Chemical Processes in the Bleaching of Paper in Library and Archival Collections
by MICHAL ĎUROVIČ & JIŘÍ ZELINGER
In the process of conservation and restoration of artefacts on paper kept in archives, libraries,
museums or galleries, various impurities, colour stains produced by microorganisms or stains
caused by water must be removed from the paper. If such stains or impurities cannot be removed by
washing in w:\tcr or some other suitable solvent, bleach is applied. Bleaching is also used in cases
in which a yellow or brown shade of paper, reflecting natural ageing, is to be removed.
Next to deacidification, bleaching is the most widely discussed topic in conservation research.1he
following text tries to stress the purely chemical side of the relevant processes. It reports on the
relevant experiences in the workshops of eastern Europe. For linguistic and other reasons, these
experiences have not yet received suitable attention.
Bleaching not only improves the aesthetic impression made by the exhibit undergoing conservation:
a suitably chosen bleaching agent raises its resistance against natural ageing. Some chromophores
are removed that may act as precursors of photo-oxidation or accelerate the acid hydrolytic
decomposition of the glucosidic bond in cellulose.2
The bleaching methods used at present in conservation procedures are predominantly modified
industrial bleaching methods. Many compounds possess bleaching activity, but the methods known
so far negatively affect the physical, chemical and mechanical properties of paper subjected to
bleaching. This is why a view is generally accepted that bleaching should be used only in
exceptional and most urgent cases, when a document, a map, a book or a piece of graphic art has
been soiled or has yellowed to the point that it is not legible. Since the conservator often works with
unique works of art, the choice of the most suitable bleaching method ought to be based on a
detailed knowledge of:
• the chromophore systems causing colour changes in paper;
• the effects of the particular bleaching methods on such chromophore systems;
• the effects of the particular bleaching methods on the physical, chemical and mechanical
properties of paper; and
• colour reversion of paper undergoing bleaching.
Moreover, in the case of historical, but also of recent collections kept libraries, archives,
museums and galleries, the choice is made more complicated due to the presence of inks (such as
handwritten additions), stamps or paints coloured pieces of graphic art. etc.
This is why this study summarizes the existing knowledge of chromophore systems in historical and
modern papers and of possible procedures for destruction that may be used in conservation work.
REVIEW OF CHROMOPHORE SYSTEMS OF PAPER
Colour changes (yellowing) of historical and modern papers are due to ail o; the components
present in them.3-5 Cellulose, hemicellulose, lignin, ions of metals, resin etc. may promote
yellowing, darkening or browning of paper during its natural ageing. The following sections focus
on the chromophore systems of the individual components of paper are in greater detail.
Chromophores of cellulose
Cellulose, a linear homopolysaccharide consisting of anhydro-β-D-glucopyranose units linked in
positions 1 and 4, is a white compound. The glucopyranose units (a, Fig. 1) contain three hydroxyl
groups: the primary group on the carbon atom C6 and two secondary groups on the carbon atoms C2
and C3. In addition, the first unit in the polymer chain has a secondary hydroxyl group on the carbon
atom C4, and the last unit has an aldehyde group in the semiacetal form on the carbon atom C..
Oxidation or photo-oxidation (a combined effect of oxygen and ultraviolet light) takes place during
natural ageing, and ketone, aldehyde and carboxylic groups are formed from hydroxyl groups.6-9
The ketone groups may arise on the carbon atoms C2 or C3 (b, c, d). Aldehyde groups may be
formed on the carbon atom C6 (e) or C2 and C3 (f), due to which the bond between these atoms
breaks up. The hydroxyl groups on the carbon atoms C1 C2, C3 and C6 may be oxidized up to
carboxylic groups (g, h, i).
The decisive effect on the photo-oxidation of hydroxyl groups on the carbon atoms C2, C3 and C6 is
exerted by the presence of water in the fibre.10,11 Until a
Fig. 1
certain critical moisture is reached (for cotton this amounts to 5-6% per absolute dry basis), photooxidation is inhibited. Daruwalla et al.10 explain this phenomenon by a decrease in the energy of
incident photons due to the loss of energy consumed by the splitting of hydrogen bonds between the
hydroxyl group and molecules of water. This reduced energy is smaller than the activation energy
of initiation reactions. At low relative humidity (RH) values (virtually 0%), the hydroxyl group on
C6 is oxidized, and only at higher RH values do the secondary hydroxyl groups on C2 and C3
undergo oxidation in their turn.
The influence of specific functional groups of cellulose on its colour change during natural ageing
can be summarized in the following points8'":
• Anhydro-β-D-glucopyranose is very stable and does not change colour during ageing.
• Neither reducing hydroxyl end groups nor their oxidation to carboxylic groups gives rise to colour
changes during ageing.
• Ketone groups on the carbon atoms C2 and C3 are those predominantly responsible for colour
changes in cellulose.
• At low RH, aldehyde groups on the carbon atoms C2 and C3 do not cause colour changes, but at
higher RH they cause yellowing.
