What is "foxing"?  I was just reading in my PBS newsletter about PBS planning to acquire pictures that demonstrate water damage and foxing, but it didn't explain "foxing."

http://en.wikipedia.org/wiki/Foxing

This thread talks about foxing and there's a link to an image.

Foxing is a discoloration on paper, usually on the edges of books. It is caused by a reaction of the paper fibers with contaminants in the air. Usually it is the

acid in the paper (used to whiten it) that reacts. You can see it best on very old documents, a brownish color starts at the edges of the sheet and moves inward over time (very long time). It is a natural chemical reaction, usually shows up more the cheaper the paper is. On books it usually shows up on the top edge of the book, sometimes even works down to the inside a little in very old books. Sometimes shows up too on the insice front and back covers, where the paper reacts with the glue used to hold the text pages to the cover.

I don't believe PBS considers foxing a problem to make a book unpostable, but obviously water damage does.

wc