Biological factors of foxing in postage stamp paper

Extracts of naturally and artificially blotched postage stamps reacted positively to phenolreagents in three different tests. Microscopic examination of blotches revealed sparse living and dead hyaline hyphae. Six species of Aspergillus and one each of Penicillium and Gliocladium, isolated from foxed stamps, were capable of producing pigments in agar media and filter paper, sometimes more intense when grown in pairs than alone. All fungal isolates showed some degree of cellulolytic ability; all but A. fumigatus and G. roseum were capable of germinating, growing, sporulating, and producing pigments at high osmotic pressures, and all were capable at low osmotic pressure. Of three species inoculated on new stamp paper, one, A. terreus var. aureus, developed hyphae and fluorescent yellow and orange-brown pigments and produced pigments at constant relative humidity of 32.5% over an eight month period. Ferrous and ferric salts appeared to be unnecessary for production of pigments diffused into agar media, liquid medium, or filter paper, but in some isolates they intensified the color. Stamps inoculated with A. terreus var aureus conidia and irradiated with a dose of 2.5 Mrad remained free of blotches throughout the two year experimental period.