ZFB - ZENTRUM FÜR BUCHERHALTUNG GmbH
Bücherstraße 1 | 04347 Leipzig
Telephone 0341 25989 - 0
e-mail [email protected]
www.zfb.com
Z E N T R ZUEMN TF R
ÜU
R MB UF C
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
On the treatment of aged papers
An explanation of the aging mechanisms, deacidification processes, and current findings
regarding the incorporated alkali reserve.
Dr. Manfred Anders und Katharina Schuhmann
1
Chemical decomposition reactions.................................................................1
Influence of ageing on the strain behaviour of paper.....................................3
Origins of acids in paper..................................................................................4
Comparison of paper characteristics according to type of
manufacturing process................................................................................... 6
Paper deacidification..............................................................................................7
2.1
2.2
2.3
3
January 2016
The ageing of paper .............................................................................................. 1
1.1
1.2
1.3
1.4
2
Overview of processes available on the market.............................................7
The ZFB||2 process....................................................................................... 8
Effects of process engineering and paper characteristics on
measurement procedures for quality tests of deacidification....................... 8
Alkali reserve – an explanation of the KUR project................................................. 9
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Definition of the alkali reserve ...................................................................... 9
Relationship between the alkali reserve and the pH-value............................ 10
Standards regarding the amount of the alkali reserve................................... 10
Does a high alkali reserve promote cellulose decomposition?...................... 11
DIN recommendations and further investigations regarding the influence of a high alkali reserve on the brittleness of paper........................................ 12
How high is the alkali reserve in the deacidified originals?............................13
Summary and conclusions..............................................................................13
Sources / References............................................................................................................15
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
1
The ageing of paper
Paper, a material manufactured out of plant fibres, is generally subject to the deterioration of its raw material. The progression of this natural deterioration is commonly referred to as the ageing of paper. All of the
processes which bring about a negative change in the physical and optical and characteristics of paper are
referred to as ageing processes. The particularly decisive factor is the influence of the ageing processes on
the cellulose which forms the main component of paper. Possible mechanisms leading to the decomposition
of cellulose fibres can be classified as follows:
Table 1: Deterioration mechanisms of cellulose
Decomposition
Process
chemical
• acid hydrolysis
• alkali decomposition mechanisms
• oxidation
• cross-linking
biological
• enzymatic hydrolysis due to microorganisms
physical
• mechanical strain
• climatic fluctuations
In the following report, it is the chemical decomposition reactions in particular which will be examined more
closely regarding their contribution to the phenomenon known as the “deterioration of paper.” Primarily,
during in the course of the (chemical) ageing of paper, a loss of fibre stability and elasticity of the fibre
structure take place. The paper becomes increasingly brittle and fragile until it cannot be used any longer
without damaging it unless countermeasures are taken. It literally falls to dust in one’s hands. In addition,
an optical change takes place (in the case of papers containing lignin) which leads to an increasing yellowing
of the paper.
The temporal course of the ageing process is essentially determined by the characteristics of the paper conditioned by the manufacturing processes as well as by the storage conditions of the paper. These will also
be examined in more detail later.
1.1
Chemical decomposition reactions
The chemical decomposition reactions described in the following section initially take place in the amorphous areas of the cellulose within the paper, since these are more sensitive to outside influences due to the
smaller number of hydrogen bonds between the fibrils and the fibres. In this process the average degree
of polymerization (DP) decreases very quickly at first. Due to the decomposition of the amorphous areas,
the crystallinity of the cellulose correspondingly increases, which leads to an embrittlement of the fibres.
Only after the decomposition of the amorphous areas has taken place are the crystalline areas attacked, in
the course of which the DP decreases only slowly. The aforementioned reactions usually take place simultaneously and are mutually dependent.
1
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Acid Hydrolysis
According to the current state of knowledge, the main cause of paper deterioration, also known as “acid
corrosion,” is due to hydrolysis of the cellulose which is catalysed by acid. This involves splitting the 1.4-β
glycosidic linkage as illustrated in Figure 1. This results in an irreversible breakage of the cellulose chains and
ultimately to a shortening of the cellulose fibres, which is in turn attended by a corresponding loss of fibre
stability.
Illustration 1: Reaction mechanism of acid hydrolysis [Anders 2000, p. 44]
•
•
•
•
Acids catalyse the hydrolysis – they are not used up in the hydrolysis, but remain present and
unaltered after the reaction.
The rate of hydrolysis rises with an increasing concentration of hydrogen ions or when the
pH-value falls. Thus, in an acidic milieu of around e.g. pH 5, the reaction takes place 1000 times faster than in an alkali milieu of pH 8.
Organic acids (products of oxidative cellulose decomposition) accelerate the hydrolysis.
The reaction process is accelerated by increased temperature and humidity.
Oxidation
In addition to acid hydrolysis, oxidation processes also contribute significantly to the decomposition of cellulose. In pure cellulose, the formation of carbonyl, carboxyl, and peroxide groups always takes place if
oxygen is present. If exposed to light, these oxidation processes are always accelerated. The carboxyl groups
of oxycellulose and the organic acids which form out of them (acetic acid, formic acid) again accelerate the
acid hydrolysis.
Cross-linking
The term cross-linking means the formation, schematically shown in Illustration 2, of new bonds between
the decomposition products which arise due to hydrolysis and oxidation of the cellulose. The amorphous
areas of the cellulose give the paper in its original condition the necessary bendability and elasticity. Due to
the formation of new, rigid bonds in these areas, a loss of these characteristics occurs, which leads to the
paper becoming increasingly brittle.