• Carboxylic groups on the carbon atoms C2 and C3 support colour changes only very little, and
their effect does not depend on RH.
• If in anhydro-β-D-glucopyranose carboxylic, ketone and aldehyde groups exist side by side on the
carbon atoms C2, C3 and C6, the colour change during the ageing is more pronounced than if these
functional groups occur separately.
Chromophores of hemicellulose
Hemicellulose is mainly heteropolysaccharides with short side chains.13,14 Macromolecules
of hemicellulose consist of a limited number of saccharides: D-glucose, D-mannose, D-galactose,
D-xylose, L-arabinose and D-galacturonic, D-glucuronic and 4-O-methyl-D-glucuronic acid. Lramnose and L-fucose participate in the structure of hemicellulose to a lesser extent.
Similary to cellulose, hemicellulose in the unoxidized form also does not cause colour changes in
the paper during the accelerated ageing. Rollinson15 found that water-soluble colour compounds
formed during the accelerated ageing, contributed to the colour change in paper by 50-60%. Of the
total amount of such water-soluble compounds, 23% were oligosaccharides composed of simple
saccharides, typical of hemicellulose. Giertz & McPherson16 detected a high content of carbonyl
groups in the orange-coloured γ-fraction of the hydroxide extract of sulfate paper subjected to
accelerated ageing.
They concluded that colour compounds in the γ -fraction are degradation products of hemicellulose,
and that paper with a major content of these hemicellulose is more susceptible to yellowing. Like
Sjöström & Eriksson,17 Rapson & Corbi18 also concluded that 4-O-methyl-D-glucuronic acid and 4O-methyl-D-glucuronoxylan are most sensitive to colour changes. According to Rapson & Corbi,18
yellowing is also caused by poly-D-galacturonic and D-arabinouronic acid. The authors stress,
moreover, that these compounds cause yellowing only if oxidized or hydrolysed. Burgess19 &
Beélik20 indicate the possibility of formation of furanaldehydes from monosaccharides arising by
the hydrolysis of hemicellulose. According to these authors, furfurals are precursors of colour
changes in paper. Furan aldehydes from monosaccharides are formed via elimination of the
hydroxyl or alkoxyl group in the β-position to the carbonyl group (Fig. 2).21
Fig. 2
The ketoaldehyde thus formed is the most important intermediate in the process of formation of
furan aldehydes.
The effect of the particular types of hemicellulose and their chromophores on colour changes in
paper can be summarized in the following points:
• Hemicellulose in the unoxidized form does not cause colour changes.
• Hemicellulose is less resistant to oxidation or hydrolysis than, cellulose, and paper with a higher
content of hemicellulose is more subject to yellowing.
• The main chromophores of oxidized hemicellulosc are aldehyde groups on carbon atoms C2 and
C3.
• Of the individual mono- and polysaccharides which form hemicellulose, oxidized uronic acid (in
their monomeric form) and 4-O-methyl-D-glucuronoxylan support the yellowing most.
• Furan aldehydes arising from monosaccharides (degradation products of hemicellulose) may also
be precursors of colour changes.
Lignins are an aromatic compound of several higher plants. The basic unit of a three-dimensional
macromolecule is the C9 unit of phenylpropane:
where R1 is OCH3 or H,
R2 is OCH3 or H, ~C (biphenyl, phenylcumaran)
R is H or alkyl, aryl, acyl etc.
The basic (C9) units of lignin are mutually linked by C —C or C —O
bonds.
Lignin is an amorphous compound that is very sensitive to oxidation. Chromophore structures
present in lignin cause yellowing of paper made from groundwood pulp (the first newspaper made
from groundwood was printed in 1845).4'8'22
Substituted benzene ring22
The basic chromophore of lignin is the benzene ring itself with its system of conjugated double
bonds. In the case of lignin, the benzene ring is replaced by a side chain propyl. This alkyl
substituent causes an insignificant bathochromic shift, which is unimportant with respect to colour
properties. The benzene ring has two absorption maxima: 203.5 nm and 254 nm. The introduction
of the propyl group brings about a shift to the wavelengths 206 nm and 260 nm. The hydroxyl and
alkoxyl substituents in positions 3, 4, and 5 are of greater importance. Phenolic hydroxyls and
alkoxyls in position ortho and para are typical auxochromes, causing both a bathochromic and a
hypsochromic shift, as corroborated by the absorption maxima of 4-hydroxy-l-propylbenzene (λmax1
=
233 nm, λ.max2 = 278 nm) and 4-hydroxy-3-methoxy-l-propylbenzene (λmax1 = 227.5 nm, λmax2 =
282 nm). If, however, these substituents are in the meta position, their auxochromic effect on the
second absorption band is suppressed (antiauxochromic effect), so that λmax2 of 4-hydroxy-3,5dimethoxy-1-propylbenzene (λmax1 = 230 nm, λ.max2 = 273.5 nm) is lower than λ.max2 of 4-hydroxy-1
-propylbenzene.
