Foxing is the scattered brown or rust-coloured spotting that appears on the paper of older books. It is caused by a mix of damp, mould and impurities in the paper reacting over time, and it is especially common on nineteenth and early twentieth century paper, on endpapers and on the edges of the text block.
How to spot it
Look at the endpapers, the title page and the first and last few leaves, where air reaches the paper most. Foxing shows as small freckles or larger blotches, often heaviest near the edges. Light foxing is easiest to see under daylight; heavy foxing is obvious from across a room. A musty smell often goes with it.
What it does to value
Light foxing on an old book is expected and lowers the grade a little. Heavy foxing that reaches the text or plates, or foxing on a modern book where the paper should be clean, lowers value more, because buyers can see it at once and it tends to spread in poor storage. Plates and illustrations matter most: a foxed frontispiece hurts more than a foxed endpaper.
Keep books in a dry, steady room with air moving around them to stop it getting worse. Our guide to caring for collectable books covers storage, and book condition grading explains how foxing is described in a listing.