Foxing — those brownish yellowy spots you often see in old books — appears mainly to occur in machine-made papers from the 19th century. Surprisingly the cause seems to be unknown. The contenders are either impurities in the pulp or size leading to fungal growth, or the presence of iron leading to what in effect would be rusting, or some problems with the bleaching process. Such testing as has been done has unfortunately found no evidence of fungi. It does show acid and iron relatively higher in relation to the rest of the sheet, but nobody seems sure whether the iron is a result of the foxing rather than the cause. The process does seem to be accelerated by humidity. Foxing doesn’t affect the integrity of the paper, and methods of “curing” the problem seem likely to damage the paper (e.g. spot bleaching), so it’s better just to accept the splodges as merely an aesthetic problem.

The fact that we don’t make papers nowadays that fox seems to suggest a manufacturing problem. The paper industry makes constant process and cleanliness improvement, and even if we don’t know what the cause of foxing is it seems to be something we are no longer up to. The Oxford English Dictionary‘s earliest reference to foxed paper is from 1848, but they do have one from the previous year referring to foxing in timber. To me that rather tilts the likelihood of causation towards an “impurities in the pulp” explanation. It seems odd that nobody has done the research though: I guess there’s no financial incentive to find out now that we don’t seem to do it any more.

Among other uses of the verb to fox are the following: to delude (as we’d use it in school, where we’d also use the same word to mean to unearth wrong-doing — it all depended on context); to have your nose turn red by excessive drinking; to turn sour in fermenting (of beer); to repair boots or shoes by renewing the upper leather; to trim a horse’s ears!