Carbonyl group and double bond4,22
The following two chromophores of lignin are carbonyl (IIa) and the double bond (IIb). Their
influence becomes apparent if they are conjugated with the benzene ring, i.e. in the position alpha.
In the position beta the effect of these chromophores is insignificant. In the
case of alpha-carbonyl and hydroxyl group, in the position ortho a chromogen is formed, the last
absorption band of which overlaps the visible range.
Hon & Glasser4 have proved that light-induced degradation (300-400 nm) and the subsequent
coloration of lignin arc initiated by the absorption of light by alpha-carbonyl groups. These excited
groups split off hydrogen atom Iron; free hydroxyl groups in lignin, which gives rise to phenoxy
radicals, and these in turn form colour compounds.
Chromophores of quinoid character4,22
Another type of lignin chromophores are compounds of quinoid character, derived from quinones
(IIIa,b), hemiquinones (IIIc) and quinonemethides (IIId) (Fig. 3):
According to Leary23, orthoquinones arise in the photo-oxidation of guaiacylpropane.
Photochemically generated phenoxy radicals of guaiacylpropane are sensitive to oxygen or
hydroxyl radicals, which cause demethoxylation combined with the formation of quinones. Harkin
& Obst24 investigated the
enzymatic oxidation of a model compound of lignin, i.e., 2,4,6-trimethoxyphenol. The results of
their study led them to conclude that enzymatic oxidation may lead to the formation of quinones
from guaiacylpropane and syringylpropane.
Organometallic complexes4,22
Organonictallic complexes also contribute to the coloration or wood-containing fibres. Basically,
this relates to three types of complex compounds:
 phenoxides of simple phenols that are present in both lignin and flavanoids(see next
section);
 phenoxides of ortho-diphenyls, that occur, for example, in catechin (two hydroxyl groups in
the ortho position on the benzene ring); and
 phenoxide-carbonyl complexes, formed between the carbonyl group and a sterically
advantageously situated hydroxyl group, such as in flavones or in lignin.
Complex-forming metals may be represented cither by trivalent (Al3+ , Fe3+ ) or by bivalent (Fe2+,
Cu2+, Mn2+) ions.
Nonlignin polyphenols can be obtained from wood and wood fibres by extraction with polar
solvents after the prior removal of terpenoids, fats and waxes with nonpolar solvents. These
compounds have a phenylbenzopyran structure. Flavanoids rank among the most characteristic
compounds possessing such structure. Their main chromophores are conjugated carbonyls, double
bonds and quinoid structures, with the hydroxyl group being the main auxochrome.
Chromophores of resins26-28
A linear dependence exists between paper yellowing and the resin content. Resins present in paper
are a component of the wood substance and arc used in sizing. Resin acids, the main chromophore
of which is the conjugated double bond, can be divided into the primaric and abietic type. The
conjugated system of double bonds in resin acids of the abietic type is very sensitive to oxidation
with ambient oxygen.
Many micro-organisms (bacteria and fungi) possessing cellulolytic activity also produce colour
compounds (yellow-green, yellow, red, black, etc.) firmly fixed on the cellulose fibre and often
difficult to remove. Such colour compounds not only damage the respective book, document or
piece of graphic art aesthetically but may also become precursors of further degradation processes.
Bacteria, i.e., unicellular lower plants, require fur growth and reproduction a high RH (70-80%), an
optimal temperature, nutrients and neutral or slightly alkaline medium.29 Since the pH of paper in
most library and archives collections is 4.0-6.0, no optimal conditions are provided for their growth,
and their occurrence is limited. Nevertheless, Gallo30 reporits the occurrence of representatives of
the genera Cellvibrio, Cellfalcicula and Cytophaga. Representatives of these genera possess
considerable cellulolytic activity, which in many cases is related to the formation of yellow or
orange stains
.
Colour compounds produced by fungi
Fungi, i.e. microscopic mushrooms, unlike bacteria, need a mild acid medium to grow; in most
cases, such a medium is indeed provided by paper deposited in libraries and archives. This concerns
approximately one hundred kinds of microscopic fungi belonging to the class Ascomysetes
(Chaetomium, Mjxotristum, etc.) and Deuteromycetes (Trichoderma, Aspergillus, Penicillium,
Fusarium, etc.). Many of these fungi are cellulolytically active and many produce colour stains.
These colour stains are either colour fungal mycelia or pigments produced by those30 The various
staining depends not only the kind of the growing fungus, but also on:
• the kind of paper and its degree of destruction;
• climatic conditions (RH, equilibrium paper moisture, temperature);
• the duration of the effect of fungus;
• the coexistence of other kinds of fungi;
• the presence of trace amounts of metals (such as Fe or Cu);
• the acidity of paper (an optimal pH for the production of pigments is 4.8-5.6).
