Foxing, a fungal infection of paper

IRREGULAR yellowish-brown patches, or ‘foxing’, often occur on old books and prints, but their cause is nevertheless uncertain. One possibility is fungal infection 1–11 , suggested by the destructive pigmented lesions seen on mildewed cloth 12 ; but foxed paper is almost invariably intact and never looks mouldy to the naked eye. Chemical causes have therefore been suggested, such as dampness 13 affecting impurities in the paper 14 , connected in some way with either poor bleaching 15 by chlorine 16 , or poor washing 17 , or the accumulation of visible iron salts in the lesions 2,7,8,11 . If the cause were indeed fungal infection, the most reliable method of proof should be microscopy rather than culture, considering that fungal spores are ubiquitous and to be expected in virtually any sample of paper. Moreover, with these unusual lesions that may have taken half a century to develop, a positive culture might merely reflect a fast-growing contaminant and not the true cause. The isolations reported 1,3–5 are therefore difficult to assess, added to which some came from paper that was frankly mouldy 1 . We find that fluorescence microscopy invariably reveals fungal mycelium in foxed lesions whose properties can be explained by fungal growth occurring in an unfavourable environment.