paper fibres, with the main reactive species given in brackets. In order of
increasing damage they are:
● alkaline stabilised hydrogen peroxide (HOO/HOOH),
● chlorite/chlorine dioxide (ClO 2 /ClO 2 ),
● pH 9.0 hypochlorite (OCl),
● pH 4.5 hypochlorite (Cl 2 /HOCl), and
● pH 7.0 hypochlorite (HOCl/OCl).In hypochlorite solution, three species are present which can produce
bleaching: chlorine, hypochlorite ions (OCl) and hypochlorous acid (HOCl).
In pH neutral conditions, HOCl is produced. This is considered the most
harmful species for bleaching as it produces aldehyde groups in oxidised cel-
lulose that may later produce yellowing on ageing. The safest bleaching
occurs above pH 9.5–10.0. When the hypochlorite ion is the dominant
bleaching species, the damage to cellulose is least and the product of oxida-
tion is the carboxyl group that does not cause colour reversion on ageing.
When initially bleaching an acidic paper, care should be taken that the paper
does not alter the pH of the bleaching solution outside the desired range.
Burgess gives details of bleaching procedures.
Hydrogen peroxide is a versatile and popular bleach in both textile and
paper conservation. Research in the paper industry has shown that although
hydrogen peroxide is a good bleach, higher levels of cellulose degradation can
occur where there are traces of metal contaminants, e.g.iron and copper.
Alkaline hydrogen peroxide gradually decomposes-evolving oxygen bubbles,
and this can be an unwanted and damaging side-effect with some objects.
Although, many conservators use hydrogen peroxide solutions in the range
0.5–3.0% just made alkaline with a few drops of ammonia solution, others
prefer to use a fully buffered and stabilised solution. Such recommended
solutions for bleaching at pH 8–9 may be made quite complex containing
magnesium sulfate, sodium silicate, hydrogen peroxide and a buffer solution
containing sodium phosphate, boric acid or sodium carbonate for pH 8, 9 and
10 baths, respectively.
Hydrogen peroxide is mostly used in aqueous solutions but it can be dis-
solved in diethyl ether. If hydrogen peroxide aqueous solution is placed in
contact with diethyl ether, the ether layer floats on top as it is less dense. On
shaking the layers together, some of the hydrogen peroxide goes into the
ether. The pigments, lead white (basic lead carbonate) and red lead may react
with hydrogen sulfide in ambient air and blacken by the formation of a thin,
black, lead sulfide layer. The sulfide can be oxidised by applying the ethereal
solution of hydrogen peroxide when the lead sulfide forms white lead sulfate
and the original appearance is usually regained. The desired reaction occurs
46 Chapter 3