Analysis of Historical Documents from a Viewpoint of Paper Science

Analysis of Historical Documentsfrom a Viewpoint of Paper Science
YOON-HEE HAN*, TOSHIHARU ENOMAE†, AKIRA ISOGAI‡,HIROFUMI YAMAMOTO** and MICHIHISA HOTATE***

JSPS Postdoctoral Fellow, †Associate Professor, ‡Professor**
Professor of Historiographical Institute*

Paper Science Lab., Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences
The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, JAPANCurrently affiliated with*

Historiographical Institute, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8654, JAPANenomae@psl.fp.a.u-tokyo.ac.jp (Toshiharu Enomae)

ABSTRACT
Restoration of historical documents and arts has become important to inherit cultural properties. Most of historical
documents were recorded on paper. Therefore, restoration of ancient papers is demanded and techniques for thispurpose must be developed and sophisticated. In our study, several nondestructive methods for analyzing ancient paper
have been developed. Image analysis using fast Fourier transform with suitable modifications was applied to optical
micrographs of traditionally-handmade Korean and Japanese papers. This analysis determines the angle and anisotropyof fiber orientation of paper surfaces. Fiber orientation of traditionally-handmade Korean and Japanese papers wasfound to show their own characteristics in accordance with the motion of a papermaking screen made of bamboo splints.
Consequently, the information on fiber orientation was found to be possible to distinguish the flow-sheet forming
typical of Japanese paper and still- sheet forming typical of Korean paper. Moreover, the anisotropy was always higher
for the screen side than for the top side, thus meaning that surface fiber orientation is possible to distinguish the twosides of paper of which papermaking history is unknown. An application of this technique to actual historical
documents evidenced that wrapping papers were used as envelops with a lateral side up, namely, after rotating 90
degrees. A variety of cultural habits in writing letters was revealed by discrimination of the two sides.

INTRODUCTION
History of paper manufacturing began in ancient China.
In East Asia, this technology was handed down to Korea
at some time between the 2nd and 4th centuries
according to an ancient Korean record 1) and finally to
Japan in 610 according to the Japanese oldest
historiography. In these modern days, a lot of paper isindustrially produced from wood pulp fibers, but papermaterials for Korea and Japanese ancient art anddocuments was made of bast fibers. As for papermaking
tools, following is the Japanese characteristic tools used
nowadays 2) and it seems that paper craftsmen have usedsimilar tools with the same principle for hundreds ofyears. The forming screen (corresponding to the forming
wire in the modern papermaking) is made of finely split
and beveled bamboo splints woven together with silkthreads, leaving faint characteristic "chain lines" in thefinished sheets. A craftsman clamps the forming screen
into the light wooden frame to use it as a mold. Korean
2006 Pan Pacific Conference

and Japanese papers, called Hanji and Washi,
respectively, in their own countries, might appear to be
similar to each other. However, they have many different
aspects nowadays and the differences date back to themedieval times or earlier.
From a viewpoint of the screen motion, traditional sheet
forming processes are classified basically to two
methods; the still-sheet forming method and the flowsheet forming method. In the flow-sheet forming method,a mold is dipped into a vat containing fibers suspended
in water. The filled mold is lifted above the water andthen swished back and forth or sideways. Several dips ofadditional fibers and sloshes-off of the excess fibers form
one sheet. This Asian unique method of sheet formation
contrasts with the still-sheet forming method where themold is dipped only once and allowed to drain naturally.
The still-sheet forming is commonly used in the Westand some areas of the East 3).