The chemical composition of pigments differs, however; antraquinones and carotenoids are most
frequent.30 In cultures of some fungi of the genus Aspergil-
lus, red-brown erythroglaucine (l,4,8-trihydroxy-3-methyl-6-methoxyanthra-quinone) was isolated,
and catenarin (1.4,6,8-tctrahydroxy-3-methylantraquinone) was found and subsequently isolated in
some fungi of the genus Penicillium.;" Some lower and higher fungi produce emodin (l,fi,8trihydroxy-3-niethylantraquinonc}.''1
Carotenoids or tetraterpenoids, if their polyunsaturated chain is sufficiently conjugated, become
chromogens for the visible part of die spectrum32 Low-molecular-weight polyterpenoids were
isolated from some fungi {Aspergillus fumigatus] but also from some bacteria.
In conservation the term foxing is used for cases in which sheets of paper arc covered with rusty
stains. Views on the origin of such stains differ. Liams & Beckwith33 detected an elevated iron
content in the stains (in both organic and inorganic form), and they attribute (he phenomenon-'to the
effects c: microorganisms. On the contrary, Carter34 asserts the main cause of the stains is residues
remaining after bleaching with chlorine compounds.
Baynes-Cope35 using a book dating back to the middle of the 16th century, tries to prove the
inconsistency of views in which the formation of rusty stains is assigned to bleaching with chlorine
compounds (which began in 1780) or to the introduction of iron hollanders into the production of
paper (starting about 1682). He characterizes foxing as brown rusty stains, probably of microbiological origin, which does not rule out the presence of Fe cations3+, which may form
coordination compounds with macrocycles, such as porphyrins.
REVIEW OF POSSIBLE DESTRUCTION PROCEDURES OF CHROMOPHORE SYSTEMS
Polčin22 suggests several procedures for destroying chromophore systems of lignin, cellulose,
hemicellulose, nonlignin polyphenols or accompanying compounds:
• Interruption of the conjugated connection between chromophores of the chromogcnic system. *
• Interruption of the connection between the auxochrome and chromophore. Without the
auxochromic effect of the hydroxyl or methoxyl groups, the wood chromophores have no colour.
The function of the hydroxyl as the auxochrome consists in the transfer of the unpaired electron
from the oxygen atom of the hydroxyl through the system of double bonds of the benzene ring on
alpha-carbonyl in the side chain. If an electron-attracting group
such as the carboxylic group, is attached to the hydroxyl, the electron transfer to the carbonyl is not
feasible, and the function of the auxochrome is paralysed.
• Change in the chemical structure of the auxochrome.
• Elimination of the chromophorically active substituent. for example, by breaking (he double bond
by oxidation, by splitting-oiTthc whole side chain (the Dakin reaction).
• Change in the chemical structure of the chromophore to a structure that is chrome; chromophore
ically inefficient.
Of the alternatives listed above, the procedures that could be considered for application in
conservation and restoration work are, predominantly, the change in the chemical structure of the
chromophore (selective oxidation methods, reduction methods or a combination thereof),
elimination of the chromophorically active substituent and interruption of the connection between
the chromophore and auxochrome.
Oxidation methods
The oxidation methods most current in conservation work arc bleaching with calcium and sodium
hypochlorite, chlorine dioxide, chloramines, potassium permanganate and stabilized hydrogen
peroxide. In their extensive literary review of the bleaching methods, Lienardy & van Damme'
mention some other agents, such as sodium peroxoborate, ozone, sodium chlorite and solar
radiation.
Sodium hypochlorite and calcium hypochlorite
Hypochlorites were used in textile bleaching for the first time in the eighteenth century and were
used on an industrial scale in paper bleaching until the year 1910, when they were replaced by
chlorine.
Hypochlorites are stabilized with an excess of alkali (pH 9.5—10.5); otherwise they are hydrolysed
in aqueous solutions, giving rise to hydroxide and hypo-chlorous acid, which is further decomposed
to chlorates, chlorides, and oxy-gen.°6'37 The presence of chlorine, hypochlorous acid or the
hypochlorite anion in aqueous solutions of hypochlorites depends on their pH: at pH < 2 only
chlorine is present, at pH 4.0-6.0 only hypochlorous acid is present in solution and at pH > 9.5 only
the hypochlorite anion is present there.
Hypochlorites arc nonspecific oxidation agents (redox potential +0.94 V). Lignin chromophores are
oxidized and chlorinated by the hypochlorites. The
latter attack both the alpha and beta position of phenylpropane skeletons or free hydroxyl groups.
Hypochlontes also oxidize cellulose. 1 he removal of chromophores resent in oxycellulose,
however, is connected with the splitting of the p-D-glucopyranose ring. During the reaction
between hypochlorii" and cellulose, the following processes take place:
e oxidation of the primary hydroxyl group to the alddivdc and carboxylic group;
a nc!
• splitting of the glucosidic bond and oxidation of the reducing end group to the carboxylic group.