2
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Illustration 2: Schematic representation of cross-linking in amorphous areas [Giovannini 2004, p. 154]
1.2
Influence of ageing on the strain behaviour of paper
As explained above, the chemical decomposition reactions lead to a loss of the stability of individual fibres due to oxidation and hydrolysis as well as to a reduction of elasticity of the fibre composition due to
cross-linking. Rigid bonds have formed between the now markedly shorter fibres.
If one considers the behaviour of the tensile strength of different papers, new papers react in such a way
that after an initially reversible (visco-) elastic and subsequently irreversible plastic elongation, in which
the fibres slide past one another, the bonds between the fibres finally rupture (see Ill. 3). By way of contrast, aged papers exhibit distinctly less viscoelastic and plastic elongation portions. The entire elongation at
rupture is distinctly less than is the case with new paper. The tensile strength (without previous folding),
however, is only slightly smaller (compare Ill. 3). [Note: Should (Bansa-Hofer) folding be carried out before
the tensile test, aged papers show distinctly less tensile strength.]
Tensile force
The rigid intermediate fibre bonds cause the fibres to rupture before they can glide past one another after
undergoing only an elastic elongation. The differing rupturing behaviours are shown particularly clearly in
the SEM images of the ruptured edge of a new and an aged paper provided in Illustration 3.
New paper
Old paper
Elongation
Illustration 3: Tensile strain diagram of aged and newer paper without folding [Anders 2000, p.67]
3
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Illustration 4: SEM images of ruptured edge of a wood-pulp paper before (left) and after (right) accelerated ageing
[Anders 2000, p. 86]
1.3
Origin of acids in paper
In answering the question regarding the origin of the acids in paper, one must distinguish between acid
sources which are endogenous, in other words contained within the paper due to manufacturing processes
or within the type face, and those which are exogenous, meaning that the environment affects the paper.
When examining handmade rag papers, they exhibit very good flexibility and tensile strength even after several centuries have passed, predominantly alkali pH-values and no to only very slight yellowing. The ageing
process takes place only very slowly in these papers, which is due to the very good quality of the fibres. The
raw materials for such papers included rags from such materials as cotton, flax, hemp or other natural fibres,
from which almost pure, very long cellulose fibres could be obtained. These fibres possessed a high stability
which corresponded to their length. Lignin, which leads to the yellowing of paper, was not present or was
only present in very slight amounts. Manufacturing processes using fresh spring water as well as storing the
fibres in limewater imparted the paper an increased share of earth alkali ions, whose salts further link the
cellulose chains to each other and buffer against external acidic influences. The surface sizing which was
employed at that time with slightly acidic to neutral gelatine or bone glues had hardly any damaging effects
on the ageing behaviour.
In contrast, industrial papers which were manufactured using acidic processes in the time between around
1850 and 1990 are particularly critical regarding their endogenous acid sources. These papers had already
exhibited an acidic pH-value at the time of their manufacture. The individual sources of acid will be examined more closely in the following sections. Table 2 provides an initial overview to the correspondingly
categorized sources of acid.
Table 2: Possible sources of acid
Endogenous sources of acid
Exogenous sources of acid
• acidic sizing (alum → sulphuric acid)
• Hemicelluloses
• Lignin (decomposition products)
• carboxyl groups
• Iron gall inks
Acidic corrosive gases from the environment
• sulphur oxide
• hydrogen halide
• nitric oxide
4
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Endogenous sources of acid
With respect to rag papers, there is only one (indirect) endogenous source of acid: the iron gall ink which
was widely used at the time for writing and which led to so-called “ink corrosion.” Gallic acid obtained from
gallnuts was used to manufacture these inks. An excess of iron (II) sulphate was added which created a soluble, colourless iron (II) gallic acid complex. Only after some time had passed after the writing was applied
to the paper did the typical black, water-insoluble typeface in the form of iron (III) gallate appear due to
oxidation processes. Regarding the ageing of paper, several damaging effects have arisen over the course of
time in the area of the typeface. First of all, the gallic acid in the ink as well as the sulphuric acid arising from
the contact of ferrous sulphate with humidity both serve to promote hydrolysis. Furthermore, iron (II) ions
have a catalytic effect on the oxidation of cellulose and/or oxidize into iron (III) oxide hydrate (rust). To treat
the objects which have been damaged by ink corrosion, more elaborate restoration methods are required
than the paper deacidification which is the topic of this report and these methods will not be discussed in
any more detail here.
Since around the time of the first industrial paper manufacturing in about 1850, hard water was not used
for the production of paper, but rather only soft water was employed so as to protect the paper machinery.
Therefore, industrially manufactured wood-pulp papers do not contain a “home-made” alkali buffer as do
rag papers. In addition, surface sizing using gelatine/bone glue was replaced by an acidic rosin sizing. This
involved adding rosin acids (abietic acid, colophony) directly to the suspension fibres. These acids make the
cellulose fibres water-repellent so to speak, yet do not adhere to them sufficiently. Therefore, the rosin
acids were first converted into water-soluble rosinates and finally fixed onto the fibres in a complex with aluminium ions. For this fixation, it was necessary to add an excess amount of potassium aluminium sulphate
(alum) into the fibre suspension. Excess alum quickly formed sulphuric acid in the paper when it came into
contact with water.
