This is post #24 of #Blogchatter A2Z. This includes blogging everyday in April for 26 days, except on Sundays. What’s special about it is that every day’s post will be corresponding to each letter of the alphabet. 

My theme for 2021 is Bookish TriviaIf you liked this post, don’t forget to “roll” me on Blogchatter’s website! All my #BlogchatterA2Z posts 2021 can be found here.

(I couldn’t find a bookish word that starts with “X,” so I chose to write about foxing.)

Foxing in books
Source: Photo by Irina Savanova on Unsplash

Foxing refers to the disfiguring small, irregular and sometimes fluorescent reddish-brown spots that form on paper and paper products such as certificates, postage stamps, photographs, old books and currency notes, especially those produced between the 15th and 20th century.

It occurs due to a combination of chemical, physical and biological factors. However, it is understood that an attack by microbes begins the degradation of paper.

For book collectors, foxing reduces the value of the book but this age-related damage does not destroy the integrity of the pages.

Leonardo da Vinci drew his self-portrait with red chalk on paper, which is strongly affected by foxing spots. It is kept in the Royal Library in Turin. Conservationists are attempting to study the types of fungi infesting the paper to determine how to treat it without damaging the portrait.

Foxing depends on the intrinsic properties of the paper. Some paper will never develop these rusty spots. Other papers with the spots are called “foxed” papers.

Books from the 18th century and 19th century are especially susceptible to foxing.

What causes foxing in books?

There could be two causes of foxing:

Growth of fungi, mildew and bacteria

Excessive humidity, usually above 75%, encourages the growth of fungi and mildew on the paper. The fungi feed on the paper and any dirt or organic material on its surface, such as food stains, finger marks or squashed insects. This, in turn, leads to foxing.

Types of fungus that grow on paper are Aspergillus, Cladosporyum, Penicillium, Pithomyces, Ulocladium and yeast.

Sometimes, bacteria from the Bacillus genus have also been found in foxing spots. Conditions in paper manufacturing mills are conducive for the growth of these bacteria.

Montemartini Corte et al. (2003) discovered that foxing spots contain around 58% fungi, 30% yeasts and 12% bacteria.

Foxing on books
Foxing spots

Oxidation of metal impurities

Foxing could be a result of oxidation paper due to the presence of iron, copper or other metal impurities present in the wood pulp from which the paper was made.

Iron ions Fe(II) and Fe (III) catalyze the oxidation of paper and cause the foxing stains.

Damp air causes iron impurities to turn into rust.

The name “foxing” is probably derived from the chemical ferric oxide used to coat paper or the chemical iron oxy-hydroxide (FeOOH) that is rust.

It is also possible that a combination of factors is involved in the development of foxing in books.

How to fix foxing

The foxed document is cleaned with a solution of 5%-10% bleach, plant extracts, or fungicide to remove the stains. This process could damage the ink or the paper, so it’s usually better to accept the blotches as an aesthetic problem.

Conservationists physically remove visible mold from paper by wiping, HEPA vacuuming or gentle brushing.

Microwaving or freeze-drying may also be used to eliminate the fungus. However, this method only serves to prevent the spores from reproducing. It may still be harmful for humans and animals and cause allergies.

Foxing books
Source: Photo by Jessica F on Unsplash

Laser treatment at particular wavelengths can remove foxing stains.

Thus, an effective mold remediation strategy involves physically removing the mold from the paper, treating the reddish-brown stains and then fixing the conditions that encouraged the growth of the mold.

Another method is to scan the paper and process it with a high-level image processing program. Thus, the document becomes digitized, leaving the foxing behind.

How to prevent foxing

The best way to prevent foxing is to store books and documents properly in a cool, dry place.

In libraries and museums, a 50% humidity is maintained to prevent the onset of foxing.

If you live in a humid place, you can use a dehumidifier to protect your books.

Use acid-free paper to wrap old or rare books because foxing changes the pH of paper and the extra acid will cause more damage.

Here’s a video about how mold is removed from library books:


  8. Pinar G, Tafer H, Sterflinger K, Pinzari F. Amid the possible causes of a very famous foxing: molecular and microscopic insight into Leonardo da Vinci’s self‐portrait. Environ Microbiol Rep. 2015 Dec; 7(6): 849–859.

5 responses to “Foxing”

  1. Thank you I’ll check this out. 😊

  2. Rare or valuable books are taken to a book conservator for careful treatment. For regular books, you can try this:

    1. Thank you I’ll check this out. 😊

  3. Though I have seen these spots, never heard this term before. Thank you for enlightening us.

  4. Have seen these spots but never knew what they are called. Where does one take the books to be treated if these are observed at home?
    Deepika Sharma

    1. Rare or valuable books are taken to a book conservator for careful treatment. For regular books, you can try this:

      1. Thank you I’ll check this out. 😊

  5. […] tanning is used interchangeably with another term, foxing. But they are different phenomena. Foxing typically involves the work of fungus to create spots on […]

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