Lewin & F.pstein38 found that, in the neutral and acid pH range, carbonvl group are predominantly
formed, while carboxylic groups arise in the alkali TV region. Koudelka & Svoboda12 arrived at
similar conclusions.
Hypochlorites cause degradation of paper undergoing bleaching. Oxidation proceeds mainly in
amorphous domains of fibres, and the rate of degradation depends on pH of hypochlorite solution
used. Burgess & Hanlan39 investigated the degradation of cotton during bleaching with hypochlorite
as a function of pH of bleaching solutions. They found that the highest degradation is brought about
by sodium hypochlorite in the neutral range of pH. In the acid (pH 4.5) and alkaline (pH 9.0) range
the degree of degradation is smaller. According to Lewin & Epstein38 hypochlorite bleaching may
also be accompanied by the destruction of cellulose due to the alkalis present. Carbonyl groups
formed during the oxidation on the carbon atoms C2, C3; and C6 of 38-D-glucopyranose represent the
(3-glucooxycarbonyl structure sensitive towards the degradation effect of the alkalis.
Strofer-Hua40 stresses the role of the OH radical during oxidative bleaching processes. Because this
effect is minimized using an alkaline hypochlorite solution, lie recommends the general use of this
process in the paper restoration workshop.
The hypochlorite bleaching reduces the colour stability of paper subjected to bleaching.8'12'19
Koudelka & Svoboda12 investigated the yellowing of hypochlorite oxycelluloses and the effect of
functional groups thus formed on the process. In accordance with other reported datn,1'8'19 they
concluded that colour reversion depends on pH of the bleaching solution. The greatest degree of
yellowing was observed with paper using solutions of neutral pH (carbonyl groups are formed that
support the yellowing). Burgess'9 reports that bleaching of wood-containing paper with a high lignin
content is accompained by the
formation of unstable chloro-quinoid structures supporting the colour reversion.
Chloramine B and chloramines T
Chloramine is often employed in the conservation work in the bleaching of engravings, pieces of
graphic art and watercolours, because the degradation of colours and pigments is minimal and the
process of bleaching can be easily checked.35'41
Chloramine T (sodium X-chloro-p-toluenesulfonamide) and chloramine B (sodium Nchlorobenzenesulfonamide) are oxidizing agents having redox potential + 0.9 V. In aqueous
solutions the chloramides are hydrolysed and hypochlorite is formed, which undergoes further
oxidation. Similar to hypochlorites, in aqueous .solutions of chloramines, chlorine is also present in
several forms: as Cl2 (pH<2), hypochlorous acid (pH 4-6) and the hypochlorite anion (pH>9.5).
The bleaching component in aqueous solutions of chloramine is represented by sodium hypochlorite
formed by hydrolysis, and the effect of chloramines of chromophores in paper subjected to
bleaching is therefore similar to that of hypochlorites.
Santucci42 was the first to point out the negative effect of chloramine residues remaining in the
paper after bleaching. He studied changes in the average degree of polymerization of Whatman
chromatographic paper after bleaching with a 2% solution of chloramine T and accelerated ageing.
The degree of polymerization of the samples from which the remaining chloramines had not been
removed by washing with distilled water decreased by 50%, whereas that of washed samples
remained unchanged. Unlike Santucci,42 Daniels43 argues that washing with water is insufficient to
remove chloramine residues from paper, because they form a insoluble complex with the aluminium
present in paper that cannot be removed from paper by water alone. Moreover, the complex
preserves the oxidative properties of chloramine. This is why Daniels suggested that the bleaching
with chloramines should be followed by a reduction stage (antichlorine) in which the residues of the
bleaching agent are reduced. The whole bleaching process must be completed by thorough washing
with water. Burgess41 investigated in detail the possibility of a reduction removal of residues after
bleaching with chloramine T. Using the results of his study, she suggested an order in which the
ability to remove chloramine residues from paper increased:
Tetrahydridoborates arc more effective in aqueous solutions than in alcoholic ones.
Chloramine residues that have not been removed from paper cause further bleaching of the paper
support.19'41 This is why any Further changes in papers undergoing bleaching depend on the kind o!
antichlorine used or on its ability to remove residues after bleaching. According to Burgess,19
tetrahydridoborates not only destroy the chloramine-aluminium complex, but they also reduce
carbonyl groups in cellulose, thus reducing the rate of colour reversion.
Chlorine dioxide
Chlorine dioxide was first used in conservation by Gettens in 1952.
