Changes in Brittle Paper during Conservation Treatment
by JOZEF HANUS
Aging is an irreversible process that deteriorates the mechanical properties of paper. Paper becomes
brittle due to the influence of internal and external factors. The most important internal factors are
established during the manufacture of the paper and include the kind and quality of the fibers, sizing
materials, coatings, the presence of acidic and metallic compounds and other components of the
sheets. The external factors are related to the deleterious influence of the storage conditions or use,
such as temperature and humidity, light, air pollutants, microbial attack, atmospheric oxidation, etc.
The strength of brittle paper can be improved by conservation treatment. Most frequently this
involves washing, deacidification in aqueous or nonaque-OUS solutions and resizing. The term
resizing refers to a process that coats or impregnates the paper with a material, resulting in paper
strengthening. Various different impregnating agents and evaluating of paper aging have been
tested in many studies.1-10
A recent study11 evaluated some materials used as impregnating agents. Of this group,
methylcellulose was the most promising material available, based on imparted strength, pH and
color changes after dry oven aging and general working properties.
The study also recommends further testing of the effects of treatment on papers of varying
composition and age, on papers that are particularly brittle or have deteriorated, etc.
This research represents an attempt to follow some changes of mechanical properties of the brittle
paper from 1886 during conservation treatment involving washing and deacidifying by magnesium
bicarbonate and the Wei T'o process and to evaluate the strength imparted to it by resizing with
methylcellulose and gelatin.
MATERIAL AND METHODS
The book Works of Virgil, (translated by John Dryden, Philadelphia: Lippin-cott, 1883), has been
provided for this experiment by courtesy of the National Library of Canada, Ottawa.
The paper of the book was very brittle and yellow, and the size of the book page was 18.5 x 12 cm.
Conservation treatment of the brittle paper included 2x20 min washing in hot water, aqueous
deacidification by a solution of Mg(HCO3)2 for 25 min or nonaqueous deacidification by the Wei
T'o system (hand spraying). After the samples were dried, resizing followed as a final step of the
treatment with methylcellulose and gelatin as impregnating agents - the process used in the National
Archives of Canada, Ottawa.
Preparation of Mg(HCO3)2
Magnesium hydroxide (Mg(OH)2), 125 g was added to 18 liters of cold water through a filter into a
tank. After the tank was closed, bubbling of CO2, was continuing until the pressure reached 242
kPa. The solution was then stirring for 50 min. After this procedure, the Mg(HCO3)2 solution was
ready to use.
Wei T'o nonaqueous commercially available solution was used for hand-spraying deacidification.
Methylcellulose Methocel A 15C available from Dow Chemical Company and Gelatin (Purified
Gradem, Gelatin, Granular, 1000 Bloom G-7, Type B) from Fisher Scientific & Allied Company
were chosen as impregnating agents.
Samples were prepared by immersing paper strips into the 0.5% and 1% aqueous solutions of
methylcellulose, 1% aqueous solution of gelatin and by brushing (both sides) with 2.5% solutions
of methylcellulose. The immersion time for the paper was 5 minutes. The treated samples were
allowed to drip dry on a sheet of Plexiglas for 10 min and to dry on air at ambient temperature.
Before testing, all speciments were equilibrated at 20±2ºC, 50 ± 4% relative humidity according to
TAPPI test method T402.
Mass changes of the paper
Untreated samples, samples treated by deacidification agents Wei T'o and Mg(HCO3)2, deacidified
and sized samples by methylcellulose and gelatin (strips 9.5 x 1.5 x 0.0125 cm) were weighed after
drying and equilibrated according to TAPPI T 402. A Mettler analytical balance AE200 was used.
On the base of experimental results, the weight of treated and untreated papers
was calculated. Loading factors arc reported as changes comparing deacidificd papers by Wei T'o
and Mg(HCO3)2 and deacidified and sized samples to untreated paper.
Thickness
The thickness of the samples was determined by measuring paper strips through the whole width of
the page according to TAPPI T411. It has been expressed as an average value of 40 measurements
on several pages (random selection).
Instron stress-strain tensile test
Instron Model 1011 was used to measure tensile strength, elongation and tensile energy absorption
at a speed of 4 mm/min. The samples of the paper were cut in the machine direction to strips of 70 x
15 mm, thickness 0.125 mm. TAPPI test method T494 was used with the modification in sample
length to 30 mm gauge because of the small size of the book pages.
Folding endurance
Folding endurance tests were performed on an M.I.T. Folding Endurance Tester, model TiniusOlsen Testing Machine, Willow Grove, PA, USA according to TAPPI T 511 on samples cut in the
machine direction. The size of the samples was 15 x 1.5 cm. A standard tension of 0.5 kg was used.
In all cases the paper and impregnating agent ruptured simultaneously.
pH measurements
pH was measured by a cold extraction method according to TAPPI T 509 using Orion Research
microprocessor pH/millivolt meter 811 with Electrode Orion Mode No. 91-02.
RESULTS AND DISCUSSION
Mass changes of the paper
The calculated changes in g/m2, density (g/cm3) and loading factors (g/m3) expressed as a
percentage of untreated paper are shown in Table 1.
