Foxing is a term describing the age-related spots and browning seen on vintage paper. while the exact cause of the fungal growth is uncertain, it is best to avoid humidity and acidic papers or mats on the piece.  While looking through your old vintage artworks, if you find discoloration of any kind bring them into Custom Frame Services and we will do our best to repair what we can and preserve the piece carefully.
For more information on foxing, mildew, and restoration of artwork please visit these links.
Foxing and Mildew Information
Photo restoration information

Foxing on Old Photo

The circles show where the foxing has started to damage the piece