147

Analysis of Historical Documents from a Viewpoint of Paper Science

Generally, fibers in a suspension flow tend to be oriented
in the flow direction. The motion of the screen in the two
methods of sheet forming is differently reflected in thefiber orientation of paper. Namely, fibers are likely to beoriented greatly by the flow-sheet forming, but hardly by
the still-sheet forming. In order to evaluate the extent(anisotropy) and direction (angle) of fiber orientation
objectively, a new nondestructive image analysis
technique was developed and applied to micrographs ofpaper surfaces 4). If this principle is applied the other way
round, Hanji and Washi manufactured in the past mustshow their characteristics to the fibers flow, namely, thescreen motion at the time of sheet forming 5 ).
Consequently, fiber orientation can be a criterion toclassify ancient papers according to the sheet forming
method. This application covers not only oriental papers
but also western papers.
The purpose of this work is to estimate the sheet-forming
methods used for manufacturing papers of historic
documents and thus the papermaking technology in thosedays. In addition, we expect that this image analysis
technique developed in this work will be utilized to
select a repair paper that matches the original paper as apart of cultural properties by conservators.
EXPERIMENTAL
Image analysis for fiber orientation
Statistically, fiber alignment is expressed as a fiber
orientation distribution as a function of angle. For thehandmade papers analyzed in this work, the left and right
direction of the squared mold from the craftsman's viewis assumed to be 0 °. The major direction of fibers wasdetermined following the next steps4). First, amicrograph as a 256 gray level image was trimmed to asize of 1024 by 1024 pixels (Fig. 1(a), for example) forfast Fourier transform (FFT). Second, the images werebinarized using a dynamic threshold method of image
partition (Fig. 1(b)). This binarization process can correct
the shading of images photographed under non-uniform
illumination 6 ). FFT was computed with these binary
images to obtain power spectra like Fig. 1(c) calculated
from Fig. 1(b). The central bright region extending
relatively in the horizontal direction in Fig. 1(c) relates tothe fiber orientation. In the process of evaluating fiber
orientation, amplitude (a square root of power) was
added in the radius direction from the origin and its mean
was determined for each angle from 0 to less than 180 °above the horizontal-axis. If the mean amplitude values
are plotted as a function of angle in the polar coordinates,
fiber orientation diagrams like a curve drawn with thesolid line in Fig. 1(d) is obtained. This curve wasapproximated to an ellipse drawn with the dotted line
using another technique of FFT. The ellipse has thelonger (L) and shorter (S) axes at right angles to eachother. The angle of θ (0 ≤ θ < 180 º) and the magnitude

148

of elongation calculated as length L divided by length S
represents the direction and anisotropy of fiber
orientation, respectively. In order to determine theoverall fiber orientation for the identical sheet, the mean
amplitude values of all the images within the sheet wereaccumulated for each angle and the overall orientation
angle and anisotropy were calculated in the same manneras the procedure for a single image.
SamplesContemporary handmade papers
Contemporary papers manufactured in the traditional
methods in South Korea and Japan were examined. The
samples used were Mino paper with a basis weight of 61
g/m2 (symbol H, Washi) prepared by S. Hasegawa, Jangji
of 57 g/m2 and Uiryeong of 52 g/m2 (symbols J and U,
respectively, Hanji). Those samples have a mean
thickness of 0.158, 0.143 and 0.141mm for samples H, J
and U, respectively. These two kinds of Hanji were madeat craft studios in the traditional papermaking districts
Uiryeong and Jangji, Korea. Mino is a name of the citywith a lot of Washi production.
Model papers hand-made in laboratory
Traditional sheet forming methods were simulated
1. Still-sheet forming: a mold is dipped into a fiber
suspension once, lifted with the horizontal position kept
and left still until the excess water has drained away.
2. Flow- and still-sheet forming: one operation of theflow-sheet forming followed by one operation of thestill-sheet forming after a quick motion of mold dipping.
3. Flow-sheet forming: the craftsman's side edge of amold is dipped into a fiber suspension once, lifted andslanted so an excess suspension flows out over the far
side edge, and subsequently dipped into the fiber
suspension once again, lifted, shaken back and forth for ashort time and slanted so an excess suspension flows out
again over the far side edge; after the removal of thedilute portion of the fiber suspension, the mold with thewet web remaining on the screen is returned to thehorizontal position to complete the draining process.
The fiber suspension with a consistency of 0.5 % was
prepared from mulberry fibers after beating to 1000
revolutions with a PFI mill. Two litters of a 0.09%
solution of Neri extracted from lightly-hammered rootsof Abelmoschus manihot immersed in water was added
to 40 litters of the fiber suspension. Bamboo splints ofthe screen are 0.1 mm thick and woven at 28 pieces per
Sun (30.3 mm).
Ancient document papers
Japanese ancient documents, — the Shimadzu Family
documents — owned by the Historiographical Institute,
University of Tokyo, were examined. The documents arecorrespondences exchanged between the Shogunate
Government of Edo and the Shimadzu Family in
Satsuma (currently, Kagoshima prefecture, Japan). The
2006 Pan Pacific ConferenceAnalysis of Historical Documents from a Viewpoint of Paper Science

