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Since 1998, in, the paper conservation department of the Instituto Português de Conservção e Restauro, we have been documenting works on paper which present partial or total discolouration and which do not fit the general pattern. The discolouration does not seem to be caused by light or water but by prolonged contact with air. Visually this type of discolouration turns the paper more opaque and is not reversed by the usual treatments. In the research literature there is reference to photo-oxidation, foxing, ‘tidelines’ and other discolouration related to iron gall ink corrosion or from contact migration of unstable volatile organic materials, normally related to local high humidity absorption. Work on visual characterization, photographic documentation (UV and transmitted light) and simple analytical testing was carried out in order to support the empirical observation that the agent that caused the oxidation was air – the humidified and polluted air found inside a building. Parallel to this, tests were made with different oxidizing and reducing agents to reverse the opacity. The paper also discusses the reason why the discolouration is accompanied by a loss of transparency.
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