What are conservation or preservation mats and why should I use them?At My Personal Framer we prefer to use 100% conservation/preservation mat board.
A little about paper mats
The standard paper mats do not protect your art. They are non-purified wood pulp which can actually burn the paper they sit on. The acids in the wood pulp board reacts with light and air to discolor the art. Have you ever seen old prints that have a gold rim on the paper just inside the ma? This is caused by wood pulp acids attacking paper. Paper mat top colors are dyed and not pigmented. Dyes are very light sensitve and fugative. This means that the colors will fade significantly in short periods of time. Next time you see a framed piece and the mat looks dull and muted compared to the art-that is fading. The colors fade in order of Yellow, then Red, Blue lasts the longest and you often end up with a blue print and mats that don't match the art anymore.
Conservation and preservation mats
These mats are either cotton rag or wood pulp mats also, but they have had all the nasties removed from them and then they are buffered to be neutral. The top color of these mats are also pigmented. Pigmentation means that the actual mineral is ground and mixed with a ground to stablize and make them adhere to the surface.
from Wikipedia (Ok, this is the really technical stuff)Acidic vs. "acid-free"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. (I use the term Preservation or Conservation instead of "Acid-free" as there is some confusion about what constitutes "acid-free")
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:
I. 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.
II. 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.
III. 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 qualitypaper matboard available.
IV. 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 and the core is either recycled fiber. 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"-qualityfoamcore 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.
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs fromfluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.
Many materials selectively absorb certain wavelengths of light. Materials that humans have chosen and developed for use as pigments usually have special properties that make them ideal for coloring other materials. A pigment must have a high tinting strength relative to the materials it colors. It must be stable in solid form at ambient temperatures.
For industrial applications, as well as in the arts, permanence and stability are desirable properties. Pigments that are not permanent are called fugitive. Fugitive pigments fade over time, or with exposure to light, while some eventually blacken.
Pigments are used for coloring paint, ink, plastic, fabric, cosmetics, food and other materials. Most pigments used in manufacturing and the visual arts are drycolourants, usually ground into a fine powder. This powder is added to a vehicle (or binder), a relatively neutral or colorless material that suspends the pigment and gives the paint its adhesion.
The worldwide market for inorganic, organic and special pigments had a total volume of around 7.4 million tons in 2006. Asia has the highest rate on a quantity basis followed by Europe and North America. In 2006, a turnover of 17.6 billion US$ (13 billion Euro) was reached mostly in Europe, followed by North America and Asia.
A distinction is usually made between a pigment, which is insoluble in the vehicle (resulting in a suspension), and a dye, which either is itself a liquid or is soluble in its vehicle (resulting in a solution). The term biological pigment is used for all colored substances independent of their solubility. A colorant can be both a pigment and a dye depending on the vehicle it is used in. In some cases, a pigment can be manufactured from a dye by precipitating a soluble dye with a metallic salt. The resulting pigment is called a lake pigment.
Mounting: why is it important
Sally Dallas, Artisan Framer