Korean ancient documents from the 11th to 16th
centuries were examined. The documents include the
Korean Buddhism sacred book and several ancient
documents mainly in the Goryo Dynasty period and the
Chosun Dynasty period conserved in HOAM museum orby an individual in Korea.
Daitokuji is a temple in Kyoto and conserves a lot of
historic documents. The number of the analyzed
Daitokuji document papers ranging from the 12th to 18th
century was 106. The side was not distinguished between
the screen side with the higher anisotropy and the top
side with the lower anisotropy, but was distinguishedbetween the written side and the non-written side.
Formal letters using only one sheet for writing texts
accompanies a blank sheet as a backing customarily.
Method of acquiring surface images
A digital microscope DG-2, Scholar Co. Ltd., Japan wasused to acquire reflected light images of paper surfaces
with a lens of 100 times magnification. For thecontemporary handmade papers and the model papers
hand-made in laboratory, five images were captured fromeach of the nine evenly spaced locations; these locations
were horizontally located in groups of three along the top,
middle, and bottom of a sheet. Images of 1024 × 1024
pixels corresponding to 1.66 × 1.66 mm2 were subjected
to FFT followed by the calculation of fiber orientation
distribution. For ancient document papers, 10 images
from evenly spaced 10 locations on either side of theancient document papers were taken and subjected to thesame processes of the image analysis.
For every image, the bottom side corresponds to theposition from which the craftsman held the wooden
frame with a bamboo wire. Therefore, a fiber orientation
angle of 0 degrees, for example, is equivalent to the leftand-right direction from the craftsman’s view. The
ancient document books or papers were placed so thatthe writing direction comes to be vertical in the captured
image.

axis means the orientation angle. The shape of thisellipse shows a high anisotropy and the orientation angle
is almost 90 °. Fig. 2(a) shows the anisotropy andorientation angle for the contemporary papers. For thescreen side, all the samples give high anisotropies
ranging from 1.16 to 1.22 and their orientation angles of
all about 90 ° as Fig. 2(b) shows. For the top side, Washi
H gives a fairy high anisotropy, while Hanji J and U
show random fiber orientation as shown by theanisotropies almost equal to unity. The orientation angles
for the top side corresponding to anisotropies less than
1.05 is not presented in this figure because such angles
have no significance. The orientation angle of 90 °means that the craftsman slanted the mold and let thefibers flow in the back-and-forth direction as he regularly
does. The general tendency of our data about the two-

(a)

(c)

(d)

Screen side
Top side

1.2

1.1

1.0

RESULTS AND DISCUSSION

2006 Pan Pacific Conference

(b)

1.3

H (Japanese)

J (Korean)

U (Korean)

210
Orientation angle, degree

Analysis of contemporary handmade papers
Fig. 1 shows an example of the FFT image analysis forfiber orientation distribution of contemporary paper H.
Photograph (a) is a representative micrograph of thesample. Its binary image (b) was subjected to FFT tocalculate power spectrum (c). The diagram of overall
fiber orientation (d) includes two drawings; the solid line
shows a practical plot of the distribution and the dottedline shows its approximated ellipse with the longer axis
in the horizontal direction. The direction of the shorter(a)Fig. 1 Micrograph of screen side of Washi H with digital optical
microscope (a), binary image (b), FFT spectrum (c) anddistribution diagram of overall fiber orientation

Anisotropy

paper-made formal letters corresponded from the
Shogunate Government (Edo era) ranging from 1606 to1859 were selected for examination.

180

(b)

Screen side
Top side

150
120
90
60
30
0
H (Japanese)

J (Korean)

U (Korean)Fig. 2 Fiber orientation anisotropy (a) and angle (b) of Korean
and Japanese contemporary papers. Error bars represent
95 % confidence intervals as in other figures.

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Analysis of Historical Documents from a Viewpoint of Paper Science
1.3

Anisotropy

1.3

(a)

Screen side
Top side

1.2
1.1
1.0
Still

Flow and Still

Flow

Sheet-forming method
Orientation angle, degree

210
180

(b)

Screen side
Top side

150
120
90
60
30
0
Still

Flow and Still

Flow

Sheet-f orming method

Fig. 3 Fiber orientation anisotropy (a) and angle (b) of model
papers hand-made in laboratory by three different methods ofsheet forming.

Analysis of ancient document papers

150

(a)

Screen side
Top side

1.2
1.1
1.0
1606

1647 1706

1752 1752
A-1) A-2)

1752 1752
A-3) B-1)

1752 1807
B-2)

1859

1752
B-2)

1859

Year

Orientation angle, degree

Analysis of model papers
To assure the relationship between the mold motion andfiber orientation, typical motions of the traditional sheet
forming were simulated in laboratory. Fig. 3(a) and (b)show the anisotropy and orientation angle of the model
papers. The flow-sheet forming method shows thehighest anisotropy and the still-sheet forming shows thelowest anisotropy as we expected. Approximately,
anisotropies higher than 1.10 and those lower than 1.05
mean strong fiber orientation and random fiber
orientation, respectively. The sequential flow- and stillsheet forming method brought about fairly highanisotropy for the screen side and very low anisotropy,
that is, the random orientation for the top side. This
result agreed reasonably with the relationship that can bepredicted from the screen motion. The significant twosidedness was observed with the model paper preparedby the flow-sheet forming method. This paper consists of
two layers formed by twice flow-sheet forming actions.
The degree of anisotropy is related to the dehydration
rate. The first layer of a fiber mat is formed usuallyduring quick dehydration and ends of fibers are easily
trapped by the bamboo splints of the forming screen withthe fibers easily oriented. But, this first layer reduces thedehydration rate of the second fiber suspension.
Therefore, when the excess of the second fiber
suspension is made to flow out, fibers are hardly retained
because of few trapping chances.

