Foxing marks on paper edges and book pages in Hypatia by Dora Russell ca 1925 (C) Daniel FriedmanFoxing Stains on Books, Papers, Photographs: cause, cures, prevention

  • FOXING STAINS on books & papers - CONTENTS: Definition of Foxing Stains on Books, Papers, Photographs: cause, cure, prevention. What causes rusty stains or foxing marks on books and paper? How experts examine foxing stains on paper product. Definition & basic chemistry of foxing stains on books, papers, photographs. What are the Foxing Fungi? fungi identified as growing on or in paper materials included the following. How to remove & prevent Foxing stains: Cures for Foxing on Paper, Books, Photographs
  • POST a QUESTION or READ FAQs about the cause, cure, & prevention of reddish-brown foxing stains on books, papers, photographs & similar materials

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Foxing stains on books, papers & photographs:

This article defines book & paper foxing - those reddish-brown stains found on some old books, papers, photographs, and other paper products. We explain the causes of foxing stains, the chemistry and mold components of foxing, and we describe what foxing looks like, how it is cleaned from books, papers, or photographs, and how foxing can best be prevented by book and paper restorers and paper conservators.

We provide and cite an extensive list of authoritative references about foxing: cause, cure, and prevention.

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Foxing Stains on Books, Papers, Photographs: cause, cure, prevention

Foxing stains on book page edges (C) Daniel Friedman

[Click to enlarge any image]

Article Series Contents

Question/Comment: what causes rusty stains or foxing marks on books and paper ? Foxing definition, chemistry, causes, treatments or removal methods, and prevention

Are foxing marks on books and paper always caused by mould or are other factors also at work?

Also, I have a couple of books with shiny pages that have become sticky during a humid summer and have developed orange lines along the edges of some of the pages.

Is this a reaction of the acid in the books seeping through the cut edges or could mould be a factor? - Rachel 9/2/202

At left our photograph of book foxing illustrates foxing stains on the page edges of the book discussed in this article. Photograph taken in direct sunlight.

Reply: iron oxide, fungi and yeast associated with foxing on books and papers: causes, cures, prevention


Your surmise that more than mold is at work in the development of local discoloration or reddish-brown foxing marks on paper and books is correct in that at the core, foxing is caused by exposure of those materials to high humidity, and the brown stains characteristic of foxing marks, according to some researchers are typically found to contain high levels of Iron Oxy-Hydroxide (FeOOH) in one or more chemical forms.

However as paper expert Paolo Calvini corrects the record (in detail below),

  1. Iron Oxide (FeO) has nothing to do with foxing. It represent the "scaling" of Iron and is generally inactive on paper documents.
  2. Perhaps the author confuses it with Iron Oxy-Hydroxide (FeOOH) responsible of "rusting". Rust can be easily detected, and can not properly labeled as foxing.

Paper chemistry as well as chemistry of inks and other materials comprising the book, paper, stamp, or other foxed document are also important components in the foxing problem. (Carter explains that the general yellowing of some paper products is distinct from the localized red-brown stains associated with foxing. [36][37])

Dr. Calvini, generously commenting on this article continues:

  1. The chemical origin of foxing is thought to be due to the fact that that Iron Ions [i.e., Fe(II) and Fe(III)] catalyse the oxidation of paper, yielding the yellow-brown stains. These stains belong to the organic chemistry of cellulose and gelatine.
  2. Perhaps we were not clear in our articles, but we mean that "sometimes" the (dead) fungal bodies can be mechanically removed, thus explaining their absence in several SEM analyses. The FTIR analysis shows that fungi stained the fibres, and these stains can't be removed mechanically.

