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Foxing stains: 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. Also see BOOK MOLD, Moldy Book Cleaning.
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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 are typically found to contain high levels of iron oxide (FeO) in one or more chemical forms. 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. )
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  (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. 
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.
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. 9] Yet according to at least some of those experts, the biotic or even chemical origin of these stains remains unclear.
I'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.
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.  [Forensic microscopic images of these stain materials are forthcoming. - Ed.]
Research reported by Arai et als established the fungal basis of foxing stains.  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. 
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.
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, [17a] 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.  op. seq. Fungal species commonly identified as found growing on or in books, papers, photographs, stamps, etc. include at least the following:
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.
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.
Watch out: Separately, Tronson warns
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, Moldy Book 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
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).
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.
Frequently Asked Questions (FAQs) about foxing stains on books, papers, photos: causes, cures, prevention
Comment: correcting the record on Foxing and Foxing Stain Removal Techniques
I have read the article about
In the article published by Inspectapedia 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  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 [17a][17b]
I cannot thank you enough for taking the time to send me your note.. I have edited the article above to reflect your warnings.
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