Chlorine dioxide has 19 valency electrons. It readily forms free radicals. which makes it unstable. Ii
explodes if mixed with oxygen; its aqueous solution are comparatively stable. Chlorine dioxide is
highly toxic to all living organisms. It oxidizes and chlorinates lignin and resins; under suitable
conditions ii docs not react with cellulose and hemicellulose. In an acid solution its complete
reduction results in five oxidation equivalents;
In the bleaching with chlorine dioxide, the pH value becomes an important parameter of the
bleaching process. In an alkaline medium it reacts with hydroxide anions;
The reaction cannot be used in bleaching, because chlorates are not a bleaching agent. For this
reason paper should not be deacidified before bleaching.44 At pH 4.0, reaction D is very slow and
only 10% ClO2 decomposes; at pH 7.0 90% decomposes. At higher pH (> 7.0), not only lignin but
also cellulose and hemicellulose are oxidized. The formation of chlorates (reaction D) can be
suppressed by adding chlorites. In an acid solution chlorine dioxide, hypochlorous acid and
chlorous acid are in equilibrium with elementary chlorine. These compounds act as oxidation
agents.
Chlorine dioxide reacts minimally with cellulose and hemicellulose but reacts readily with phenol
and its derivatives. An analysis of the reaction products of lignin model compounds showed that the
reaction with chlorine dioxide is
accompanied by dimethylation, splitting of the benzene ring, chlorination as well as by the
formation of ortho- and para- quinoncs.45
Burgess39 and Voelker46 investigated in detail the effect of chlorine dioxide on physical and
chemical properties. Under optimal bleaching conditions chlorine dioxide does not decrease the
average degree of polymerization to any considerable degree and, consequently, the mechanical
properties remain unimpaired.
Chlorine dioxide removes lignin and resins, thus essentially reducing the colour reversion of papers
subjected to bleaching. Burgess39 found that the colour reversion of papers made from rags (high
content of alpha cellulose) depends on the degree of oxidation of the latter, which is minimal for
CIO, For wood-containing paper, the colour reversion depends on the amount of chlorine remaining
in paper (chlorine-lignin bond) more than on the oxidation itself. As chlorine dioxide reacts with
lignin very readily, the amount of chlorine remaining in wood-containing paper is as much as ten
times higher than in rag paper. This is why bleaching with ClO2 must be followed by applying a
suitable antichlorine.
Hydrogen peroxide
Hydrogen peroxide is applied in conservation in the form of aqueous or aqueous-alcoholic
solutions. Its main advantage consists in its specific effect on lignin and lignin compounds, and it
reacts minimally with cellulose. Aqueous solutions of hydrogen peroxide are weak acids:
The bleaching effect of peroxide is based on the oxidation ability of the peroxide
The number of peroxide anions arising by dissociation depends on the temperature and pH of
solution. The optimal pH of bleaching is 9-10. A considerable disadvantage of hydrogen peroxide
consists in its sensitivity to the effect of some metal ions (Fe3+, Mn2+ and Cu2+) and enzymes.
Oxygen formed by the decomposition initiates the radical reaction of cellulose chains, causing its
degradation.48 Also, oxygen is absorbed by lignin and undergoes dimerization through the
mediation of phenoxy radicals formed in the reaction. The chromophores thus formed are resistant
to further bleaching.22,47 This is why hydrogen
peroxide is stabilized in. most cases by an addition of sodium silicate and magnesium sulfate39,47,48
These stabilizers form coordination compounds with metal ions, thus inactivating the latter.
Moreover, a colloid suspension of magnesium silicate is also formed, which adsorbs metal ions on
its large surface.47 The higher pH value has a stabilizing effect, too.
Hydrogen peroxide reacts mainly with lignin chromophores and nonlignin polyphcnols.47 A general
scheme of the reaction between alkali peroxide and lignin compounds, based on the nucleophilic
addition of the peroxide anion to the carbonyl group or to conjugated carbonyl structures, is given
in Fig. 4.
The alpha carbonyl group reacts with hydrogen peroxide under mild conditions only if there is free
hydroxyl group in the position para.49'50 Methoxyhydroquinone is the reaction product in this case
(the Dakin reaction). The double bond in the alpha position to the benzene ring reacts with peroxide
only if there is aldehyde or ketone carbonyl in the vicinal position to the double bond. The final
oxidation product is arylaldehyde, which undergoes further degradation via Dakin's mechanism.50
Under normal conditions, quinone structures are oxidized by alkali peroxide with great difficulty'.
Burgess51 used gel-permeation chromatography to investigate changes in the molecular weight
distribution of various types of paper after bleaching with stabilized hydrogen peroxide. She
observed that modern rag papers are not degraded with peroxide, whereas historical papers showed
a mild degradation of the cellulose fibre..
Bleaching with alkaline peroxide improves the colour reversion of paper undergoing bleaching.8'9
However, if optimal conditions are not observed (stabilization and pH), aldehyde, dialdehyde or
ketone groups may be formed in cellulose and the colour reversion may increase considerably.8
Bezúch & Masarykova52 found that, although alpha-carbonyl groups in lignin are degraded during
the bleaching with peroxide, new, secondary chromophores having the alpha-carbonyl structure are
formed. They recommended, therefore, that the peroxide stage should be followed by the reduction
stage (best with NaBH4), which removes the carbonyl groups formed, thus improving not only the
yield of bleaching, but also the colour reversion.
