Accelerated Ageing of Paper: Some Ideas on its Practical Benefit
by HELMUT BANSA
INTRODUCTION
Accelerated ageing tests are widely used in conservation research, either to predict how long a
paper would last or to what extent a certain treatment would prolong the period during which an
object, or its material, will keep the qualities needed to be used for its intended purpose. In the
context of a library and archive the "object" is a book, an archival file or a piece of art, the
"material" is mainly paper.
Accelerated ageing is to expose the object in question to a higher level of energy, mostly heat, but
also to light or other radiation and aggressive pollutants. It is presumed that the higher energy
promotes the processes that together interact to produce symptoms recognised as "ageing", that is
harming a material's useful qualities. Moreover it is presumed that these processes take place in the
same relationship to each other whether at a high or low energy level. It is also presumed that the
reaction, (i.e. the ageing), of a material is proportional to the level of energy used to age it, that is
the higher the energy level, the more rapid the decay, and this in direct proportion.
It is well known that these last two assumption are not true.
CELLULOSE DECAY
Among the processes that, together combine within a material to cause the deterioration recognised
as "ageing", hydrolysis and oxidation prevail. In a practical assessment, which professional
chemists may consider to be an over-simplification, but which might, however, be helpful to
conservators, their pattern can be explained as follows.
Hydrolytic decay is quite simple. If there is a surplus of hydrogen ions, i.e. in acidic surroundings,
the oxygen bridge between two cellulose monomers are attacked and broken; the hydrogen ion is
connected to the oxygen of the bridge
thus forming a hydroxyl group. In this way the concentration of hydrogen ions is reduced below the
level specific for the pH governing the system, water molecules are split into hydrogen and
hydroxyl ions, this latter becomes surplus and immediately connects to the "open end" of the
neighbouring monomer. Only H+ and OH" = H2O is consumed; in terms of chemistry the acid,
which is necessary for the process, catalyses it. This process might also happen at the oxygen in the
pyranose ring; the result is comparable, i.e. breaking off the chain molecule, reducing the degree of
polymerization.
Oxidation of cellulose is a much more complicated and diverse process. Models for it are to be
found in the technical literature1. Without discussing them in detail, it can be stated that, which
oxidized groups are formed, whether one or the other prevails, is highly dependant on the
conditions within the system, i.e. the paper, and among these conditions temperature is important.
As a result of some oxidation processes keto- and aldehyde groups are formed. These groups are
highly reactive; they are prone to crosslinking, which is the third chemical process of cellulose
decay. This also, most probably, depends on the conditions within the system, i.e. the paper, and
again temperature is important.
DOUBTS
The only aim of this excursion into cellulose chemistry is to demonstrate that the assumption on
which accelerated ageing is based is highly unlikely, because the chemical processes that occur
together in parallel and which can influence each other to cause "ageing", take place at different
temperatures in different ways.
More doubts in the reliability of accelerated ageing may arise form looking at the two "rules of
thumb" that are found in the technical literature on conservation research. The first is:
• Raising the temperature by 10°C is equivalent to doubling the ageing speed, or, vice versa,
reducing the temperature by 10°C is equivalent to doubling the time. That would mean:
105°C equivalent to
3 days
65°C equivalent to
1 month 18 days
95°C equivalent to
6 days
60°C equivalent to
2 months 12 days
85°C equivalent to
12 days
.........
.........
20cC equivalent to
3 years 2 month
This contradicts the other "rule of thumb":
• 105°C for 3 days accelerated ageing is equivalent to 25 years natural ageing at 20°C (18°C).
The two statements would be in accordance if the increasing rate of rule 1 was 2.54 instead of 2 or
if the temperature rate provoking the double speed was 7.4 instead of 10. But this is playing with
numbers. Both rules of thumb are obviously wrong.