The main problem of acidic paper is not ink corrosion, but industrially manufactured paper (from about
1850 to 1990). In addition to cellulose, the wood pulp used to manufacture paper contains 8-24% hemicelluloses and 22-32% lignin. Hemicelluloses naturally contain uronic acids, which contribute to the catalysis of
the hydrolysis process. Lignin forms organic acids in oxidative decomposition reactions, in particular oxalic
acid. Furthermore, the phenols contained in paper oxidize to discolouring quinoid compounds, thus causing
the wood-containing paper to go yellow.
Depending on the chemical pulping process, in which the lignin is separated from the wood pulp, and the
bleaching process which is applied, the wood pulp which is used even today to manufacture paper is disadvantaged with respect to oxidative factors and contains carboxyl groups. Globally, paper is predominantly
manufactured today using the sulphate process of alkaline solubilized, long-fibred kraft pulp. However, since
this process was still associated with a strong odour until only a few years ago, pulp was manufactured in
Germany for a long time using the (acidic) sulphite process. Up to a certain point, carboxyl groups are also
required for increasing the wet tear resistance and are therefore necessary for ensuring the web tension
and velocity which are required for effective machine operation.
Exogenous sources of acid
If the ambient air contains harmful gases (sulphur oxide or nitric oxide), these are absorbed by paper and
form bonds with water (humidity) acids in the paper. These then catalyse the acid hydrolysis, as described
above. A combination of sulphur oxide and nitric oxide is particularly critical since their damaging influences
intensify each other.
5
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
1.4
Comparison of paper characteristics according to type of manufacturing process
The preceding descriptions of decomposition reactions as well as possible endogenous and exogenous sources of acid which promote or catalyse cellulose decomposition in the course of ageing should clarify what
influence the various manufacturing processes, the materials and equipment used as well as storage conditions have on the ageing behaviour of different types of paper. In Table 3 the characteristics of papers
produced using different types of manufacturing or produced in different eras have been summarized.
Table 3: Paper characteristics according to type of manufacturing process and era
Rag paper, aged
Industrial wood pulp
Modern paper
paper (1850-1990) aged (from 1990 on) new
Novo test paper, new
• hardly any organic
acids
• possible sulphuric acid
due to iron gall ink
• alkali content due to
spring water and
storage of fibres in
limewater
• slightly acidic to
neutral sizing
• alkali pH-value in the
core
• organic acids
(oxidation, hemicellulose, lignin decomposition
• sulphuric acid (acidic
sizing)
• acidic pulping
• acidic pH-value,
according to progression of the ageing ≈
3 pH
• negative alkali reserve
• hardly any organic
acids, primarily
alkaline pulping
• no sulphuric acid
(only in the case of
extreme exogenous
conditions)
• usually 10-15% fillers
(kaolin, CaCO3, TiO2)
• DIN EN ISO 9706:
alkali reserve >1.64
m% MgCO3
• TP1, wood-free:
100 % cellulose,
alkaline pulping
• TP2, wood-containing,
cellulose and wood
pulp
→ organic acids
• 10-15 % filler (kaolin)
• acidic sizing
→ sulphuric acid
• 4.5/5.5 pH
• negative alkali reserve
→ good ageing
resistance due to
high quality of fibres
and alkali buffer
→ still flexible and
usable today
→ point of rupture:
inter-fibre bonding
→ not ageing resistant
→ fragile
→ brittle
→ low tensile strength
→ low durability
→ yellowed
→ point of rupture:
fibre
→ largely ageing
resistant
→ flexible
→ not brittle
→ high tensile strength
→ good expansion
behaviour
→ point of rupture:
inter-fibre bonding
→ not ageing resistant
→ unaged: mechanical
properties still like
modern paper
•
Rag papers are very ageing resistant due to the very high quality of their fibres as well as their
alkaline pH-value (limewater, spring water) and are also highly flexible and stable even after many centuries have passed.
•
Industrial wood-pulp papers are subject to a very rapid and autocatalytic ageing process due to
their high pH-value imparted by the manufacturing process (acid sizing, hemicelluloses, lignin → sulphuric acid, organic acids; after only a few decades these papers become increasingly fragile,
brittle and yellowed.
•
Modern papers today largely contain no or only very slight amounts of wood pulp and are predominantly acid free and ageing resistant; they are flexible and elastic and react to tensile force with
fibres that glide past one another until the inter-fibre bonds rupture.
6
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
•
To test the success of mass deacidification treatment, the company KLUG sells conservation test
papers (Novo 1+2) which are designed to simulate the characteristics of acidic wood-pulp papers
and acidic wood-free papers; the acidic pH-value is only regulated via the alum sizing (sulphuric
acid); these papers receive hardly any organic acids such as those contained in naturally aged
papers.
2
Paper deacidification
Methods which deacidify paper are generally employed to treat industrially manufactured acidic papers dating from between 1850 and 1990. The target of the deacidification is neutralization of the acids contained
within the paper. Additionally, an alkali reserve should be introduced into the paper in order to protect it
from a new formation of acids and to protect it from the effects of external acids. In this way, the decomposition of cellulose should be markedly decelerated, thus lengthening the useful life of the paper. A more
detailed discussion of the alkali reserve will follow in Section 3.
2.1
Overview of processes available on the market
In all major mass deacidification processes available today, alkaline magnesium or calcium compounds such
as magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2) or calcium carbonate (CaCO3) are introduced
into the paper. Over the course of a longer period of time, these aforementioned magnesium compounds
transform into alkaline magnesium carbonate, whose precise composition can, however, fluctuate. Therefore, the paper contains a mixture of magnesium oxide and/or magnesium hydroxide as well as alkaline
magnesium carbonate (“magnesium alba” = magnesium hydrogen carbonate), which together form the
alkali reserve.