Potassium permanganate
The method of bleaching using potassium permanganate consists of two stages: the oxidation stage
(oxidation of cellulose, or lignin, or impurities, in which brown insoluble manganese dioxide is
formed, which is precipitated on paper fibres) and the reduction stage, in which manganese dioxide
is reduced
Fig. 4
to colourless manganese oxide. The reducing agents most often used are sodium dithionite and
potassium disulfite.33
Potassium permanganate is a nonspecific oxidizing agent (E0MnO4-/MnO2 = + 1.695 V) that oxidizes
chromophores and degrades the aromatic ring of lignin to insoluble aliphatic fragments. Fheder54
successfully used potassium
stains from paper.
After bleaching with potassium permanganate, the physical and chemical properties of paper
deteriorate. Sobucki55 reports that, after oxidation with a 0.16% solution of permanganate lasting 4
h, the average degree of polymerization decreased by 66%. the tearing load decreased by 25%, the
folding resistance by 13%, and the copper number increased four times. The emission spectral
analysis of paper bleached according to ref. 53 showed that, after reduction and thorough washing,
the paper still contains 0.1 -d.01% Mn, an amount sufficient to catalyse oxidative degradation. In
his literary review, Neuvirt5 reports, however, that the effect of manganese on the rate of oxidation
is not completly unequivocal, because in some cases it acts as the catalyst and in others as the
stabilizer.
Problems of colour reversion of papers during bleaching with permanganate Lave not attracted
much attention. Only Polčin22 in a study on wood-containing fibrous material reports that Mn (II)
ions may contribute to the coloration of this material by the formation of colour complex
compounds. On the contrary, according to Rapson & Spinner,8 Mn (II) ions inhibit the colour
inversion. Further colour changes may be brought about by manganese oxide, which remains in the
paper after bleaching with permanganate, if the papers thus conserved arc again subjected to
bleaching. Manganese oxide is oxidized to brown manganese dioxide. Baynes-Cope56 suggested a
procedure for the removal of the oxide thus formed by using hydroxylamine hydrochloride acidified
with hydrochloric acid. After the reaction the papers should be thoroughly washed with water.
Reduction methods
The reduction methods include anionic and cationic ones. In the former, anion (sulfite, dithionite)
donates electrons; in the latter, the donor is the cation of a polyvalent metal, the more positive form
of which is more stable. If we bear in mind the electronegative character of cellulose, it becomes
obvious that the cationic reducing agents react much more readily and quickly than anionic agents.
Nevertheless, anionic agents, particularly dithionites and tetrahydridoborates, predominate in
conservation practice.
The dithionite ion consists of two SO2- groups, with the sulfur atoms situated at a distance of 0.239
nm, which causes considerable lability of the whole ion.
Dithionites decompose either by oxidation or by spontaneous decomposition.57,58 In an
acid medium the oxidation-reduction reaction proceeds with the potential E0 = -0.08 V:
In an alkaline medium, with the potential E0 = —1.12 V:
The high reducing ability in an alkaline medium cannot be employed in practice in the bleaching of
lignin chromophores, because resistant secondary chromophores are formed in such a medium. This
is why an optimal pH should be found, at which redox potential of dithionite is sufficiently high and
the rate of decomposition is minimal (for ZnS2O4 : pHopt = 5.5 — 6.0; for Na2S2O4 : pHopt = 5.5 —
6.5).57 Zinc and sodium dithionites are used in industry; in the conservation work sodium salt is
mainly used. Dithionites predominantly bleach the wood-containing fibrous material. Polčin57,58
found that, in a weakly acid solution, dithionite reduces quinones and quinonemethides. In
cinamaldehyde and flavonoid structure, only double bonds are reduced. Chromophore systems'
benzene ring-carbonyl or benzene ring-double bonds are virtually resistant to the reduction with
dithionites. Reduction also decreases the colour of organoferric complexes (Fe3+ — > Fe2+) but the
reaction is reversible and the bleaching effect of dithionite is thus paralysed.
Voelker46 reports that the average degree of polymerization of paper and the content of lignin and
hemicellulose are not decreased by bleaching with dithionites.
With reducing agents the colour reversion becomes a serious problem, because reduced
chromophores may be reoxidized, and the brightness resumes the original values of unbleached
paper.27,59 This is why dithionites are mainly used as antichiorine agents in the oxidizing bleaching
in conservation practice.
Tetrahydridoborates
Tetrahydridoborates are often used in conservation, because with sulfur-containing reducing agents
they are not only more effective bleaching agents, but also more effective as antichiorine agents.
19 41,52,59
,
Moreover, reduction with tetrahydridoborates raises the resistance of cellulose fibres to the
acid hydrolysis, photo-oxidation, the effect of alkalis and ultraviolet radiation.59 In both aqueous
and alcoholic solutions the tetrahydridoborates undergo decomposition:
The rate of decomposition is much faster in aqueous solutions than in alcoholic, ones. Since
hydrogen cations accelerate the decomposition, paper undergoing bleaching must he deacidified
before the reduction with tetrahydridoborates.