How experts examine foxing stains on paper products

Foxing example on book paper (C) Daniel FriedmanFoxing and other stains on paper are typically examined by several methods including

  • Direct visual examination in daylight or under daylight-frequency artificial light (photos included in this article) At left our photograph illustrates extensive foxing stains on the right-hand paper just inside the cover of the book used for these foxing illustrations. The role of chemistry of glues and other materials in the selective appearance of foxing stains in books and on other papers might be inferred by the foxing stain pattern shown at the right side of our photo.
  • Microscopic examination using transmitted light microscopy (photos included in this article)
  • Fluorescence, detected by exposure to UV radiation (“Wood light” instrument: Vilber Loumart, VL-15L, 15W–365 nm, tube power 30W). The procedure adopted follows the protocol presented in Zotti et al (2008)). [17a]
  • Light spectroscopy, or Fourier Transform Infrared (spectroscopy) - (FT-IR or FTIR), a term applied to a variety of types of spectroscopy including optical spectroscopy, infrared spectroscopy (FTIR, FT-NIRS, FTIR spectroscopy) - cf Zotti 2008 [17a]
  • Culture media might be used to further study some fungal species collected from a surface but with the caveat that most fungi will not grow on culture media and those that do may appear morphologically quite different from their growth on other substances (see MOLD CULTURE TEST KIT VALIDITY)

Brown stains on various paper goods may also be found to contain conidia and hyphae of fungi, but keep in mind that many of the fungi commonly identified as growing on or in paper products (listed below) do not themselves produce darkly-pigmented fungal materials. Some experts add that bacteria play a role in paper deterioration [29] (and possibly foxing), and indeed, fungi and certain bacteria are about the only natural organisms in the environment capable of breaking down cellulose.

Our own forensic microscopy lab work as well as that of other experts confirms an interesting diagnostic observation: the color and appearance of stains (black, brown, white, etc) that we see macroscopically or with the naked eye can be quite different from the color and appearance of the staining materials (often fungi) when examined at high magnification under the light microscope. [16]

In any case, and for those rusty reddish-brown foxing stains on paper, it appears that both moisture and several species of mold are at work together, conspiring in the accumulation of iron oxide stains or foxing stains on paper, as we explain in more detail here. It also is likely that paper and ink chemistry, such as pre-existing levels of iron in the paper, inks, bindings, or other materials is important in the development of foxing stains.

Definition & basic chemistry of foxing stains on books, papers, photographs

It is useful to note that the term foxing, used to describe brown or reddish brown discoloration appearing on books, papers, some photographs, derives its name from The F and Ox in Ferrous Oxide, or iron oxide deposits that are attracted to areas in the paper substrate. Exposure of a book or other paper materials to water or more often high humidity over the materials' life is the key or gating factor in the development of foxing stains.

This same exposure to humidity is also a determining factor in the development of fungal growth in or on books and papers as well as on other materials such as book bindings, glues, cloth covers, and of course on other indoor building materials and surfaces.

Biotic foxing on paper is indeed a widely-recognized book and paper conservation concern. [1][2][3][4][5][6][7][8]9][10][11][12][14] Yet according to at least some of those experts, the biotic or even chemical origin of these stains remains unclear.

What do foxing stains look like on books or paper?

Foxing stain macroscopic photo (C) Daniel FriedmanI'm not an expert book or paper conservator, but our photo (above left) is indeed a typical example of modest foxing marks or stains on both the page edges and within pages as well. The example is from our copy of Hypatia or Woman and Knowledge, by Dora Russell, third impression, printed by Mackays Ltd., Chatam, London, in the mid 1920's.

Examining small brown spots of "foxing" stains from a sample page macroscopically we observe a classic fungal colony pattern - as if a colony has grown from a single spore center.

But examining these samples by light microscopy in acid fuchsin and in KOH (not the optimal mountant chemicals for this purpose) as well as dry without a cover slide and using reflected as well as transmitted light did not produce good images of fungal colonization, just a few fungal spores resembling Cladosporium sphaerospermum.

At left our photo illustrates preparing a large sample (triangular cut) of one of these brown stains.

Further work is in process, and of course other experts have examined foxing stains using other methods including FLIR. [16] [Forensic microscopic images of these stain materials are forthcoming. - Ed.]

