| Resumo: | Deterioration of graphical documents can occur naturally as a result of aging, but it can be accelerated by poor manufacturing constituents or conditions. One indicator of deterioration can be discoloration or staining of the surface. Among these paper discoloration processes is foxing, which has been actively researched since the 1930s. Our attention herein is focused on trying to elucidate its causes and to establish protocols for detection, prevention and treatment. This foxing phenomenon occurs in the form of small isolated patches of discoloration that are typically rust, brown or yellowish toned and limited in size, with sharp or jagged edges; it is often punctiform and sometimes circular. A long discussion about the origin of foxing has been undertaken, being the major causes attributed to fungal activity or to chemical origin. In fact, sometimes biological attack is the only factor observed, while in others metal-induced degradation is present and interaction between both factors can also be found. During the last few decades, different analytical techniques have been used for the study of the foxing processes; nevertheless there is still much to be done.
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