The third attempt to provoke and support distrust in accelerated ageing tests arises from a closer
look at the Arrhenius relationship, which is often quoted in the technical literature as a scientific
base for accelerated ageing. There are several written forms of the equation, one of them is: ln(k) =
In (A) - (Ea/R)*l/T, where
A = the ,,frequency factor", a constant,
R = the gas constant, pertinent to ideal gases,
Ea = the activation energy, a constant specific for a certain substance,
T = temperature (expressed in degrees Kelvin),
k = amount (concentration) of a specific substance changed per time unit.
The doubts in the usefulness of the Arrhenius relationship for accelerated ageing of paper arise from
common sense:
• Paper is not a certain substance, but a mixture of many: cellulose, lignin, sizing agent, filler, etc.
• In the context of Arrhenius plots any cellulose of a certain DP must be considered to be a
separate "specific substance", having a specific activation energy: CellDPx, CellDPy, CellDPz,
hemicelluloses, oxycelluloses etc.
• None of these substances is an ideal gas.
• The usual parameters checked for Arrhenius plots are not the concentration of substances, but the
influence these substances have on mechanical and optical properties.
• The "certain substances" that constitute paper, do not disappear during ageing, but are changed to
others: CellDPx to CellDPy, CellDPy to CellDPz, CellDPz to hemi- and/or oxycellulose, etc. All these
"new" substances contribute to mechanical and optical properties, in a positive or a negative way.
In light of this closer look the question arises whether the application of the Arrhenius equation to
predict the useful lifetime of paper is appropriate.
USE OF ACCELERATED AGEING
Accelerated ageing has been the object of intensive research. Older results have become Standards2.
From the fact that there are four, and that they are hardly ever used as they are given, but always
with variations, it becomes evident that
there are many and grave doubts in the reliability of ageing tests. Just recently three new and very
carefully made research reports3, 4, 5 have been published, they give sound and carefully considered
ideas on how results of accelerated ageing tests may and must be understood. In one of these
reports5 the purpose of accelerated tests is also defined, i.e. - purpose no.l - to classify papers into
stability groups; stable and unstable, and possibly a third group, moderately stable, between these
two; and - purpose no. 2 - to gain a certain idea on the long term effect of a conservation treatment.
From the viewpoint of practical paper conservation only the second one is relevant.
SIMILARITIES AND DIFFERENCES BETWEEN ACCELERATED AND NATURAL AGEING
In the new literature it is stated that there are some, or even many, processes that take place during
natural as well as during accelerated ageing. In both, e.g. hydrolysis, oxidation and crosslinking
take place. This might be seen as a banal observation. More pertinent is that also the final
degradation products are qualitatively very much the same: some mono-carbon acids, such as lactic,
acetic and formic acid, some dicarbon acids such as succinic and mainly oxalic acid. The proportions of these final degradation products will differ depending on the temperature and the relative
humidity of the system, and most probably there will also be distinct differences of the
intermediates: hemicellulloses, oxycelluloses, more or less acidic poly- or oligosaccharides like
alginic acid. etc. An interesting statement of the new research is that there seems to be quite a
similar reaction mechanism in the temperature range of 70 to 90°C, but that the reaction mechanism
dominating at temperatures below 70°C seems to be quite different.
SYSTEMS IN PRACTICAL USE
As said above, there are several standards for performing accelerated ageing tests, and again as said
above, they are rarely used as given. In the technical literature the following parameters are found
as promoting the speed of the processes that, occur in parallel and combine or interact to form what
we call the "ageing" of a material:
• Elevated temperature without humidity control: 60, 80, 90, 100, 103, 105°C -1, 3, 7, 10, 12, 13,
24, 100 days.
• New variant: samples enclosed in airtight glass tubes at temperature between 70 and 100°C for a
period of up to 30 days.
• Elevated temperature and controlled humidity: 50, 59, 60, 70, 80, 90, 100, 120°C - 2, 30, 38, 50,
65, 70, 100% RH.
• New variant: 80°C, humidity changing between 30 to 60% RH every hour.