The classification of mass deacidification processes is carried out according to the manner in which the
deacidification agent enters into the paper. In the so-called dry or fine particle deacidification process, fine
particles of the deacidifying agent (MgO/CaCO3) are blown via an air current between the pages. However,
these kinds of processes have not been successful on the market since various independent investigations
have shown that the deep penetration of the deacidification is not sufficient and that the deacidification
agents are only effective more or less on the surface of the paper.
In the case of solvent methods, the deacidification agents are completely dissolved in an aqueous or nonaqueous carrier fluid. The books to be treated are saturated with the treatment solution which – in contrast
to the dry methods – transports the deacidification agent into the core of the paper. Non-aqueous solvent
methods utilize various magnesium alkoxides as a deacidification agent which neutralizes the sulphuric
acids contained in the paper by forming magnesium salts. Subsequent to treatment, during the course of a
reconditioning phase, the surplus deacidification agent forms the aforementioned magnesium hydroxide after coming into contact with water (humidity) or forms alkaline magnesium carbonate (“magnesium alba”)
which serves as the alkali reserve after coming into contact with the CO2 from the ambient air over a longer
period of time. In the dispersion process, the deacidifying agent in solid form is dispersed very finely within
a carrier medium. Generally, the books to be treated are then saturated with the dispersion, usually under
vacuum pressure. The decisive factor here is that the size of the particles is as small as possible which allows
the deacidifying agent to penetrate through the pores of the paper into its core.
7
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
2.2
The ZFB||2 process
The ZFB||2 method is a dispersion process in which a mixture of magnesium oxide (MgO) and calcium carbonate (CaCO3) are applied for deacidification. Both compounds are dispersed very finely within the carrier
medium heptane. The total amount of both of these alkali substances which remains in the paper after it
has been neutralized represents the alkali reserve. Magnesium oxide first transforms by reacting with water
into magnesium hydroxide (Mg(OH)2) which then in turn transforms over the course of a longer period of
time into alkaline magnesium carbonate (hydroxycarbonate) by reacting with carbon dioxide in the ambient
air.
2.3
Effects of process engineering and paper characteristics on measurement procedures for quality tests of deacidification
In Germany, the success of deacidification treatment is usually tested and evaluated according the guidelines of the so-called DIN recommendations.1 Among other things, these guidelines provide target value
ranges with respect to surface pH and extract pH, the alkali reserve, and tensile strength after a Bansa-Hofer
folding. One factor which exerts considerable influence on the success of the treatment as measured according to these standards is the process engineering of the deacidification process employed. When carrying
out deacidification treatment on test papers, the different modes of action of the processes as described
above generally lead to correspondingly different results for the aforementioned parameters. Should a deacidification process achieve a particularly high value in one of the parameters, this fact does not necessarily
mean that this technique is intrinsically better. For example, if one considers the dry process method, surface accumulations of the deacidifying agents will exhibit comparatively high results for surface pH-values
as well as for the alkali reserve. However, the pH-value taken from the cold extract results in a smaller value
than that which was measured on the surface, since the deacidifying agent does not penetrate into the core
of the paper, as has been documented by many independent investigations. The extract pH-value is formed
by an average of the paper core, which is partially still acidic, and the alkali deacidification agent on the
surface of the paper.
Furthermore, the characteristics of the untreated paper as a starting point are also decisive for the deacidification level which can be achieved. Aged, industrially produced, acidic papers contain both sulphuric
acid due to the alum sizing and organic acids produced in oxidation processes as described in Section 1.
The incorporated deacidifying agents CaCO3 and MgO as well as the magnesium hydroxide which forms out
of them enable neutralization both of strong acids such as sulphuric acid and the organic acids which are
usually weaker. Weak organic acids are never completely dissociated but are always present in a chemical
balance, leading to the well-known buffer effect which sinks the pH-value. If the ageing process of a paper
is already very advanced or if the concentration of organic acids in paper before deacidification treatment is
very high, then such papers can exhibit a slightly acidic pH-value of between 6.5 and 7.0 even after undergoing deacidification treatment despite the incorporated alkali reserve (see 3.2).
1
R. Hofmann and H.-J. Wiesner, “Empfehlung zur Prüfung des Behandlungserfolgs von Entsäuerungsverfahren für säurehaltige Druck- und Schreibpapiere”, in: “Bestandserhaltung in Archiven und Bibliotheken”, Publisher: DIN Deutsches
Institut für Normung e.V., 5., revised and expanded edition, Beuth Verlag 2015
8
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
3
Alkali reserve – an explanation of the KUR project
At the moment, the alkali reserve is the subject of many questions such as “Can one infer that a high pHvalue means that a high alkali reserve exists?”, or “Is a high alkali reserve dangerous for paper?” Therefore,
this section is designed to provide an overview regarding the topic of the “alkali reserve”. The central
discussion involves the risks which are associated with a high or a low alkali reserve. This discussion refers
particularly to the newest results of research supplied by the “KUR Project” for the support of sustainability
in mass deacidification.
3.1
Definition of the alkali reserve The alkali reserve (alkali reserve) of deacidified paper is the amount of alkaline substance which remains in
the paper after neutralizing the acids which were originally present in the paper:
Alkali Reserve = absorbed amount of deacidification agent - amount of acid
Measuring the alkali reserve is performed by an acid-alkali titration using an extract of very small cut pieces
of paper sample in accordance with ISO 10716. This method is prescribed by DIN guidelines. It is not possible to precisely measure the alkali reserve without destroying some material. Determining the alkali reserve
in originals, therefore, requires that one sheet of the original documents is sacrificed.
