Two significant differences between foxed and clean areas of a paper are the higher proportion of acid and iron in the former, although there does not seem to be any clear and definitive relationship between iron and foxing. Insofar as the acid is involved, it is not clear whether this is produced chemically or as a byproduct of the life function of the organisms present. Iron is attributed to impurities present in the paper, and this conclusion seems to be based largely on the fact that it is seldom found in papers produced before the introduction of papermaking equipment made of iron, e.g., the beater, and improvements in techniques and other forms of chemical treatment. But what role iron has in accelerating foxing, or causing a change from the invisible to visible state, has yet to be demonstrated.
The other factor which controls foxing is relative humidity (R.H.), since these fungi will not develop if the R.H. falls below 75%. The fact that foxing generally starts from the edge of the leaf and spreads inward would seem to indicate that something in the atmosphere is relevant, although air borne organisms may be adequate as an explanation for this effect. In addition, it must still be explained how the center of the leaf is affected most in occasional instances. Perhaps the most logical explanation is that infection by air borne organisms (or by organisms that are natural to the paper) may occur if the conditions, and especially the R.H., are favorable, and that growth, resulting in the generation of fox marks, then occurs. The acid subsequently renders any iron in the paper soluble and therefore visible, with its color being intensified by the presence of organic matter.