• Irradiation: daylight, sunlight, artificial light 300-600 nm, Xenon light 65000 W, gamma
radiation, radioactive radiation - 23, 30, 35, 50, 60, 70, 80, 90°C -50, 60, 65% RH - 3, 6, 7, 8, 12,
30, 28, 156, 185 days.
• Controlled atmosphere: SO2, NO2, NOx, O2, exhaust-fumes, inert gas (argon) -20, 22, 23, 28, 50,
60, 65, 70, 80, 90, 100, 105, 150°C - 0, 50, 65, 80, 95, 100, 105% - 1, 3, 4, 10, 7, 24, 28, 32, 35, 42
days.
For research in mass deacidification, as it has been done quite intensively during the last 10 years
most often 90°C 50% RH for 12 days has been used. The variant of a regularly changing humidity
suggested by Käβberger3 (80°C, 30/60/ 30% RH, 1 h cycle) is not totally new; it has, for example,
been used with other figures, i.e. 90cC 35/80/35% RH and changing cycles of 3 hours, for the
research on fighting corrosive ink done during the last eight years in the Netherlands Institute for
Cultural Heritage in Amsterdam.
The joint Canadian and US American research project5, which is aiming to define stability classes,
suggests a totally new method of accelerated ageing, i.e. to age paper samples in airtight sealed
glass tubes with an internal volume of 145 ±5 ml at 90°C. In order to maintain the aspired relative
humidity inside the heated tubes the paper inside the tube must be packed quite densely: 4 g in each
tube, that means 37 strips 12 x 1.5 cm for fold measurement or 42 square sheets 4x4 cm for tear
measurement in the case of paper weighing 60g/m2, more paper is required for thinner and less for
thicker papers. The starting point for this new method of accelerated ageing is the discovery - also
this not totally new - that paper sheets inside stacks, within the book block, e.g., suffer from
accelerated ageing more than the outside sheets and more than sheets, which hang freely. The
reason for this is simple, final degradation products mentioned, such as lactic, acetic, formic,
succinic and oxalic acid, are volatile, at least at high temperature. They can evaporate from free
hung sheets and from the outside sheets of a stack, but they remain and provoke further decay in the
inner block, at least for the relatively short periods that are used for accelerated ageing. Whether
this is true also for the much longer periods of natural ageing, or whether it is more likely that these
acids diffuse equally within the stack, i.e. a book block and even out of it, might be the topic of
further research. Obviously these acids remain in the airtight sealed tubes and provoke further
decay. It might be doubted that this would be near to natural ageing. Another problem of this newly
suggested method might be to provide appropriate tubes. These must be made of a material which is
not only be airtight, but also chemically resistant and thermally stable, etc.
THE MOST APPROPRIATE METHOD
The first method of accelerated ageing is more than 100 years old. Later many others were
developed, most of them during the last 50 years and the last one just recently. In the publication of
Henk Porck4 there is list of publications, which refers to the question of which method is the most
apt; there are 80 relevant titles, since 1980, and Henk Porck's list is not even complete. Two new
methods have been suggested just recently, and there is no reason to suppose that they will be the
last. Neither the older nor the new tests really imitate natural ageing, and again none of them
provoke other reactions than those that, completely all exist and combine to form what we call
"ageing". The differences between the reactions in all these methods of accelerated ageing are
negligible in comparison to the difference between these reactions under artificial ageing and what
actually happens during natural ageing.
There are two publications - possibly there are more - where two fundamentally different methods
of accelerated ageing, i.e. dry heat at a temperature over the boiling point of water and moist heat at
distinctly lower temperature, have been done on several same pairs of paper6, 7. From the vast
amount of data given it can be seen that the course of changing chemical, mechanical and optical
properties of paper provoked by the two ageing procedures can be very much the same and very
different, that a longer ageing period at low temperature can provoke higher or lower change than a
shorter period at a high temperature. Parallel decay prevails, but there are many exceptions, and
there is no regularity in them. A look at the DP measurement (Figs. 1 and 2) which represents one
single chemical change, i.e. breakings in the cellulose chain, may support this statement. A look at
mechanical strength (Fig. 3), which is influenced by more than just one chemical change within the
material, shows even more diversity. Undoubtedly it would be possible to explain them all by
analysing the components of the several samples in complete detail and follow their specific
chemical changes caused by heat and humidity, and it can not be doubted that the relevant
discoveries would be of great scientific interest. However, it seems doubtful whether they would be
of any practical use for paper conservation - not to mention the expense of the research.