The DIN recommendations require that the alkali reserve is specified as a mass percent (mass %) of magnesium carbonate (MgCO3). For example, an alkali reserve of 1.6 mass % MgCO3 would mean that 100 g of
deacidified paper contains as much alkaline substance as corresponds to an amount of 1.6 g MgCO3.
In addition to indicating the alkali reserve in mass percent of magnesium carbonate, it is also possible to
indicate the mass percent of calcium carbonate in mol or mmol per gram or kilogram of paper. These specifications can all be converted to each other as follows:
1 mass % calcium carbonate (CaCO3) = 0.84 mass % magnesium carbonate (MgCO3) = 0.1 mmol calcium or
magnesium per gram of paper.
•
An alkali reserve protects the cellulose from any further decomposition arising due to acids which
form during the course of the oxidation processes. Additionally, it provides a buffer against exogenous sources of acid. In addition to the aforementioned air pollutants, these include non-deacidified library and archive materials which are stored in the same room as the deacidified material.
9
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
3.2
Relationship between the alkali reserve and the pH-value
The alkali reserve cannot be estimated based on the pH-value, since the pH-value is not solely dependent on
the alkali reserve but on several factors. The most important factor for this is the chemical composition of
the paper: old papers contain different decomposition products of cellulose and lignin which form a buffer
mixture together with the deacidification agent and can significantly lower the pH-value. This can lead to
papers with a high alkali reserve exhibiting a relatively low pH-value and papers with a low alkali reserve
exhibiting a high pH-value. Furthermore, the measured pH-value also strongly depends on the measuring
method which was employed. Surface pH-measurement does not destroy the object and is also simpler to
carry out than extract pH-value measurement, but it is less accurate and also more prone to errors. Only
when the surface pH-value is clearly within the acidic range (≤ 6) and the pH measurement has been carried
out completely properly, may one assume that the paper has been insufficiently deacidified.
•
When comparing the quality criteria provided by the DIN recommendations (surface/extract pHvalue, alkali reserve, tensile strength) the alkali reserve is the most important measurement for
the success of deacidification treatment. By measuring alkali reserve before and after deacidification, the amount of alkaline substance which has been incorporated can be assessed. However, the
pH value, in particular the surface pH-value, is influenced by several factors.
3.3
Standards regarding the amount of the alkali reserve
The international standards (ISO 9706: 1994, ISO 11108: 1996, ANSI Z39.48: 1992, DIN 6738: 2007-03) for
manufacturing ageing-resistant papers require a minimum alkali reserve of 1.7 mass % MgCO3.
The Library of Congress, the leading mass deacidification provider in the USA, considers an alkali reserve of
0.83 to 2.5 mass % MgCO3 (converted) to be sufficient.
In Germany the so-called DIN recommendations are authoritative, which require an alkali reserve of 0.5 to
2.0 mass % MgCO3 in deacidified paper (see point 3.5). However, these guidelines only apply for test books
which are treated together with the originals – for the originals themselves, no guideline values are prescribed, which is based on the fact that it is very difficult to compare heterogeneous original papers.
3.4
Does a high alkali reserve promote cellulose decomposition?
The recent research project entitled “Nachhaltigkeit der Massenentsäuerung von Bibliotheksgut” (The Sustainability of Mass-Deacidification of Library Stock) examined the important question of whether a high
alkali reserve promotes the decomposition of cellulose. This project will hereafter be referred to using the
name of its support organization, the “KUR Project”2 . In the KUR Project during the period from 2008 to
2010, independent scientists in a task force led by Professor Antje Potthast (Universtität für Bodenkultur
Wien) examined books taken from the Deutsche National Bibliothek (DNB – German National Library) and
the Berliner Staatsbibliothek (Berlin State Library) which had been deacidified between 1994 and 2006. The
results were published in several scientific journals and one dissertation whose main author is Ms. Kyujin
Ahn. Books and papers deacidified using the ZFB||2 process were not an object of investigation for the
KUR Project since this process has only been on the market since 2012.
2
KUR-Projekt “Nachhaltigkeit der Massenentsäuerung von Bibliotheksgut“, supported by the German Federal Cultural
Foundation in the KUR programme for the conservation and restoration of mobile cultural assets, duration 07/2008 10/2010
10
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Investigations regarding the alkali reserve in deacidified books
Kyujin Ahn and her co-authors determined the amount of the alkali reserve in samples taken from deacidified books and from non-deacidified duplicates. Furthermore, they submitted samples from the books to
an accelerated ageing process and determined the average molar mass of the cellulose before and after
the ageing. Regarding these measurements, the following rules apply: the higher the average molar mass,
the longer the chains of cellulose and the higher the stability of the paper. The less the average molar mass
of cellulose in the artificially aged paper decreased, the better the paper was stabilized by deacidification.
Furthermore, Ahn et al. also investigated whether and to what extent β-elimination – a special decomposition reaction of cellulose, which above all plays a role in the case of papers which had suffered oxidation
damage before treatment, is influenced by mass deacidification and how this β-elimination affects the molar mass of the cellulose.
