Evaluation of Hand-made Nepalese Paper for Lining Paintings
by D. G. SURYAWANSHI & O. P. AGRAWAL
Hand-made Nepalese paper has long been used in Indian conservation laboratories for lining paper
paintings and for other repair work. This type of paper is manufactured as a cottage industry, mostly
by various people residing in Nepal and surrounding areas. For repair it was used chiefly because
the Japanese mulberry tissue paper was not available in the country and had to be imported. On the
other hand, the Nepalese paper was freely available in the markets of several cities of India,
particularly Waranasi. Furthermore, it had very good wetting quality, a rough texture and long
fibers, some of the desirable properties for the paper intended for lining old miniature paintings
(Fig. 1). Despite its wide use as a lining material, the Nepalese paper had not been studied
scientifically to evaluate its physical and chemical properties after aging. A study was undertaken at
the National Research Laboratory for Conservation of Cultural Property in Lucknow, India, for the
evaluation of hand-made Nepalese paper as a lining and repair material in conservation laboratories.
The samples were selected at random from the stock purchased by the Laboratory from the
Nepalese market.
PROPERTIES OF LINING PAPERS
A good quality paper for restoration work should be durable, have good dimensional stability,
should be light weight and yet strong, should have good spreading capability when wetted, should
be neutral or slightly alkaline and should be resistant to the normal atmospheric variations of
humidity. In order to assess the efficiency of Nepalese paper, its physical properties such as tensile
strength, tear strength, folding endurance and bursting strength were ascertained before aging and
after aging. Its chemical properties and the nature of raw materials were also
Fig. 1. Strength properties.
examined. On the basis of this study, it is seen that the Nepalese paper has excellent durability and
other qualities required for lining paper materials.
PHYSICAL PROPERTIES
The paper sheets received in the Laboratory were approximately 45X60 cm. The appearance of the
paper was rough, with a district texture, unequal in thickness, very light cream-colored and slightly
old in appearance. There were distinct screen marks on the paper, indicating that the paper was
hand-made. The rejects and the specks were also present in the texture of the paper sheet, which
indicated that the pulp was prepared mechanically.
SAMPLING
Paper samples were collected and prepared as directed in the TAPPI Standard2 T 400 and ASTM3 D
585. The fundamental principle of sampling is that the
sample selected must be representative of the entire lot of the material for which the analysis or
testing is to be conducted.
GRAMMAGE
The most common specification of paper is its weight. Because paper is used in sheet forms and its
area is more important than its volume, its weight is expressed per unit of area rather than per unit
of volume, as is the normal practice in the case of other 3-dimensional materials. In the case of
paper, weight is expressed as grams per square meter in the metric system, and this is termed its
grammage. The grammage of the hand-made Nepalese paper was determined in accordance with
the TAPPI method. In this particular case the grammage was very low: 27.9 g/m2.
THICKNESS
The thickness of the paper was determined by using a micrometer as the perpendicular distance
between two circular plane discs. The paper is very thin, the thickness of 4 uniform sheets being
37.4 μm.
APPARENT DENSITY
Density is expressed as grams per cubic centimeter and is calculated by dividing the weight in
grams per square meter by the thickness in micrometers. It is frequently termed apparent density
because it includes the air space in the paper and is calculated from the measured thickness.
Apparent density (g/cm3)=4X27.9/374=0.298
BULK
The bulk number is important for book papers. This is the number of sheets required to give a
thickness of 25.4 mm when under a pressure of 250 kPa. It indicates the thickness to be expected in
a book when under the pressure exerted in a binder. It is just the reverse of the apparent density
(calculated by dividing 1 by apparent density).
Bulk (cm3/g)= 1/0.289 = 3.46
STRENGTH TEST
To determine the strength of the paper, 4 main physical strength tests, tensile strength, tearing
strength, bursting strength and folding enducance were carried out. For all these tests, strips of
paper were prepared and the measurements done according to the standard methods of TAPPI.
TENSILE STRENGTH
The tensile strength of a paper strip is the maximum tensile stress applied to rupture the paper strip
under prescribed conditions. This test was performed with the Tensile Strength Tester No. 88
manufactured by Karl Frank, Mannheim, Rheinau, Germany. The results are recorded in Table 1.
TEARING STRENGTH
Tearing strength is also important to determine the resistance of paper to rupture by a given force
expressed in grams. It is found that alkaline-cooked pulps have higher tear strength than the
corresponding acid-cooked pulps. The other determining factors in tear strength are the fiber length
and interfiber bonding.
Tests were conducted by the instrument manufactured by Kamal Metal Industries, Ahmedabad,
India Model-A. The results are given in Table 1. The Nepalese paper has high tearing strength,
indicating that the fiber length as well as fiber bonding is excellent.
BURSTING STRENGTH
Bursting strength was determined by the Bursting Strength Tester Model-B manufactured by Kamal
Metal Industries. The results of bursting strength are mentioned in Table 1.
