foxing

Localised discolouration of paper, usually appearing as random rust-coloured spots. Caused by both mould growth or metal impurities within the paper and exacerbated by high humidity, dampness and airborne acids.

 

A watercolour with disfiguring foxing. Photo courtesy of Tony Rich, Art Conservation Framers. 

The irregular size and shape of these foxing spots suggests a fungal cause. Photo courtesy of the State Library of Victoria.