HOW TO INTERPRET AGEING TESTS
For practical use the following statements, derived from the above reported observations and
deliberations, may be pertinent. They are not totally new8:
• The method of accelerated ageing does not matter a great deal. It is much more important to make
the correct use of the results.
Fig. 1: Diagrams of DP measurement on new samples arranged in a way that parallel (linear)
trendlines were achieved. It can be seen that 6 days of dry ageing at 105°C can be equivalent to
more than 170 and less than 50 days of moist ageing at 80°C. The numbers for the diagrams are
taken from ref. 6 (diagr. 1 - 4) and ref. 7 (diagr. 5, 6).
Fig. 2: Diagrams of DP measurement on modern cardboards and historic paper demonstrating that
the course of change during dry and moist accelerated ageing can be very much the same (rows 1
and 2) and very different (rows 3 and 4). The numbers for the diagrams are taken from ref. 6
(cardboards) and 7 (historic papers).
Fig. 3: Diagrams of mechanical strength measurements on modern cardboards and historic paper
demonstrating that the course of change during dry and moist accelerated ageing can be very much
the same (rows 1 and 2) and very different (rows 3 and 4). The numbers for the diagrams are taken
from ref. 6 (cardboards) and 7 (historic papers).
• Accelerated ageing tests never allow absolute, they only allow relative conclusions.
• Accelerated ageing can only give some information on whether one certain quality, one certain
conservation treatment done in order to improve the actual quality, influences the ageing behaviour
for the better or worse.
• The relative change, the observation that a certain quality brought into the paper by a certain
conservation treatment changes the ageing behaviour of the object for the better or worse, must be
true not only for one pair of objects that are exactly the same except for the very quality in question,
but also for more samples, and these different samples should be as different from each other as
possible.
Transferring these statements to paper production, in order to define stability classes, could be done
by
• defining two standard papers,
• one of undoubtedly high permanence: pure rag of high DP, slightly alkaline production, a certain
(quite small) amount of (precipitated) CaCO:r,
• the other of undoubtedly low permanence: high lignin (and hemicelluloses) containing fibre, acid
production, high amount of inert filler (clay)
• describing by fixed numbers for the reduction rate of apt chemical, optical and mechanical
qualities occurring during accelerated ageing according to a certain, arbitrarily, but reasonably fixed
method, the place of the paper in question between the two standard papers.
The author does not maintain this to be a practical method. But he insists on it being a scientifically
sound one, possibly the only scientifically sound one among those based directly on accelerated
ageing.
SUMMARIES
Accelerated Ageing of Paper: Some Ideas on its Practical Benefit
After demonstrating the many inconsistencies in what is believed to be the basis and the result of
accelerated ageing, suggestions are made as to how to perform accelerated ageing and how to draw
conclusions from it: The method (dry or moist heat) does not matter a great deal. More important
for this is that any method can only provide relative results: one paper is more stable than another, a
certain component brought into the paper by a conservation treatment, or a variation of this
treatment, influences its stability for the better or worse. Finally, an idea is given as to how this
principle can be used for defining stability classes for paper production.
Le vieillissement accelere du papier : quelques reflexions quant a son interet pratique
Apres avoir demontre les nombreuses incoherences relatives aux fondements et aux resultats
communement admis a propos du vieillissement accelere quelques reflexions sont faites afin de
savoir de quelle façon le vieillissement accelere doit etre pratique et quelles conclusions doivent
etre tirees de ces resultats. La methode (chaleur seche ou humide) n'importe guere. Le plus important est que chaque methode ne peut fournir que des resultats relatifs : p. ex. qu'un papier est
plus resistant qu'un autre, qu'une certaine composante apportee au papier par un traitement de
restauration a une influence positive ou negative sur sa stabilite, qu'une variante de la methode de
traitement agit plus ou moins qu'une autre. Enfin une reflexion est faite sur la question de savoir
comment on pourrait utiliser ce principe pour definir des classes de stabilite dans le pro-cessus de
production du papier.
