Bleaching in Paper Conservation

AbstractChemical bleaches are powerful chemical agents that destroy colour by breaking and reforming covalent bonds within organic colouring matter. They are, consequently, the most potentially damaging chemicals used in paper conservation treatments and their use should only be considered when all other means of removing discolouration have been fully explored. The colour of organic compounds depends upon the presence of a number of multiple bonds linked together in a conjugated system known as a chromophore. Rupture of the conjugated system renders it colourless. This is the process that occurs in bleaching. Chemical bleaches may be either oxidising or reducing agents. The tendency of paper to yellow with age is know as brightness reversion. Lignin is an important source of colour, especially in regard to the yellowing of paper based on mechanical wood pulps. In non ligneous papers, yellowing is commonly associated with the oxidation of cellulose and associated materials, such as hemicelluloses, resulting in the formation of carbonyl groups. The most powerful oxidising bleaches are based on the hypochlorite ion, OClˉ, and include bleaching powder, sodium hypochlorite , calcium hypochlorite and chloramine-T. The preparation, chemical properties and use of these agents are discussed in detail. Attention is draw to the importance of keeping the pH above 9 during treatment. This is in order to minimise the formation of carbonyl groups. After bleaching with these agents, thorough washing and deacidification is recommended. The oxidising agent chlorine dioxide has certain advantages. However, in a conservation context, its use is highly questionable due to serious health and safety issues. Hydrogen peroxide is a moderate bleaching agent and its preparation, chemical properties and use are discussed in detail. Stabilising agents are recommended when using this bleach. Sodium borohydride is the only reducing agent commonly used in paper conservation. Not only is it an effective bleaching agent it also has the benefit of increasing paper stability by re-ducing the carbonyl groups present in oxidised cellulose. Its preparation, chemical properties and use are discussed in detail.