Acid Free Mats & Backing
Over time, framed art can be damaged by paper mats and foam core backing. Obviously, this is not an ideal situation. There are tell-tale signs of acid burns from mats and backings. If you remove the mat and see a thin brown line around the artwork, or if you see that the bevel of the mat has turned brown, it is an acidic mat. If brown spots are appearing in your artwork, the foam core backing could be acidic (you could also have mold in your artwork).
Today, framing suppliers offer conservation grade acid-free paper mats and foam core backing. They are pH buffered to neutralize the acidity that naturally occurs. This means that your art can sit comfortably inside its frame without worry.