Bad quality framing materials like mats and backing have lignin in them, a component of the trees that the paper was made from. Unfortunately, lignin is acidic and reacts with light to cause “burning” of other paper with which it comes into contact. You will often see a yellowing of the surface paper of the mat and the art, especially along the bevel of the mat. The mat out-gasses along the bevel, and these gases interact with light to discolor the art. Bring your art to us for evaluation and possible upgrading of your mat, backing, and glass to conservation quality materials.