Foxing is an age-related degradation of paper that causes rusty brown spots or blotches on the pages of books and prints. The exact cause is debated among conservators, but it is generally attributed to fungal growth or the oxidation of iron impurities in the paper pulp. It lowers the grade of collectible books and requires chemical bleaching or specialized conservation to remove.
Heard on the job
Check the endpapers on that first edition for foxing before we grade it near mint.