Research reported by Arai et als established the fungal basis of foxing stains. [31][32][33][34] while more recently, Zotti et als, using FLIR, cultures, and other methods, identified Cladosporium sphaerospermum, Penicillium purpurogenum, Aspergillus melleus, Pithomyces chartarum, Aspergillus sclerotiorum as among the most common fungi associated with foxing marks or stains on paper. Those researchers found these fungal genera/species present before and after microwave treatment of foxed papers. [16]

Further work by the same authors found that these biota were absent after mechanical "rubbing" of the stained areas to remove the apparent stain - which surprised me. My own preliminary work on the brown foxing stains such as those shown here suggests that at least in some cases the foxing stains involve materials that reside within the matrix of wood pulp fibers comprising the paper itself, making successful mechanical surface treatments difficult.

In an earlier work the same lead author, Zotti, who along with Arai has done extensive research on foxing, noted that the while fungi and some yeasts are found in foxing marks on books and papers, the dominant genera and species appeared to be in the Penicillium group.

Ten species representative of filamentous fungi genera and one yeast form were identified. Of the fungal species, the one most frequently represented was Penicillium, with 5 different strains, while for each of the other genera there was only one species. [17]

What are the Foxing Fungi? fungi identified as growing on or in paper materials included the following

At least ten species of fungi and one yeast have been identified as growing on/in or "hosted by" paper, [16][17][17a][29] Indeed, a literature search confirms that among scholarly and research papers we researched for this article, not one authority detected the presence of either of the two types of mildew (Oidium-Erysiphe - powdery mildew, or Peronosporaceae - downy mildew) among the various fungi found growing on books, papers, photographs, stamps, or other paper based works. [1] op. seq. Fungal species commonly identified as found growing on or in books, papers, photographs, stamps, etc. include at least the following:

  • Aspergillus sp.
  • Aspergillus melleus [brownish stains, visible microscopically]
  • Aspergillus sclerotiorum [pale brown stains on paper, visible microscopically]
  • Cladosporyum sp. [Cladosporium sp. - ed.]
  • Cladosporyum sphaerospermum [pale brown stains on paper]
  • Penicillium sp. [at least 5 different species or strains]
  • Penicillium purpurogenum [brownish-yellow stains; pale whitish stains, visible microscopically]
  • Pithomyces chartarum [often appearing black on surfaces, brown or dark brown stains visible microscopically]
  • Ulocladium alternarie
  • Yeast [species to be identified]

How to remove & prevent Foxing stains: Cures for Foxing on Paper, Books, Photographs

Conservationists and paper or book restorers typically use a range of chemical or plant extract treatments to attempt to neutralize the reddish stains associated with foxing. Removing ferrous oxide (FeO) from the paper appears to be a key step in both removing the stains and preventing their recurrence. Where appropriate, such as in visibly moldy books and papers, mold needs to be physically removed from paper and book surfaces first, typically by gentle brushing, wiping, or HEPA vacuuming. Some conservationists also attempt to "kill" the mold using microwave or freeze-drying methods.[16]

Watch out: killing mold simply means that most of the remaining mold spores, if placed in a culture media, will not reproduce. But depending on mold genera/species such spores may still be harmful to humans or other animals and may retain allergenic or even toxic chemicals such as mycotoxins found in some fungi. For this reason, a proper mold remediation strategy should always involve physically cleaning mold from materials and surfaces that can be cleaned, and correcting the conditions that caused the mold growth in the first place. See MOLD KILLING GUIDE for details.

  1. Gas bleaching: bleaching foxed documents with chlorine dioxide gas, combined with de-acidification [35]
  2. Laser treatments for foxing: Sarantopoulou et als. have explored and demonstrated successful removal of foxing stains in paper using lasers at particular wavelengths. [27]
  3. Plant extract and paper amendment treatments for foxing: Tronson has developed a refined foxing removal method that considers the details and condition of the paper itself and uses plant extract treatments to remove the iron oxide staining material as well as additional steps to restore and preserve the paper fibers, sizing, and other components. [25]
  4. Anti-fungal treatments for foxing prevention: S. Sequeira et als have researched and report on the efficacy of antifungal treatments for the prevention of foxing damage to valuable books and papers. The focus is on prevention, not removal or cure for foxing stains. [28]
  5. Chemical treatments for foxing: For the removal of foxing stains on paper such as stamps and other paper materials, Coleman recommended a now-obsolete procedure that used, in order of increasing severity, immersion in solutions of washing soda, chloramine, sodium chlorite, and potassium permanganate plus oxalic acid. [26]

    Watch out: Calvini has warned at this article's FAQs section of this article that procedure #5 above is obsolete, dangerous, and can cause blindness. We list it here only so that readers can recognize it and thus stay away from it.

