Guide for identification and treatment of mould and foxing in libraries and archives

And now the million dollar question: Foxing is mold… or what?
There is diversity of opinion… to me, not necessarily.
Foxing may foster microbial presence, but it is not indicative that a paper with foxing spots has actually –or not– mould. A sheet without them may well be fungi infected.
Literature is still vague as to the causes of foxing formation: some may say it is as well microorganisms, while according to others, it is not. The last attribute its origin on the oxidation of metal particles, as reminds us John Swart (thanks!).

What is foxing?

Foxing appears as dark spots, brownish (never any other colour which isn’t darkening of paper) in paper or cloth documents, but not on leather stuff.
What we cannot deny is that oxidation plays a significant role, and its consequent acidification and the derived cellulose weakening propitiates microbial growth.
Evidence also shows us that environmental exposure to oxygen is –if not its direct cause– a remarkable catalyst, since usually it is most pronounced in the most exposed areas (edges of the sheets, covers, book-edges…).

Foxing also has a very direct relationship with the type of paper, thus some supports are more susceptible to suffer it than others. The image below is a good example of it.

A third aspect favors the foxing apparition, which is high relative humidity. This is because the current pollutant components in air suffer acid decomposition in the presence of moisture (CO₂, S-², among others).

Foxing acidifies, but ony if the paper were in a very low pH, this increase of acidity would have appreciable consequences on the mecanical strength of the paper.

The “contagion” is never further than to its immediate neighbors (acidity migration), because this damage lacks the possibility to move, while microorganisms sort of do, expanding its activity. Besides, foxing “only” takes place where conditions are favorable. However, the pollution transfer due to air may appear as a similar sympthoms as the contagious effect of an infection.

How to deal with foxing

Ideally preservation mesures should prevent from favouring any of it. We can cope foxing by deacidification: Stopping oxidation, reversing acidity, and reducing microbial activity, as this latter shuns alkaline pHs.
Since damage directly resulting from foxing is not alarming, it can also be coped simply using conservation folders or envelopes made with alkaline paper, to prevent its increase.

Foxing vs mould

Confusion between mold and foxing is natural, as the same conditions that favor foxing (moisture, slightly acidic pH) are ideal for microbial growth. Moreover the consequences of foxing (weakening and decomposition of cellulose) enhance the idyllic setting for fungi and bacteria (cellulose supports, acidity, moisture… I shall stop or else I do not respond!).

Mould stains, unlike foxing’s, can present a wide variety of colours (purple, yellow, black…). The colour of the stain is not indicative of the type of fungus. The stains may be accompanied by the hyphae of microorganisms (mold). The microbiological attack degrades cellulose weakening it: in the worst cases it acquires a cottony touch, removing its entire rigidity. Unlike foxing, which can be more selective affecting one support more than an other, microorganisms are able to colonize almost all kinds of media (although intensity may vary depending on the type of media). Organic media (paper, leather…) will always be preferred by mould.