The measurements of Ahn et al. demonstrate that while the β-elimination is in fact promoted by incorporating an alkali reserve, this undesired reaction, however, practically only develops on the side chains and
the ends of the cellulose chains. In this process, the average molar mass of the cellulose remains unchanged
so that the β-elimination does not exert a negative influence on the stability of the cellulose. On the other
hand, acidic hydrolytic decomposition, which leads to the splitting of the cellulose chains and thus to a weakening of the paper, is suppressed by introducing an alkali reserve.
Ahn et al. draw the following conclusions based on their measurements: “Larger amounts of an alkali reserve up to 1.5 mass % MgCO3 considerably reduce the loss of a weighted average molar mass (Mw) of
cellulose following accelerated ageing.” And furthermore: “It can be concluded as well that a higher alkali
reserve leads to more stable cellulose in the long run. The positive effects of a higher alkali reserve far
outbalance the negative effects. The fear that incorporating a higher alkali reserve might bring about additional damage, which has led to the rather moderate amounts of alkaline substance being incorporated
in European mass-deacidification processes, could not be confirmed in this study. On the contrary: raising
the alkali reserve to amounts from 1.0-1.3 mass % MgCO3 – in contrast to the lower thresholds of the
standards, meaning 0.3 % for the Swiss quality standard and 0.5 % according to the current DIN guideline,
would significantly improve the cellulose stability, while taking into consideration that there are also
other factors which influence the concentration of the deacidification agent.”
Investigations regarding a higher alkali reserve on model papers
The statement quoted above, however, only refers to books with an alkali reserve up to 1.5 mass % MgCO3
because there were hardly any samples exhibiting a higher alkali reserve to be found among the deacidified
books. In order to investigate the effects of an alkali reserve in the range above 1.5 mass % MgCO3, Ahn et al.
conducted additional experiments using two model types of book papers in which the alkali reserve values
had been increased up to a level of 5.1 mass % MgCO3.
These investigations demonstrated that an increase of the alkali reserve initially leads to an improved stabilization of the cellulose. After a certain threshold value has been reached – the level of which depends
on the type of paper – a further increase of the alkali reserve has practically no additional effect; in other
words: high alkali reserve values (above 1.5 mass % MgCO3) are sometimes useful and sometimes not, but
are never harmful.
11
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
3.5
DIN recommendations and further investigations regarding the influence of a high alkali
reserve on the brittleness of paper DIN guidelines regarding the amount of the alkali reserve
The DIN guidelines regarding the amount of the alkali reserve state: “The quality criteria ‘alkali reserve’ has
been fulfilled if this is between 0.06 mmol/g and 0.24 mmol/g alkaline earth (0.5 to 2 mass % MgCO3). A higher alkali reserve generally promotes the ageing resistance of the paper. However, an alkali reserve which is
too high can stiffen the paper and make it more fragile. If the paper is already fragile, the danger of stiffening
due to a higher input of alkali is more pronounced.”
Investigations on the effects of a high alkali reserve on paper stability
The assumption that a high alkali reserve leads to paper becoming brittle is based on two investigations by
Dr. Manfred Anders and Dr. Joachim Liers from 2000 and 2002. In his investigation on papers which had been
deacidified by the papersave process, Liers determined that in the case of mechanically intact papers, even
an alkali reserve of up to 4.38 mass % MgCO3 did not lead to brittleness; however, mechanically weakened
papers already displayed brittleness at an alkali reserve of 1.73 mass %.
Liers und Anders explain the brittleness observed in damaged papers as due to the deacidification particles
being stored between the cellulose fibres. Therefore, the danger of embrittlement becomes greater, the
more inorganic particles are introduced into the paper. The amount of inorganic particles which is brought
into the paper cannot always be equated with the level of the alkali reserve, which is described in the following section.
Amount of salts introduced into paper during mass deacidification
With respect to the amount of inorganic particles introduced into the paper, the papersave process of ZFB3,
which was the process employed by Anders and Liers in their investigations described above, possesses one
great particularity. This process works with a complex of magnesium alkoxide and titanium alkoxide, which
is why titanium dioxide (TiO2) remains present in the paper after the deacidification process in addition to
the deacidifying agent of magnesium hydroxide. Chemically, TiO2 is a completely non-hazardous substance,
which is often used in paper manufacturing processes as a filler or white pigment. However, it does not
contribute to the alkali reserve.
An estimated comparison of the two deacidification processes results in a total of 1.95 times as many salts
being introduced into the paper in the papersave process using magnesium hydroxide and titanium dioxide
as occurs when the ZFB||2 process is employed, given the same alkali reserve. Therefore, the aforementioned measurements of Anders and Liers, which describe an embrittlement of pre-damaged paper given an
alkali reserve of 1.73 mass % MgCO3, are relative. In reality, the alkali reserve which was measured as having
an embrittling effect on these papers corresponds to a share of 3.37 mass % inorganic salts introduced into
the paper, thus lying clearly above the limit of 2.0 mass % MgCO3 which is recommended by the DIN guidelines.
3
Until 2012, ZFB utilized a deacidification process called papersave, which was replaced by the more modern ZFB||2
process in the same year. The ZFB||2 process was developed by ZFB.
12
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
3.6
How high is the alkali reserve in the deacidified originals?