Watch out: Separately, Tronson warns

There has been no definite cure for neutralising the mildew [it's mold not mildew - Ed] spore, chemical use not only breaks down the paper cellulose but also reactivates the ink so not only do the fibres of the paper break down, after a while and the paper starts to disintegrate and the ink or what ever medium can be rubbed off!
Bleach treated or chemical treated papers on the other hand will always be subject to the foxing returning and eventual disintegration. [25]

Warnings & Corrections About Causes of & Treatments for Foxing Stains on Paper

Reader Comment: correcting the record on Foxing and Foxing Stain Removal Techniques

I have read the article about I'm one of the authors quoted in the bibliography (16, 17: Zotti et al.).

In the article published here there are some questionable statements:

  1. Iron Oxide (FeO) has nothing to do with foxing. It represent the "scaling" of Iron and is generally inactive on paper documents.
  2. Perhaps the author confuses it with Iron Oxy-Hydroxide (FeOOH) responsible of "rusting". Rust can be easily detected, and can not properly labelled as foxing.
  3. The chemical origin of foxing is thought to be due to the fact that that Iron Ions [i.e., Fe(II) and Fe(III)] catalyse the oxidation of paper, yielding the yellow-brown stains. These stains belong to the organic chemistry of cellulose and gelatine.
  4. Perhaps we were not clear in our articles, but we mean that "sometimes" the (dead) fungal bodies can be mechanically removed, thus explaining their absence in several SEM analyses. The FTIR analysis shows that fungi stained the fibres, and these stains can't be removed mechanically.
  5. The reccommendation after Coleman [26] are out-to-date and potentially very dangerous: should not be described in detail in Inspectapedia, otherwise not trained people may apply them in a blind way, as they find only this receipt fully described.

Sincerely, Paolo Calvini [16][17][17a][17b]


Sr. Calvini:

I cannot thank you enough for taking the time to send me your note.. I have edited the article above to reflect your warnings.

As a matter of editorial policy we ask for expert technical review of our articles, while at the same time,we understand that we should not burden experts with too great a volume of requests. Had I considered that risk was involved I might have been smarter by asking for further reviews. - Daniel Friedman, Editor.

Using Microwave Oven Treatment for Foxing on Paper Products

Reader Question: any truth to effectiveness of using microwave oven treatments for mold?

(Mar 19, 2014) Foxing said:

Am a collector of cigarette cards and was talking recently about foxing it was mentioned that placing a card in the domestic micro wave and giving it a couple of blasts would do the trick and kill the mold, fungi. Is there any truth in this as it sounds plausible, and if it does work does it harm the card. - Regards David

Reply: uses of microwave treatment on moldy books


Microwaving is one of the techniques used by some book restorers to try to dry out and "kill" mold.

First, on cigarette cards with foxing stains, microwave treatment will almost certainly not remove the existing brown stains attributed to fungal attack.

Next, it might "kill" some or even all of the fungal spores, though we haven't tested that theory. We'd need to identify a suitable culture media that works with foxing-fungi (not all molds will grow in any culture of any sort), then do some testing with samples collected before and after treatment.

Third in my OPINION there may be some cigarette cards that might be damaged, either their inks that might be oxidized or other components such as metallic inks or foils that would cause or might cause a bit of excitement (sparking) in the microwave. In the most extreme case we might even oxidize or brown the card by heat.

Still it's an interesting idea. If you want to pursue it we could get some help from appropriate mycologists and paper restoration experts, and here at InspectApedia we could conduct the lab work in our forensic lab.
Let me know if you have some low-value examples that we could use for testing if you want to pursue it more scientifically.

