The types of mat board that are considered acid-free or archival are made of fibers that have had the acid chemically removed. Alpha cellulose -- wood fibers -- and rag board -- cotton fibers -- are the most common acid-free mat board types. These mats are made of 100-percent of their respective fibers and, when cut, have the same color throughout. Acidic mats are made of wood pulp and will fade and yellow over time. Archival mats are also stronger than paper mats.
Acid-free mats are bright and brilliant in color, with true hues. Acidic mats start out dingy and only yellow and fade the color more. Archival mats also have more color options than acidic mats and they will not fade or bleed. Though, ultraviolet rays will still fade both types of mats with exposure to the light unless the mats are protected by UV-protected glass.
Acid in the mat and in the air will, over time, eat away at the structural integrity of the mat, weakening its fibers. As the acid develops in the mat, the color fades and the beveled edge becomes darker. If using the mat for a piece of art or a sign that is valuable or that you want to preserve, acid-free is the best choice. If you are framing something that is for a temporary function or display, paper mats are acceptable.
A mat board directly touches your artwork, so it is important to choose the right type for the quality of art you have. Archival mats will not damage your picture, but it is important to also choose an archival tape to attach the art to the mat. Paper mats will not only yellow and eat away at your artwork, but the acid from the mat will often become glued to the artwork, ripping or destroying it when the mat is removed. Acid-free mats are highly recommended for original artwork, diplomas or anything that is of any value, whether for monetary or sentimental reasons.