Q. Some of my 10- to 15-year-old watercolors have foxing (a discoloration) along the beveled mat edge, but not anywhere else. I assume this has happened because the acid core of the matboard is leaking. Would coating the entire mat with casein–including the beveled edge–seal this acid core?
Elouise Gardiner Giles
A. Its most likely that your paintings suffer from mat burn, which is caused by the acidity in the mat moving into the paper. This leads to the paper becoming brittle and discolored with a reddish brown hue in that particular area. (Foxing is usually due to impurities in the paper and the results are scattered across the surface.) And unfortunately, trying to seal the mat from acid transfer is futile. Instead, your best bet is to replace the aged mat and backing board with an acid-free rag matboard, often called museum board. You can also get colored rag boards, but the most common is white or cream. You can then cover the mat burn with the new mat.
Ross Merrill is chief of conservation at the National Gallery of Art in Washington, D.C.