Research articles on biotic foxing and its treatments on valuable or antique books, papers, related objects

  • Calvini, Paolo, Andrea Gorassini, and Antonio Luigi Merlani. "On the kinetics of cellulose degradation: looking beyond the pseudo zero order rate equation." Cellulose 15, no. 2 (2008): 193-203.
  • Calvini, Paolo, and Maise Silveira. "FTIR analysis of naturally aged FeCl3 and CuCl2-doped cellulose papers." E-Preservation Science 5 (2008): 1-6.
  • Calvini, Paolo, Andrea Gorassini, and Antonio Luigi Merlani. "Autocatalytic degradation of cellulose paper in sealed vessels." Restaurator 28, no. 1 (2007): 47-54.
  • Calvini, Paolo, and Stefano Vassallo. "Computer-assisted infrared analysis of heterogeneous works of art." E-Preservation Science 4 (2007): 13-17.
  • Calvini, Paolo, Andrea Gorassini, Giorgio Luciano, and Enrico Franceschi. "FTIR and WAXS analysis of periodate oxycellulose: evidence for a cluster mechanism of oxidation." Vibrational spectroscopy 40, no. 2 (2006): 177-183.
  • Calvini, Paolo, Giuseppina Conio, Elisabetta Princi, Silvia Vicini, and Enrico Pedemonte. "Viscometric determination of dialdehyde content in periodate oxycellulose Part II. Topochemistry of oxidation." Cellulose 13, no. 5 (2006): 571-579.
  • Calvini, Paolo, and Andrea Gorassini. "On the rate of paper degradation: lessons from the past." Restaurator 27, no. 4 (2006): 275-290.
  • Calvini, Paolo, Andrea Gorassini, and Rosanna Chiggiato. "Fourier transform infrared analysis of some Japanese papers." Restaurator 27, no. 2 (2006): 81-89.
  • Calvini, Paolo. "The influence of levelling-off degree of polymerisation on the kinetics of cellulose degradation." Cellulose 12, no. 4 (2005): 445-447.
  • Calvini, Paolo, and Andrea Gorassini. "FTIR–Deconvolution Spectra of Paper Documents." Restaurator 23, no. 1 (2002): 48-66.
  • Calvini, Paolo, and Andrea Gorassini. "The degrading action of iron and copper on paper A FTIR-deconvolution analysis." Restaurator 23, no. 4 (2002): 205-221.
  • Calvini, Paolo, and Gianni Martinelli. "Numerical processing of Fourier transform infrared spectra: A powerful tool in paper analysis." In ICOM Committee for Conservation, 9th triennial meeting, Dresden, German Democratic Republic, 26-31 August 1990: preprints, pp. 453-455. ICOM Committee for Conservation, 1990.
  • Cappitelli, F., and C. Sorlini. "From papyrus to compact disc: the microbial deterioration of documentary heritage." Critical reviews in microbiology 31.1 (2005): 1-10.
    Pinzari, Flavia, et al.
  • Lin, Lang-Dong, et al. "Modified atmosphere and humidity packages for conservation of paper antiques." Journal of Wood Science 53.2 (2007): 121-126.
  • Margutti, Simona, Giuseppina Conio, Paolo Calvini, and Enrico Pedemonte. "Hydrolytic and oxidative degradation of paper." Restaurator 22, no. 2 (2001): 67-83.
  • Niehus, Lena, Ute Henniges, Monika Horsky, Thomas Prohaska, Antje Potthast, and Irene Brückle. "Reducing the Risks of Hydrogen Peroxide Bleaching in Presence of Iron Ions in Paper." (2012): 356-394.
  • Pinzari, Flavia, Mirca Zotti, Antonella De Mico, and Paolo Calvini. "Biodegradation of inorganic components in paper documents: Formation of calcium oxalate crystals as a consequence of< i> Aspergillus terreus</i> Thom growth." International Biodeterioration & Biodegradation 64, no. 6 (2010): 499-505.
  • Porck, Henk J., and René Teygeler. Preservation science survey. Council on Library and Information Resources, 2001.
  • Teygeler, Rene. "Preserving paper: recent 5 advances." Managing Preservation for Libraries and Archives: Current Practice and Future Developments (2004): 83.
  • Zotti, Mirca, Alice Ferroni, and Paolo Calvini. "Inhibition properties of simple fungistatic compounds on fungi isolated from foxing spots." Restaurator 28, no. 3 (2007): 201-217.