In the course of the KUR project, investigations were also carried out regarding how high the alkali reserve
was in deacidified originals and which factors influence the level of the alkali reserve in the originals. This
resulted in the following conclusions:
•
•
•
Only about half of the books had an alkali reserve of above 0.5 mass % MgCO3; there were practi-
cally no alkali reserve values above 2.0 mass % MgCO3
The amounts of acid found in the corresponding non-deacidified books lay between 0 and -0.7 mass % MgCO3 and exhibited a wide range (note: negative alkali reserve = amount of acid)
The amount of the alkali reserve is strongly dependent on the thickness of the paper: thicker
papers can easily take in twice as much deacidifying agent as thinner ones (these results from
the KUR project correspond well with the measurements of ZFB, in which the papers with
the highest absorbency can take in about twice as much treatment dispersion as those with a
lower level of absorbency.)
Ahn and her co-authors (KUR project) recommend: “In practice, a provider of mass deacidification treatments should not only consider the acidity [= amount of acid, note by ZFB] of the books before treatment,
but also the absorption of alkali reserve since different types of paper can absorb different amounts of alkali
reserve. Test papers can be designed in such a way that their acidity corresponds with the acidity of the original book papers, yet the resulting alkali reserve can differ from the originals due to the great differences
in the amounts which were absorbed.” [Original English, translation by ZFB]
3.7
Summary and conclusions
Risks of a high alkali reserve
A high alkali reserve does not promote the decomposition of cellulose, but effectively suppresses such deterioration. In the most unfavourable cases, raising the alkali reserve above a certain threshold value – which
differs depending on the type of paper – does not benefit the paper, but also does not damage it. The only
risk which is associated with a high alkali reserve is thus the risk of an undesirable embrittlement of the
paper, which is presumably caused by inorganic particles being stored between the cellulose fibres. In the
case of clear mechanical damage to the paper 3.37 mass % of inorganic particles introduced into the paper
cause to it become brittle. Papers which are mechanically intact, however, are not affected by the risk of a
possible embrittlement.
Risks of a low alkali reserve
If the mass deacidification is carried out in such a way that the test papers receive a relatively low, yet sufficient, alkali reserve, then there is a relatively high probability that the originals will receive an insufficient
alkali reserve. This results due to the pronounced differences in the acid content of the paper and its absorbency with respect to the treatment solution. In the case of originals with a high acid content and/or poor
absorbency, one will naturally achieve a much lower alkali reserve than in papers with a low acid content
and/or high absorbency.
13
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Conclusions
•
In our opinion, it is generally better to deacidify with a higher concentration, in order to ensure that
all originals are provided with a sufficient alkali reserve. The risks of a high alkali reserve are decidedly lower than those of an insufficient alkali reserve.
•
In the ZFB||2 process the concentration of deacidifying agent is adjusted so that the alkali reserve
of the test books lies at approx. 2 mass % MgCO3. In this way the papers with a high acid content
and/or lower absorbency will be provided with sufficient alkali reserve.
•
In treating strongly acidic papers the using the ZFB||2 process, the concentration of deacidfying
agent can be increased so as to ensure a higher alkali reserve in the originals; the alkali reserve of
the test papers will then correspondingly lie above 2.0 mass % MgCO3.
To evaluate the quality of deacidification, the quality criteria prescribed by the DIN guidelines (pH-value,
alkali reserve, tensile strength) must always be viewed and assessed within the context of the larger picture.
The decisive parameter in this process is the alkali reserve; the effects of the other quality criteria should
always be taken into consideration as well. For example, following a complete and successful deacidification
treatment, which leaves a high alkali reserve, a slightly acidic pH-value between 6.5 and 7.0 can occur due
to the buffering effect of organic acids. Furthermore, the alkali reserve should only be considered as too
high if the tensile strength of the treated papers has been affected when compared to the initial condition
of the paper.
14
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
Sources / References
[Ahn 2011] K. Ahn, U. Henniges; A. Blüher, G. Banik and A. Potthast, „Sustainability of mass deacidification.
Part I: Concept, selection of sample books and pH-determination.“, Restaurator 32, (3), 2011, pp. 193-222.
[Ahn 2012a] K. Ahn, G. Banik, U. Hennigs und A, Potthast, „Nachhaltigkeit in der Massen¬entsäuerung von
Bibliotheksgut“, erschienen in: R. Altenhöner, A. Blüher, A. Mälck, E. Niggemann, A. Potthast,
B. Schneider-Kempf (Hrsg.), „Eine Zukunft für saures Papier. Per¬spektiven von Archiven und
Bibliotheken nach Abschluss des KUR-Projektes »Nachhaltigkeit der Massenentsäuerung von
Bibliotheksgut«“, Zeitschrift für Bibliotheks¬wesen und Bibliographie, Sonderband 106, Vittorio Klostermann Verlag 2012, ISBN 978-3-465-03728-6, S. 29–82
[Ahn 2012b] K. Ahn, U. Hennniges, G. Banik and A. Potthast, „Is cellulose degradation due to β-elimination
processes a threat in mass deacidification of library books? “, Cellulose 19 (2012), pp. 1149–1159
[Ahn 2012c] K. Ahn, G. Banik and A. Potthast, „Sustainability of mass-deacidification. Part II: Evaluation of
alkaline reserve. “, Restaurator 33, (1), 2012, pp. 48-75.
[Ahn 2013a] K. Ahn, T. Rosenau und A. Potthast, „The influence of alkaline reserve on the aging behavior of book
papers“, Cellulose 20 (2013), pp. 1989–2001
[Ahn 2013b] K. Ahn, „Sustainability of mass deacidification of library objects“, Dissertation, BOKU – Universität
für Bodenkultur, Wien, 2013. Link zum direkten Download (PDF): URL: https://zidapps.boku.ac.at/
abstracts/download.php?dataset_id=10248&property_id=107, abgerufen 2015-11-13
(Anmerkung: Link enthält die wissenschaftlichen Artikel [Ahn 2011], [Ahn 2012b], [Ahn 2012c]
und [Ahn 2013a])
[Anders 2000]
M. Anders, “Untersuchungen zur Papieralterung und zur Konservierung geschädigter Papiere durch Entsäuerung und Festigung“, Dissertation, Universität Stuttgart, 2000.
