For your consideration, I submit to you neither ferric oxide nor foxes, but rather mold . . .
Folk-Etymology: A Dictionary of Verbal Corruptions Or Words Perverted in Form Or Meaning, by False Derivation Or Mistaken Analogy (Abram Smythe Palmer, 1890) offers this:
FOXED. A print or book is said to be foxed, when the paper has
become spotted or discoloured by damp. In Warwickshire the same term
is applied to timber when discoloured by incipient decay. It is, no
doubt, the same word as the West country foust, soiled, mouldy, and
fust, to become moldy, Scot. foze, the same. Compare fouse, a Craven form of fox. Fust is from O. Fr. fusté, “fusty,”
originally smelling of the cask (fust, from Lat. fustis). “They
stanke like fustie barrells.”—Nash, Pierce Penilesse, p. 33.
Following the mold theme, we find inside 1993’s Conservation Administration News an article titled “The Great Foxing Debate” by Karl R. Schaefer. Until I can access a library copy, I have used Google Books to piece together some snippets here:
While the phenomenon has been observed and remarked upon for well over
a century, the cause of the spots or stains has been the subject of
considerable debate among antiquarians, conservators, librarians,
booksellers, chemists, and biologists . . .
The term we are most likely to encounter today is “foxing,” but the
form “foxed” is slightly older and may provide a clue as to the
original meaning of the term as it applies to books. The etymology of
Lydenberg and Archer is the one most frequently cited. They claim the
term refers to Reynard the Fox, whose rusty red color is similar to
the color of the stains on foxed pages. This explanation seems rather
specious; it appears to make sense because of the characteristic
colors of the stains, but I suspect a stronger relationship than
simply one of color. . . .
[OED entries referenced here] It is interesting that two of the meanings listed — three if one uses
a little poetic license — have to do with souring or rotting. In
addition, one of the meanings of the somewhat later term “foxing” is a
growth of mold or bacteria in a vat of sour beer. It may very well be
that booksellers, librarians, or book owners borrowed the term “foxed”
from the brewer or the ale man . . .
I would suggest that a more likely explanation for the origin of the
terms “foxed” and “foxing,” as they apply to books, is that the terms
were understood to refer to the odor which arose from the mold, rather
than to its color. Or, more likely still, it implied both color and
odor. In support of this argument, Harrison mentions a “peculiar odor”
(page 166) emanating from foxed books. Such an observation, however
casual, lends weight to the case for a reexamination of the etymology
of the word. (How the brewers came to use the term is beyond the scope
of this article, but the possibility of a connection certainly
warrants further investigation. [ed. note: see Palmer, above]) . . . As noted above, the term itself
may very well suggest some sort of souring or decay.
Lastly, here’s a roundup of fox mold in action from the OED:
3. Of the leaves of books, also of timber; Discoloured by decay; stained with brownish-yellow spots.
1847 Timber is
said to be foxed, when it becomes discoloured in consequence of
incipient decay. Warw.
1848 A torn or foxt
and dog's-eared volume.
1937 Foxed, foxing or foxy, stains, specks or spots in paper, e.g. prints, books, mostly due to
mould or paper-mildew.
1935 The so-called ‘foxy’ appearance of paper caused by the oxidising action of mould fungi or chemical
processes of oxidation.
1958 First a word on ‘fox-marks’. These brown stains..are caused by damp.
Source: Oxford English Dictionary (login