Other Do-it-yourself Treatments for Foxing?

Reader question: 7/25/14 IAmBill said:

Are there DIY methods on removing the foxing stains because the methods shown don't seem like it. And is putting them out in sunlight one of them?


I agree that most of the foxing stain treatments discussed here are used by professionals.

  • If the document or book of concern to you is valuable you should indeed consult a professional paper conservator.
  • If its not of great value you can experiment with other approaches such as sunlight or dilute bleach that a pro would remind us will damage the paper document though for a time it may look better.

Watch out: as Calvini cites in comments on this article, some older documented treatments for foxing stains on paper, photographs, books, or other similar products are both obsolete (no longer recommended) and dangerous.

(Aug 1, 2014) IAmBill said:

I am simply asking if there are other DIY methods beyond the two just mentioned. There has to be more, right? Or are there only two?


Bill, researched information on foxing stains including comments from one of the most expert people in the field along with citations are in the article above. If there were other methods advised that research had cited or disclosed they'd be here. What would lead a reader to think our editors withheld useful information.?

I was taken to task by a world expert on this topic Paolo Calvini (in the article above) for including too many now-obsolete and some-dangerous methods for treating foxing stains. I conclude that while there may be more amateur approaches they are either ineffective or dangerous.

The ineffective or less-effective ones that might do something and that I know about are included in the article.

For sure, if you find reference to other foxing stain treatments that we have not discussed here just let me know and I'll be glad to research and comment on them.

Cross-Contamination Among Foxed Books & Papers?

8/24/15 Zack said:

May i know if foxed book could contaminate other books?



It's more likely that the conditions that caused foxing or mold growth on a book would affect others in the same environment. Not all books are equally affected - variations in materials, inks, papers, glues, bindings make a difference too.

Storing Books & Papers to Avoid Foxing

8/25/14 Zack said:

Hi, lately i don't have time to maintain and take care of my books. So i put them in traveling vacuum bag and vacuum the air out. Will this keep my books from getting foxed or mold?

I live in a humid country and maintaining the environment is very costly (i.e aircond and dehumidifier). What is the best method to keep the book in good condition as possible.



Provided the books and papers are already dry, you can store them in a vacuum bag and remove the air, though document experts have warned me about storing important documents in a vacuum. I think equally safe and easier is to place your *dry* books into a clean tightly-sealing plastic container. The remaining few mold spores and moisture molecules sealed in with the books won't cause a mold contamination.

Watch out: if you store wet or damp books and papers in any closed container, vacuumed or not, you can expect mold growth to ensue.

Reader Question: removing stains from a modern papyrus painting

I have a modern Egyptian papyrus painting from the 1970s that sustained some water damage. Before I could get it out of the frame it developed some black spots in the wet areas. I removed the paper from the frame and it's dry now, but I'm wondering about the spots. Are they mold or mildew? They don't look fuzzy or stringy. And is there any way to remove them from the paper? I have some photos I can send along tonight if those would help. - S.B. 8/21/14

Papyrus painting with water damage for restorastion advice (C) InspectAPedia SB Papyrus painting with water damage for restorastion advice (C) InspectAPedia SB

Here are the images of the water damaged Egyptian papyrus painting I mentioned in my post. It’s an early 1970s painting, I think the paint is acrylic. It’s mounted on a pane of glass and glued down on two corners. The image is based on a wall painting from the Tomb of Nakht in Thebes.

As you can see, there are black spots and a little white mold on the black hair of one of the figures. The paper is dry now and a little crispy. The spots don’t look fuzzy or stringy. There are also spots on the back in the water affected area.

I think the stains are on the surface of the papyrus, at least they don't seem to go all the way through.

Susan B.

Thanks so much for your help! - S.B. 8/25/2014


Use the email at our CONTACT link to send me some sharp photos and I may comment further.

There won't be mildew (that only grows on living plants) but there may be mold damage to the painting. If the value of the painting does not merit hiring a professional paper conservator and you are going to try a DIY project, gently wipe off the surface mold using a dry cotton or cloth. On some rice paper screens that were water damaged we recently had success using a q-tip moistened with *diluted* household bleach, dabbing onto the stains. Others have tried hydrogen peroxide. We quickly blotted then dabbed thoroughly with distilled water to prevent excessive bleaching and bleach damage.