In all cases the samples treated by Wei T'o deacidification process increased
Table 1. Mass changes of the paper during conservation treatment
in mass (3.3-8.5%). The changes of the samples treated by aqueous deacidifica-tion by
Mg(HCO3)2 were negligible or the mass of the samples decreased slightly. An interesting
fact was observed after the treatment of deacidified papers by sizing agents. In all cases,
except the samples treated with 2.5% methylcellulose by brushing, the sized sample mass
decreased in relation to the samples of deacidified paper. This observation indicates that
some small portion of deacidification agent deposited in paper was washed out by immersion of the paper at lower concentration solutions of sizing agents (0.5-1%).
Thickness of the paper
After the electric measurement of 40 random selected spots, the thickness of untreated paper
has been estimated as 0.125 ± 0.002 mm.
Folding endurance
The paper of Works of Virgil was so brittle that its folding endurance expressed as the number of
double folds at a tension of 1000 g was 0. Even after the sizing treatment, it was increased only very
slightly. The tension was then changed to 500 g. Only 2 or 3 values of 10 measurements for each
sample after the treatment were 1; otherwise they were 0 even at 500 g.
Tensile strength and elongation
Tensile strength more nearly approaches a fundamental measurement than other conventional
strength measurements on paper. However, tensile strength as measured in the paper industry is not
a true tensile strength, since it measures the breaking load per unit of width rather than per unit of
area.12
The values of tensile strength and elongation and their increase in percentage arc shown in Table 2.
The results reported are the average of 10 measurements with standard deviation.
The results obtained indicate that the smallest increase in tensile strength was caused by 0.5%
methyleellulose and 1% gelatin.
More obvious distinctions are indicated with using of more concentrated solutions of
methyleellulose. A 2.5% solution of methyleellulose caused the highest increase in tensile strength
(27.3% vs 38.1%) compared to the increase with 1%) methyleellulose (15-7%); the values of
elongation indicate that the paper samples treated with 2.5%> methyleellulose were less flexible
than those
Table 2. Changes of tensile strength (TS) and elongation (E) of the paper during conservation
treatment
treated with 1% methylcellulose (18-7%) (Fig. 1). In connection with the increasing of elongation,
it is worth mentioning the paper samples treated by Wei T'o deacidification and sized by 1% gelatin
(55% increase; Table 3).
Tensile energy absorption
Tensile energy absorption (TEA) is a very important measure of the strength of the paper. It is the
area under the stress-strain curve as the paper is stretched to rupture. Because TEA combines tensile
strength with stretch, it is a measure of the toughness of the paper.12 The values of TEA were read
out as the tensile strength was being determined.
The results confirm the tensile strength values, but with more obvious distinctions among the
different samples (Table 3).
pH measurements
pH readings of treated and untreated paper are provided in Table 4. After the treatment, pH data of
all the samples were in mild alkaline medium. The lowest pH was for paper samples treated after
deacidification by 1% gelatin (7.6-7.7). Samples treated after deacidification by the solutions of
methylcellulose (dipping) - except 2.5% methylcellulose - also provided very close pH readings
within the range of 7.9-8.1. The highest pH data (8.3-8.7) were observed on samples treated with
2.5% methylcellulose by brushing. Lower
Fig. 1. Increase of strength and elongation resulting from methylcellulose
Table 3. Changes of TEA of the paper during conservation treatment
Table 4. pH changes of the paper during conservation treatment (Cold extraction)
pH values of the samples treated with sizing agents by dipping in the solution can be explained by
the observation that, after the sizing process, the solutions of sizing agents were intensively colored
to yellow. This case was particularly very obviously observed with the samples deacidified by the
Wei T'o process. These observations are in good correlation with the results of the sample mass loss
after the sizing.
ACKNOWLEDGEMENTS
I would like to thank the National Archives of Canada and the Conservation Branch for the use of
the laboratory space and equipment. I would also like
to thank Messrs K. B. Hendriks, P. Begin and J. Iraci of the Conservation Research Division for
assistance and helpful discussions.
SUMMARIES
Changes in Brittle Paper during Conservation Treatment
The changes in some mechanical properties during a conservation treatment of brittle paper of a
book published in 1883 are reported. The paper was treated with Mg(HCO3)2 and Wei T'o
deacidification processes followed by sizing with 0.5%, 1 and 2.5% methylcellulose and 1%
gelatin.
Modifications dans du papier cassant au cours de traitements de restauration
On expose les modifications, au cours de traitements de restauration, de quelques proprietes
mecaniques du papier cassant d'un livre publie en 1883. Le papier a ete traite avec les precedes de
desacidification a base de Mg(HCO3)2 et Wei T'o suivis de reencollage avec de la methylcellulose
(0.5%, 1% et 2.5%) et de la gelatine (1%).
Einige Änderungen an brüchigem Papier als Folge von konservatorischer Behandlung
Es werden Zahlen zur Veränderung einiger physikalischer Festigkeitswerte mitgeteilt, die das
Papier eines Buches aus dem Jahre 1883 erfahren hat. Das Papier war mit Mg(HCO3) 2 bzw. Wei
T'o neutralisiert und anschließend mit 0.5. 1 und 2.5% Methylcellulose sowie 1% Gelatine
nachgeleimt worden.
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Jozef Hanus
Slovenský národný archív
817 01 Bratislava
Drotárska cesta 42
Slovak Republic