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
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.
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 literature . 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.
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
|| 20°C equivalent to 3 years 2 month
This contradicts the other “rule of thumb”:
||105ºC for 3 days accelerated ageing is equivalent to 25 years natural ageing at
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
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) = ln(A) – (Ea/R)*1/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
||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 Standards . 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 reports , ,  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 reports  the purpose of accelerated
tests is also defined, i.e. – purpose no.1 – 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 90ºC, but that the reaction mechanism dominating
at temperatures below 70ºC 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 Käßberger  (80°C, 30/60/
30% RH, 1 h cycle) is not totally new; it has, for example, been used with other
figures, i.e. 90°C 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 project , 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 4 x 4 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 Porck  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 paper , . 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 new :
||The method of accelerated ageing does not matter a great deal. It is much more
important to make the correct use of the results.
||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
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) CaCO3;
||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.
||E.g.: Margutti, S., G. Conio, P. Calvini & E. Pedemonti: Hydrolytic and oxidative degradation of
paper. Restaurator 22 (2001): 68 sq. (diagrams).
||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 of Effect of Dry Heat on Properties of Paper and
Board. American Society for Testing and Materials (ASTM-D776-87; 72 hours at 105±2 °C).
||Käßberger, M.: Vorgänge im Papier bei dynamisch beschleunigter Alterung. Diss. Graz 1998.
||Porck, Henk J.: 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.
||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 (ASTM/ISR). – 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.
||Botti, L., G. Impagliazzo, L. Residori & D. Ruggiero: Paper packaging for long-term reservation
of photographic plates. Restaurator 15 (1994): 79–93.
||Bansa, H., & R. Ishii: Aqueous deacidification – with calcium or with magnesium? Restaurator 19
||Bansa, H., & H.H. Hofer: Die Aussagekraft einer künstlichen Alterung von Papier für Prognosen
über seine zukünftige Benutzbarkeit. Restaurator 6 (1984): 21–60. – English version: Artificial ageing
as a predictor of paper’s future useful life. Abbey Newsletter Monograph Suppl. 1 (1989).
|Dr. Helmut Bansa, Herausgeber der Zeitschrift RESTAURATOR, ehemaliger Leiter des IBR (Institut für Handschriften- und Buchrestaurierung) und der Staatlichen Fachakademie zur Ausbildung von Restauratoren in München, langjähriges Mitglied der DBI-Kommission für Bestandserhaltung.
|Der Beitrag erschien erstmals in: Restaurator 23,2 (2002) S. 106-117