FOLDING ENDURANCE
The folding endurance of Nepalese paper was tested by Folding Endurance Tester 29 manufactured
by Karl Frank. This equipment produces mechanically double folds, repeated at the same point of
specimen paper strip clamped at both ends under 1 kg of tension till it breaks.
CHEMICAL PROPERTIES
The chemical properties of the Nepalese tissue were also determined.
A representative sample was analyzed according to TAPPI standard methods. The following
observations were made.
Ash and silica
For ash determination, 1 g of paper sample was ignited in a muffle furnace at 7OO±25°C for 1
hour. The sample was weighed after cooling in a desiccator and the percentage of ash determined.
To determine silica,4 the residue was treated with concentrated hydrochloric acid. The residue was
again ignited till a constant weight was achieved. In the Nepalese paper the silica content is
negligible.
α-Cellulose content
The percentage of α-cellulose content in Nepalese paper was determined according to the procedure
given in TAPPI Standard (T203 OS-74). The results are recorded in Table 2.
Table 2. Chemical properties
Lignin content
The lignin content was determined by the acid-insoluble lignin method described in TAPPI (T
222).5 The paper was treated with 38% hydrochloric acid (50 ml) followed by concentrated
sulphuric acid (5 ml). The mixture was shaken several times over a period of few hours, allowed to
stand overnight at room temperature, then diluted to 1 liter and boiled. Lignin was allowed to settle,
filtered through sintered glass crucible, washed, dried and then weighed. The percentage of lignin
was then calculated. Treatment of paper with strong mineral acids hydrolyzes and dissolves the
carbohydrates, leaving lignin as insoluble residue. The lignin content in Nepalese paper is low,
ranging between 3.4% and 7.8%.
Sizing materials
Sizing is normally applied to paper for imparting resistance to the penetration of liquids, ink, paint,
etc. The usual sizing materials are starch, glue, gum or rosin. Various spot tests were used6 to
identify sizing materials.
Starch can be detected with the 0.01 N iodine staining test.
Rosin was tested according to the Raspail test in which saturated sugar solution is placed on the
paper, followed by a drop of concentrated sulfuric acid. If rosin is present, a raspberry red color is
seen.
The extract of paper tested with tannic acid gives a precipitate in case glue is present. Gum is tested
with a saturated barium hydroxide solution with 5 ml of an aliquot prepared from paper with
ethanol and distilled water.
For Nepalese paper, all the above tests were negative except that of starch, which gave a positive
brilliant blue color with iodine solution. This indicated that only starch solution is used as a sizing
material in the Nepalese tissue paper.
Solubility
The solubility of paper in various organic solvents such as acetone, ethyl alcohol, chloroform and
toluene was determined. Solubility was also determined in boiling water and 0.5% NaOH solution.
The results are recorded in Table 3. No portion of the paper is soluble in organic solvents. In boiling
water, 1.84% is soluble.
Acidity
The Nepalese paper has been found to be slightly alkaline. The alkali content was determined by
TAPPI standard methods T 428 and ASTM D 548. In this method, approximately 5 g of paper is cut
into small pieces and refluxed for 1
Table 3. Solubility
hour with 250 ml of distilled water. After filtering and cooling, the extract is titrated against 0.01 N
HC1 solution. Simultaneously, a blank sample is titrated. The difference between the titration of the
extract and the blank gives the alkali content. The percentage is then calculated as NA2O. The
results are mentioned in Table 2.
Copper number
To determine the reducing groups in cellulose, the copper number was determined before aging and
after aging of the paper samples, by TAPPI standard procedure T 430 and ASTM D919. The
method is based on the Braidy method in which the copper solution contains a carbonatebicarbonate buffer. The copper I oxide formed by the reduction reaction is determined by treatment
with molyb-dophosphoric acid and titration of the reduced molybdenum with 0.05 N potassium
permanganate solution to a faindy pink color. The results are recorded in Table 2. The copper
number was 1.23.
The copper number was also determined after aging the paper: 1.93.
FIBER ANALYSIS
Fiber composition determination is essential for characterization of paper samples. The sampling
and slide preparation for fiber analysis were done according to TAPPI standard procedures T 401
and ASTM (D1030). Slides were prepared using the pulp slurry of paper of 0.5% consistency. The
fibers were dried on hot plate and stained with different stains.
In the hand-made Nepalese paper, fibers were of only one type of material.
IDENTIFICATION OF FIBERS
The fibers were identified by using a Metzer Laboratory binocular microscope Type-993.
Length
The lengths of at least 200 fibers were measured at X100. The average length was found to be 7.30
mm, more than other hand-made papers manufactured at Lucknow or Ahmedabad, India.
Width
The width of fibers was measured by a micrometer scale at X 100. The width is uniform throughout
the fiber length and averages between 0.019 mm and 0.028
Morphology
The characteristic morphology of the Nepalese paper fibers is mainly rsponsible for the high
strength of the paper. In the slide of the fibers it is possible to locate a thin continuous lumen in the
fibers; the node swelling is also distinct. Transverse lines are observed in some of the fibers (Fig. 2).
