Below: Note the yellowing of the bevel in the mat, and the "burn" on the artwork where the mat had been. This is caused by the acid in the paper mat's core. Although the surface and backing papers are buffered acid-free, the core is still in contact with the art and can lead to the "burn," seen here.
The
sample on the right is an acid-free mat, the left, a regular mat.
The tear shows the core of the matboard; where the acid-free mat will remain
white, the regular mat will continue to darken.