Beschleunigte Alterung: Überlegungen zu ihrem praktischen Nutzen
Nach einem Hinweis auf die vielen Ungereimtheiten in dem, was allgemein als Grundlage und
Bedeutung der beschleunigten Alterung gilt, werden einige Überlegungen angestellt, wie beschleunigt gealtert und welche Schlüsse aus ihren Ergebnissen gezogen werden sollten. Die Methode (trockene oder feuchte Hitze ) hat wenig Bedeutung. Wichtiger ist, daß alle Methoden nur
relative Aussagen zulassen: das eine Papier stabiler als das andere, eine Komponente, die durch eine
bestimmte Restauriermethode in das Papier gebracht wird, beeinflußt seine Stabilität in positiver
oder in negativer Weise, die eine Variante der Methode mehr als eine andere oder we-niger als eine
andere. Zum Schluß wird überlegt, wie dieses Prinzip auf die Beschreibung von Stabilitatsklässen
für die Papierherstellung übertragen werden könnte.
REFERENCES
1. E.g.: Margutti, S., G. Conio, P. Calvini & E. Pedemonti: Hydrolytic and oxidative degradation
of paper. Restaurator 22 (2001): 68 sq. (diagrams).
2. ISO 5630. Paper and board - accelerated ageing. - Part 1: Dry heat treatment at 105° C. Last
revision 1991 - Part 2: Moist heat treatment at 90° C and 25% RH. Last revision 1985 - Part 3:
Moist heat treatment at 80° C and 65% RH. Last revision 1986 - Part 4: Dry heat treatment at 120°
or 150° C. Last revision 1986. Part 1 is equivalent to the US American Standard ASTM (1987).
Standard Test Method for Determination ofEffect of Dry Heat on Properties of Paper and Board.
American Society for Testing and Materials (ASTM-D776-87; 72 hours at 105±2 °C).
3. Käßberger, M.: Vorgänge im Papier bei dynamisch beschleunigter Alterung. Diss. Graz 1998.
IV+205+7 pp.
4. Porck, HenkJ.: Rate of paper degradation. The predictive value of artificial aging tests.
Amsterdam: European Commission on Preservation and Access 2000.40 pp., 10 of them references.
5. ASTM research program into the effect of aging on printing and writing papers. Final
reports on accelerated aging test method development. - Accelerated aging test method
development for American Society for Testing and Materials Institute for Standard Research
(ASTMYISR). - Chemical analysis of degradation products. - Application of Arrhenius
relationship. - Proposal for a new accelerated aging
test. Ottawa: Canadian Conservation Institute. January 2001. 153 pp. Washington, DC: Library of
Congress. February 2000, revised February 2001. 362 pp.
6. Botti, L., G. Impagliazzo, L. Residori & D. Ruggiero: Paper packaging for long-term reservation
of photographic plates. Restaurator 15 (1994): 79-93.
7. Bansa, H., & R. Ishii: Aqueous deacidificatim - with calcium or with magnesium? Restaurator 19
(1998): 1-40.
8. Bansa, H., & H.H. Hofer: Die Aaaagekrafi finer künstlichen Alterung von Papier für Prognosen
über seine zukünftige BenutzbarkeiL Restaurator 6 (1984): 21-60. - English version: Artificial ageing as a predictor of paper's future useful life. Abbev Newsletter Monograph Suppl. 1 (1989).
Dr. Helmut Bansa
ElisabethstraßBe 23
80796 München
Germany
Tel. ++49 +89 2713327
Fax ++49 +89 27774401
E-mail: Restauratorjournal@t-online.de