|Impact of Environmental Pollution on the Preservation of Archives and Records: A RAMP Study (UNESCO, 1988)|
|5.0 The nature and degradation from pollution of materials in archives|
Organic pigments are discrete solid particles, usually crystalline, whose outer layers will encounter pollutants first and perhaps protect to a degree most of the colouring substance within. But with dyes and lake pigments, which are in effect dyed inorganic particles, there is no such protection and hence greater pollution effects would be expected with dyes and lakes. Dyes will be found as general colourants for papers, leathers and boards in archives, but almost all coloured images and graphic marks are likely to be composed of dyes, as are all coloured photographic images.
Coloured photographic prints are sometimes protected with lacquers. Those lacquer solvents are prone to form peroxides which can cause the magenta layer to yellow. This illustrates the power of such oxidising agents even in a photographic image free of metallic silver. As well as peroxides, and oxides of nitrogen, ozone is also active. Studies of water colour pigments in ozone atmospheres not much more concentrated than those found in some cities, show considerable fading of some pigments, especially those based on dyes. These molecules are coloured because they contain a string of alternating double and single bonds. These unsaturated double bonds are attacked and opened by oxidising agents and the colour is then lost or changed. Hence most organic colours could be at risk, but some very seriously. Some coloured compounds also change their structure according to the acidity of the environment. This is a useful property in measuring acidity in the laboratory, but not when images are changed. Other pollutant-induced changes may not be reversible.
All these coloured organic materials should be treated as materials which are very sensitive to pollution. They cannot be identified easily and therefore precautions to protect the most sensitive against pollutants, light and high relative humidities are necessary whenever there is any doubt.