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light as the conjugation is reduced or destroyed and colour is removed, i.e.
bleaching. In paper conservation bleaching, double bonds are usually destroyed
by oxidising agents, less frequently by reducing agents. If discolouration
returns several months or more after bleaching, this is known as reversion.
The presence of carbonyl groups, produced during oxidative bleaching, is
believed to be a cause of reversion.
In medieval times, undyed cellulosic textiles were bleached by “grassing”
them, i.e.they were laid out on fields of grass. The combined effect of morn-
ing dew and the sun being allowed to shine on them for several months, pro-
duced bleaching. The invention of cheap ways of producing chlorine made
grassing redundant after 1774. An adaptation of the slow, but effective, method
of grassing has been used for the last few decades for bleaching discoloured
paper. The paper is immersed in water in a shallow tray and the dish is covered
with a sheet of UV-absorbing Perspex and laid in strong sunlight or under a
bright artificial light. After several hours, the bleaching is complete. Many
conservators use a slightly alkaline solution of magnesium hydrogen carbon-
ate instead of pure water.
The mechanism for bleaching is not fully understood. Probably the dis-
coloured areas absorb light and transfer the energy to the oxygenated water
which then produces oxidising species, e.g.singlet oxygen or peroxy radicals,
which then destroy double bonds and hence the colour. A problem with other
bleaching treatments that are based on the use of oxidising solutions is that
the clean paper fibres are oxidised as well as the discoloured ones, resulting
in unwanted chain scission and oxidation. The advantage of light bleaching is
that as the discoloured areas are the only ones that absorb light, they are the
only areas that are oxidised.
Many of the bleaching processes are derived from the paper industry as
paper pulps often need to be bleached before being formed into paper products.
Much more research has been carried out on bleaching than has ever been
done by conservation scientists and the paper science literature is a good place
for any paper conservation scientist wanting more information.
When chlorine comes into contact with either sodium, potassium or calcium
hydroxide, the corresponding hypochlorite is formed. The hypochlorite ion
(OCl) can oxidise double bonds in discoloured paper producing bleaching.
Sodium and potassium hypochlorites are available in an aqueous solution and
are highly alkaline because excess hydroxide is present. Undiluted, the solutions
are so alkaline that swelling of the cellulose and unwanted alkaline degradation
may occur. Of the hypochlorites, it is usually safest to use calcium hypochlor-
ite, also known as bleaching powder, as, in practice, it produces less alkaline
conditions. Bleaching powder has to be dissolved in water prior to use.
The best type of bleaching occurs when there is least fibre damage but
adequate bleaching. The oxidising bleaches can be ranked for their damage to