The term foxing is used to denote the reddish or tannish spots, speckles, or bloches that discolor the paper of some older books. Foxing has been variously attributed to a micro-organism, iron particles in the paper, a high acid content in the paper, or a fungus reacting to metallic residue or other conditions of the paper.

Some collectors use the term foxing for all discoloration, but others find the terms tanning or toning useful. Tanning (or toning) refers to the uniform yellowing or browning of high acid content paper. It also can refer to areas that have discolored due to contact with glue, bookplates, newspaper clippings, or other non-organic material.

Endpapers, plates, and the pages facing plates appear to be particularly vulnerable to foxing, although foxing can occur on any area of the book, including page edges.

Foxed pages.

The picture at left shows a heavily foxed page, with numerous small, irregular specks.

Foxed pages.

The foxing shown in this picture appears to be fungal in origin.

Tanned book pages.

Tanning tends to start on the outside of the pages and progress inward over time, often creating a brownish or yellowish border around the page edges. In heavily tanned books, such as the one at the left, most of the page may be tanned. Many older paperbacks become tanned and brittle due to the acid content of their papers.

Tanned end papers.

Frequently, the end papers of a book are made from a different, more acidic paper than the rest of the book and they will become tanned while the interior pages remain unmarked. Often this tanning of the end papers appears worst in the places where glue has been applied to the back. Items like bookplates and newspaper clippings are frequently placed just inside the covers, and such items may cause tanning in those areas where they touch the end papers.