There are two types of mat material: acidic, and “acid-free” (neutral pH). Most old mats are typically acidic, because acid-free paper was not widely available until very recently. While most newer mats are acid-free, there are some that are not neutral pH and may still contain acid.
The difference between acidic and acid-free matting is important for the long term protection of your piece because acidic mats can cause what is called “mat burn”, brown or yellow marks on the beveled edge of the mat and the displayed piece itself. While mat burn is sometimes reversible through cleaning the piece, cleaning may not be feasible. Always ask your SF Framing design consultant about the acid content of any mat if the desired life of the piece being framed is more than 75 years.