Foxing presents as reddish-brown spots on works on paper — prints, drawings, watercolours, photographs, and books. It is caused by a combination of fungal activity and the oxidation of metal impurities in the paper, accelerated by humidity. In early stages it is an aesthetic problem; left unaddressed, it can weaken the paper fibres over time. If you notice foxing, don’t attempt to clean it. Move the work away from any damp or poorly ventilated space and contact a conservator — treatment can reduce the staining significantly, but the underlying environment needs to be addressed or it will return.