Of cellulose chromophores, carbonyl groups are reduced with tetrahydrido-boratcs.12,19,41,59 Of
lignin chromophores, reduction affects predominantly alpha-carbonyl groups, quinone arid
quinonemethide structures.
'I he reduction with tetrahydridoborates does not impair the mechanical properties of paper, but
hydrogen formed by the decomposition or released bubbles may cause damage, mainly to brittle
capers. A certain danger to paper undergoing bleaching consists also in a high pH value of aqueous
solutions, which increases by reaction E until the establishment of equilibrium (pH 9-10).
When studying the colour reversion of papers bleached with tetrahydridoborates, Burgess59 came to
interesting conclusions:
• Paper bleached with solutions of tetrahydridoborates at a higher concentration is more subject to
yellowing than that bleached using solutions at a lower concentration.
•Methanol solutions support yellowing more than aqueous solutions.
• Sodium tetrahydridoborate supports the colour reversion least of all.
• Colour reversion depends on the time of bleaching: papers bleached for 5-8 min are most
subjected to yellowing during natural ageing; hence, the minimal time of bleaching is 10 min.
Interrupted connection between auxochrome and chromophore
In his review of possible procedures of destruction of chromophore systems, Polčin21 (chapter 4)
suggests eliminating the function of auxochrome (without interruption of the chemical bond) by
binding an electron-attracting group. For instance, the effect of phenolic hydroxyl as the most
important auxochrome of wood-containing chromophores may be eliminated by esterification (for
example, by acetylation). Lyall60 and Hon61 used the method in the stabilization of lignin in woodcontaining papers. According to Lyall,60 acetylation with acetanhydride vapours is completly
ineffective. On the contrary, Hon60 asserts that acetylation with acetanhydride vapours is effective
and does not reduce the mechanical properties of paper during natural ageing. The method is very
promising, because preservation of wood-containing newsprint without any changes in its
mechanical and optical properties is a worldwide problem as vet unsolved.
CONCLUSION
Carbonyl groups on atoms C2 and C3 of β-D-glucopyranose are the main chromophores of cellulose
and hemicelluloses. 1 he double bond and carbonyl groups conjugate with the benzene ring, and
quinoid structures are the main chromophores of lignin, nonlignin polyphenols and resins. Colour
compounds produced by mould and bacteria arc mainly anthraquinones and carotenoids. The
destruction or removal of the chromophore systems mentioned above is difficult, bearing in mind
the artistic and historical values of artefacts undergoing conservation, because all chemical
bleaching methods used in the conservation of both historical and modern paper always cause some
damage This is why, before applying any bleaching method, one should know the chemistry of
bleaching and should try ail other alternatives suited for the removal of stains, impurities or the
brown shade in paper, by surface cleaning 'with soft rubber or a brush, washing and cleaning by
means of surface active compounds, organic solvents or enzymes.
SUMMARIES
Chemical processes in the bleaching of paper in library and archival collections
The article describes in detail the chemical reactions that happen during bleaching processes in the
restoration workshop. The formation and destruction of chromophores in cellulose, hemicellulose
and lignin are discussed. The result of an analytical reflection on bleaching processes from the
chemical viewpoint must be the suggestion to try all other alternatives first before choosing a
bleaching process, as it might be oxidative or reducing.
Precedes chimiqu.es de blanchiment du papier dans les collections de limes et d'archives
Get article decrit en detail les reactions chimiques qui ont lieu durant les procedes de blanchiment
en restauration. La formation et la destruction de chromophores dans la cellulose, i'hemicellulose et
la ligninc sont discutees. Le resultat de cette reflexion analytique sur les procedes de blanchiment
d'un point de vue chimique suggere d'essayer d'abord toutes les autres alternatives avant de choisir
un precede de blanchiment, qu'il soit oxydant ou reducteur.
Cliemische Vorgange brim Bleic/ien van Papier in Arckw und Bibliothck
Dor Arlikri beschreibt rccht detaiiliert die rhemischen Reaktionen einer rcstauratorischen Papierbleiche. Vor allem \vird die Bildung und Zcrstorung von Chromophoren diskutiert. Das Ergebn:^
einer soleh anaiysierenden Beirachtung vom Standpunkt ties Chcmikers mutt dcr Ratschlag soin,
zuerst alle anderen Moglichkciten zu prufen, einen Fleck oder eme Yerfarbung zu beseitigen, bevor
auf einem Bleichpro'/.ei! zuruckgegriffen wird, cr sei oxidativ odor redukiiv.
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Dipl.-ing. Michal Ďurovič
State Central Archives Chemical Research Laboratory
Karmelitská 2
11801 Prague 1
Czech Republic
Prof.ing. Jiři Zelinger, DrSc
Institute of Chemical Technology
Laboratory of Restoration Chemistry of Works of Art
Technikú 5, 166 28 Prague 6
Czech Republic