The Archival Method
True archival framing dictates that the mounted image (photos, prints, etc.) must be able to be cleanly removed from the mounting surface. All products and materials that surround the image(s) must be acid free and lignon free.
Adhesives should be kept to none to minimal in regards to mounting the image to the mounting surface. The basic idea is to prevent the framing process from damaging the image in any way. In the past this was next to impossible as the archival materials we have today simply did not exist.
All things age due to acids and pollutants in the air and walls (and even in the art itself) as well as UV damage from the sun and lighting within your home. And while we cannot control these variables, every step we take in our archival framing acts as a buffer from these things - significantly extending the life of your piece.
Archival matting refers to the use of mat materials that not only do not harm your artwork but actively combat degradation due to outside factors as well as pollutants inherent in the art itself.
Archival grade mats are cotton based rather then paper based and therefor do not contain the same acid that paper pulp does. This is important because acid in the mats can cause negative effects such as acid bleed and foxing.
It is also important that art, particularly art produced on or with paper, not sit directly against the glazing (glass or acrylic). If a piece has direct contact with the glazing it cannot be considered archival grade framing because the paper or media may become stuck to it and could not be removed without ruining the art.
All mats serve to pick the art up off of the glazing. However, if your mat is not archival grade, the damage it can do will negate the damage the glazing could do.
UV Blocking Glazing
All light sources give off some degree of UV radiation. This radiation can and will damage most art and objects it encounters.
UV treated glazing effectively blocks this radiation from reaching anything behind it. In the past this treatment applied to the glazing had the effect of making anything behind it look foggy. Today's UV treated glazing does not impede clarity and some varieties even cut down on glare.