Papyrus painting with water damage for restorastion advice (C) InspectAPedia SB

A general conservator's rule is to start with the most gentle benign approach possible, thus minimizing risk of damage to the artifact. I'd start with a damp wipe with a soft cotton cloth to see what happens.

Is there a distinction between "mold" and "mildew" appearance on books & papers?

Moldy books (C) Daniel FriedmanReader opines that book mold is "mildew"

As a book collector and once-upon-a-time mycologist I found your web site of interest.

However, I do not believe there would be uniform acceptance among professional mycologists of your distinction between "mold" and "mildew". The organisms involved are all fungi in the classical sense. Those attacking dead organic matter like cellulose are not all that different biologically from their relatives that may have a preference for the living cells of plants and animals.

The fact that some parasitic species may be facultative heterotrophs (feeding on non-living materials) supports this view. I believe your cause is best served by promoting the idea that the fungi, a diverse and highly successful breed, will exploit any environment where nutrients and moisture are available whether or not it is living or dead. Books and their bindings in a high humidity environment are sitting ducks.

... [text omitted here]. - Chris 9/7/2012

See our article BOOK MOLD, CLEANING and its section on the Distinction between "mold" and "mildew" on books and in general is questioned & answered for additional details about the importance of the distinction between mold growth on surfaces and mildew growth.

Mildew doesn't grow on books, nor on leather shoes, nor on wallpaper, nor on other indoor building surfaces, though lots of other mold genera/species might


Thank you for the interesting comments about book mold, mildew, HEPA vacuuming, and the important role of the fungi in our environment. I'm grateful to read your opinion and want to emphasize that we welcome polite, informed discussion or debate about this or any other topic found at

Indeed even among expert book restorers the term mildew is often used loosely and technically incorrectly to refer to the role of certain fungi or mold genera/species in the cause of foxing on books and papers. A few points need clarification: by no means do I suggest that mildew is not a fungus, as mildew is indeed a proper subset of the huge kingdom of fungi. But the fungi appearing on books are different genera/species from the two fungi properly named mildew. There are some important distinctions to be made.

Mildew doesn't grow on shoes, nor on paper, though many other mold genera/species can grow on these materials

I learned about the distinctive properties of mildew as a living plant pathogen among other members of the Fifth Kingdom from Dr. John Haines, my friend and mentor, when John was still serving as the NY State mycologist. Discussing some mold samples I'd brought along for us to examine, I mentioned that I'd just collected some white mildew from leather shoes found in a moldy home. Like many people I just bandied the word mildew about willy-nilly. John asked why I thought it might be mildew, allowed me to embarrass myself, and then kindly explained that mildew grows on plants, not shoes.

Perhaps it's technical nitpicking, but mildew a subset of "mold" that only grows on living plants. Mildews are a small group of fungi found among the Basidiomycota, Ustilaginales if I recall correctly, and any "mildew" if properly identified, will be either Oidium-Erysiphe or Powdery Mildew or Peronosporaceae or Downy Mildew. Mildew, then, has nothing to do with and won't be found growing on books nor on other building surfaces unless the item in the building is a live or recently-live plant.

The fungi identified as mildew include these two groups

Below I include photos of Oidium or powdery mildew that I collected from a jasmine plant (below left) that was growing indoors (we moved it outside before this photo was taken) along with a photo of the same mildew from that plant under my lab microscope (below right).

Typical library bookshelf dust (C) Daniel Friedman Typical library bookshelf dust (C) Daniel Friedman

Watch out: In a library of moldy books, if by careless language we informed building management that we thought the dominant mold present was just a bit of mildew (say Oidium) we would erroneously conclude that other than a possible allergic response or perhaps a problem for nearby houseplants, there was no health risk to building occupants. But in point of fact what I find on moldy library books is typically a potpourri of fungi dominated on book jackets by several species of Aspergillus - far more likely to present an IAQ and health hazard to occupants.

See Distinction between "mold" and "mildew" on books and in general is questioned & answered for additional details about this distinction.


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