[Blüher 2001]
A. Blüher and B. Vogelsanger, „Mass Deacidification of Paper“, Chimia 55 (2001), pp. 981–989,
URL: http://www.uni-muenster.de/Forum-Bestandserhaltung/downloads/007AnhangIIIeMass_
Deacidification_of_Paper.pdf , abgerufen 2015-11-13
[Blüher 2006] A. Blüher, „The alkaline reserve – a key element in paper deacidification“, Vortrag auf der Tagung
„Save Paper 2006“, in: „Papers given at the International Conference Save Paper“, 15-17 February
2006, Hrsg, Swiss National Library, Bern 2006, Seiten 191-206, ISBN 978-3-9523188-1-2; ein Link zum Download des Tagungsbandes (PDF) findet sich auf http://www.nb.admin.ch/
nb_professionnel/erhalten/00699/01490/index.html?lang=en, abgerufen 2015-11-13
[Buchanan 1994] S. Buchanan, W. Bennett, M. M. Domach, S, M. Melnick, C. Tancin, P. M. Whitmore, K. E. Harris and
C. Shahani, „An Evaluation of the Bookkeeper Mass Deacidification Process. Technical Evaluation
Team Report for the Preservation Directorate“, Washington, D.C.: Preservation Directorate, Library
of Congress, 1994. URL: www.loc.gov/preservation/resources/rt/bookkeeper.pdf, abgerufen 2015-
11-13
[DIN-Empfehlung 2013] R. Hofmann und H.-J. Wiesner, „Empfehlung zur Prüfung des Behandlungserfolgs von Entsäuerungs-
verfahren für säurehaltige Druck- und Schreibpapiere“, in: „Bestandserhaltung in Archiven und Bibliotheken“, Hrsg: DIN Deutsches Institut für Normung e.V., 5., überarbeitete und erweiterte Auflage, Beuth Verlag 2015, S. 13-36
[Dümmling 2015]
S. Dümmling, “Die alkalische Reserve bei der Massenentsäuerung mit dem ZFB:2-Verfahren“, ZFB GmbH, 2015.
[Giovannini 2004]
A. Giovannini, „Die Erhaltung von Büchern und Archivalien“, 3. Auflage, Ies èditions, Genève, 2004.
[Hanson 1939] F. S. Hanson, „Resistance of Paper to Natural Aging“, The Paper Industry and Paper World, Feb. 1939, pp. 1157–1164; zitiert nach [Kelly 1972]
15
ÜH
R EBRUHCAHL ET RU HNAGL T U N G
Z E N T R ZUEMN TF R
ÜU
R MB UF C
T H E PATPHE ER SPA
A VPIENRGS A
C VOIRNPGO R
C AT
O RIPOONR AT I O N
[Kelly 1972] G. B. Kelly, Jr., „Practical Aspects of Deacidification: pH and Alkaline Reserve“, ursprünglich publiziert im Bulletin of the American Institute for Conservation, Vol. 13, No. 1 (1972), p. 16-28, unter dem Titel „Practical Aspects of Deacidification.“ Wieder veröffentlicht in: Alkaline Paper Advocate, Vol 2, No 1, Apr 1989. Online verfügbar unter http://cool.conservation-us.org/byorg/abbey/ap/ap02/
ap02-1/ap02-111.html , abgerufen 2015-11-13
[KUR-Projektbeschreibung] Projektbeschreibung des KUR-Projektes „Nachhaltigeit in der Massenentsäuerung von Bibliotheks-
gut“ auf der Webseite der Kulturstiftung des Bundes, URL: http://www.kulturstiftung-des-
bundes.de/cms/de/programme/restaurierung/archiv/KUR-Programm/nachhaltigkeit_der_
massenentsaeuerung_von_bibliotheksgut_3565_38.html, abgerufen 2015-11-13
[Liers 2002] J. Liers, „Massenentsäuerung - Acht Jahre Erfahrung mit dem Papersave Verfahren“, ZFB Profile 4 (März 2002), S. 1–3; online verfügbar unter der URL: http://www.uni-muenster.de/Forum-
Bestandserhaltung/kons-restaurierung/psave-liers.html, abgerufen 2015-11-13
[LOC 2004] Preservation Directorate Library of Congress, „Library of Congress technical Specifications for Mass Deacidification“, Washington,DC, 2004, URL: http://loc.gov/preservation/resources/rt/Mass
Deacidification.pdf, abgerufen 2015-11-13
[LOC 2015] Aktuelle Webseite der Library of Kongress zur Massenentsäuerung (Mass Deacidification), URL:
http://loc.gov/preservation/about/deacid/ , abgerufen 2015-11-13
[PaperVOC] Webseite des PaperVOC Projektes: „VOCs in paper-based cultural heritage collections – source of
information or risk?“, URL: http://www.science4heritage.org/papervoc/, abgerufen 2015-02-10
[Wittekind 1994] J. Wittekind: „The Battelle Mass Deacidification Process: a New Method for Deacidifying Books and
Archival Materials“, Restaurator 15 (1994), pp. 189–207
16