expensive material available, the difference in actual material costs relative to the cost of framing is minimal.Museum Mat or Rag Mat - Still a good quality choice for conservation, it is constructed of cotton linters (short cotton fibers) and cellulose (wood pulp) middles. The cellulose is a less expensive raw material but offers sufficient conservation properties for most works.Conservation or Archival Mat Board - Constructed of 100% pure high alpha cellulose (wood pulp) and treated to be inert for up to 300 years. This is the highest quality paper matboard available.Acid-Free or Acid Free Lined - This material is usually lined with a wood based liner on one or both sides that has been treated to prevent "short term" acid burn - atop a recycled fiber core. Eventually the acid in the core will leach out to the surface which can harm the artwork.
Caution must be exercised in selecting the type of framing desired. Art work that is desired to last long term (more than 75 years) can be damaged by improper mat boards that are used intentionally to lower cost. However, non-archival quality mat boards may be suitable for a photographic print, laser print, etc. that is not meant to last long term. Additionally, prints made with traditional chemical processing of photographic film (i.e. dark room development), as opposed to computer printing, are already slightly acidic by nature and therefore are much less likely to be damaged by non-archival mats.
In addition, correct "conservation" framing includes all components, not just the mat board used directly behind the glass. Until recently, there were no truly "archival"-quality foamcore boards available, though a number of foamcore brands exist with buffered surfaces and the Nielsen Bainbridge company now produces one that is claimed to both block the intrusion of airborne pollutants and to avoid the problem of outgassing that non-archival foamboards may fall prey to; for this reason, and due to many smaller frames' shallow depths, it is not uncommon to see mat boards used as backing for a picture frame as well, though foamcore and mounting boards tend to be stiffer. It is also important, if long-term preservation is of concern, to make sure the framer is using good conservation framing technique.