The overall observations of the fiber, taking into consideration the microscopic and morphological
characteristics, show that the fibers are of a bark Daphne bholua or Daphne involucrata, species
from a subfamily Daphneae of the family Thymelaceae.
RESULTS AND DISCUSSION
Physical strength properties
The physical properties of the paper recorded in Table 1 indicate a high degree of durability. The
tensile strength and folding endurance of the paper demonstrates it to be of a good quality. Tear and
bursting strength also are good, indicating perfect interfiber bonding in the paper. The tear strength
is equal in all directions, a characteristic of hand-made paper.
There is not much decline in the strength properties of samples heated at 105±5°C for 48 hours.
There is a drop in the folding endurance and tearing strength, but even after aging they are quite
high.
Fig. 2. Photograph of a sample of hand-made Nepalese paper. It has a rough texture and long fibers.
Overall, it can be concluded that the strength of the fibers used for manufacturing the hand-made
Nepalese paper is quite good, and the paper is acceptable as a durable paper.
Chemical properties
The results recorded in Tables 2 and 3 on chemical analysis, solubility, copper number and acidity
show that the paper is not acidic but slightly alkaline. The lignin content is low. The sizing material
is starch and not an acid-producing substance.
The copper number is also low, indicating that the oxidation or the hydrolysis will be very slow in
the normal atmospheric conditions.
Another very important characteristic of the paper is that there is no change in the original
appearance after heating at 105±5°C for 48 hours.
Solubility tests (Table 3) show that it is practically insoluble. Organic solvents such as acetone,
ethyl alcohol, toluene and chloroform have no effect, which means that many of these organic
solvents can be used for various conservation
Fig. 3. Photograph of the fibers of Nepalese paper.
needs. Solubility in 0.5% NaOH is very low as compared with that of the other hand-made papers.
CONCLUSION
The physical and chemical characeristics of hand-made Nepalese paper, before aging and after
accelerated aging, show that it is a good quality paper and can be safely used in the conservation
laboratory to support and line old paintings and documents. It cannot, however, be used on the
surface, as it does not have the requisite transparency. Also, it quite often has thick bundles of
brownish fibers in the structure (Fig. 3). These, however, can be removed with tweezers at the time
of its use as lining.
SUMMARIES
Evaluation of Hand-made Nepalese Paper for Lining Paintings
The properties of hand-made Nepalese paper were tested to evaluate its suitability for repair of
paintings. Its physical properties, such as tensile strength, tear strength, folding endurance and
bursting
Fig. 4. The Nepalese paper sometimes has thick bundles of brownish fibers in its structure, which
can be removed with tweezers.
strength, were ascertained before aging and after aging. Its chemical properties and the nature of the
raw materials were also examined. We found that the Nepalese paper has excellent durability and
other qualities required for lining paper materials.
Evaluation du papier nepalais fabrique a la main pour k doublage de peintures
Les proprietes de papier nepalais fabrique a la main ont ete testees afin d'evaluer son utilisation en
restauration de peintures. Ses proprietes physiques telles que la resistance a la traction, a la
dechirure, au pli et a l'eclatement ont ete verifiees avant et apres vieillissement. Ses proprietes
chimiques et sa composition fibreuse ont egalement ete examinees. On a trouve que le papier
nepalais a une excellente permanence ainsi que d'autres qualites requises pour le doublage de
documents sur papier.
Handgeschöpftes Papier aus Nepal zum Aufziehen von Bildem aus Papier
Es wurden die Eigenschaften von handgemachtem nepalesischem Papier untersucht, um die
Eignung dieses Materials für die Restaurierung von Bildern aus Papier aufzuzeigen. Die
Festigkeitswerte wie Bruchlast, Durchreißwiderstand, Falzfestigkeit, Bcrstdruck wurden vor und
nach einer beschleunigten Alterung gemessen, ebenso die chemischen Eigenschaften und die
Kennzahlen der Fasern. Er zeigte
sich, daß Nepalesisches Papier eine sehr gute Festigkeit und auch andere Eigenschaften aufweist,
die für Aufziehmaterial von Bedeutung sind.
REFERENCES
1. Trier, J: Ancient paper of Nepal. Copenhagen: Jutland Archaeological Society Publications,
1970.
2. TAPPI - Technical Association of the Pulp and Paper Industry, Inc. One Dunwoody Park,
Atlanta, GA 30338, USA.
3. ASTM - American Society for Testing and Materials, 1916 Race St. Philadelphia, PA, 19103,
USA.
4. Grant, J. A laboratory hand book of pulp and paper manufacture. London: Edward Arnold,
1960: 68.
5. Browning, B.L.: Analysis of paper. New York: Marcel Dekker, 1977: 73.
6. Ibid, pp. 90-91 (TAPPI Standard T 419).
7. Ibid, p. 81.
D.G. Suryawanshi
O.P. Agrawal
National Research Laboratory for Conservation of Cultural Property
Sector E/3, Aliganj, Scheme
Lucknow - 226020
India