 Additionally, some types of art, such as pastels or chalk pictures, can smudge easily and should be separated from the glass for that reason.Another major function of the mat in archival framing (where the mats used are made of acid-free and lignin-free paper) comes into play during the mounting process.In archival framing, paper items are not typically glued down to the backing, as it prevents anyone in the future from being able to safely and easily remove it to replace damaged frames, backing or mats, and can make restoration of a damaged document or art piece more difficult.