Anisotropy

sidedness is that the screen side always has higher
anisotropies.

210
180

(b)

Screen side
Top side

150
120
90
60
30
0
1606

1647

1706

1752
A-1)

1752
A-2)

1752
A-3)

1752
B-1)

1807

Year

Fig. 4 Fiber orientation anisotropy (a) and angle (b) of Japanese ancient
documents (Shimadzu). Sample1752 B-1) is only envelope sheet
and the rest are all letter papers for texts or blank.

Shimadzu Family documents
Fig. 4(a) and (b) shows the results for the Shimadzu
Family documents. Anisotropy of the screen side washigher than or almost equal to 1.10 for every sample. The
1752 B-1) paper showed a relatively low anisotropy of
1.11. Only this paper was used for wrapping as anenvelope sheet, though all the others were main letter
papers or the same kind of blank sheets (ancient official
letters always consist of two sheets). The orientation
angle was all about 90 ° with an exception of 0 ° for thescreen side of the envelope sheet. This is because theenvelope sheet was used customarily with a lateral side
up, namely, after rotation of 90 °. It was found that thefiber orientation analysis can be used to confirm thiscustom when there is no evidenced directionality
recognized with ease from the appearance of the paper.
The orientation angles for the top side corresponding toanisotropies less than 1.05 are not presented because of
random orientation.
Korean ancient documents
Fig. 5(a) and (b) shows the results for the Korean ancient
documents. The anisotropy and angle of fiber orientation
was separately calculated for the written and non-writtensides, respectively, unlike the calculations made for theother samples so far. The samples in the mid 14th
century had a higher anisotropy for the written side thanfor the non-written side. This fact means that the screenside was mainly selected for writing. However, most
samples show no significant difference between thewritten and non-written sides. Basically, Korean papers
had not had the distinctive two-sidedness regarding
comfortable writing. This may be derived from still-sheet
forming typical of Korean paper. Regarding the2006 Pan Pacific ConferenceAnalysis of Historical Documents from a Viewpoint of Paper Scienceorientation angle, there were two angles calculated — 0 º
and 90 º (about 180 º is almost equal to 0 º). These twoangles were not systematically distinguished from thewritten side to the non-written side. However, thistendency contrasts with that of Japanese paper with theorientation angle indeed mostly of 90 º regardless ofwhat kind of two-sidedness.
Fig. 6 shows a result for the Daitokuji document papers.
By the typical, but non-systematic classification methodfrom viewpoints of such as fiber sources, presence ofadditives like rice powder (if present, it is called“suibara”) and presence of a striped-wrinkle pattern(“hikiawase”), papers manufactured in the medieval
times can be sorted as shown in this figure. However,there is no significance with the order of the paper
category names or the data plot order in each paper
category. In most samples and most paper categories, thewritten side had higher anisotropy. This fact means thatthe screen side was selected for writing because every
formed wet sheet is spread over a wooden drying board
usually with the screen side in contact with the board
surface and the screen side becomes smoother than the
top side. But, the Shukushi papers showed the opposite
result. Shukushi means recycled paper and has aremarkable screen mark with dark lines where residual

Anisotropy

1.3

(a)

Written side
Non-w ritten side

1.2
1.1
1.0
11C

Orientation angle, degree

210
180

L13CE14C

M14C

L14C

15C

16C

Year

(b)

Written side
Non-w ritten side

150
120
90
60
30
0
11C

L13CE14C

M14C L14C
Year

15C

16C

Fig. 5 Fiber orientation intensity (a) and angle (b) of Korean ancient
documents. In symbols, L, M and E denotes “late”, “mid” and“early”, respectively.

side.
As is mentioned earlier, Japanese writers of letters havecustomarily used two sheets if the second sheet was
blank. For such double sheet letters, two sheets were firsttaken together from a paper stack where sheets werestacked all with the smooth side face up. Text started tobe written usually on the smooth side of the first sheet.
Then, the two sheets together were turned over to go to
the second page if necessary. Table 1 shows the fiber
orientation anisotropy of the two sides of the two each
sheet as well as the estimated situation how the sheets ofthe double sheet letters in the Daitokuji document werehandled for writing. The regular pattern is that thewritten side of the first sheet and the unwritten side of
the second sheet is the smoother side, that is, the side
contacting a wooden drying board (traces of annual rings
were observed on this side and the figure of anisotropy isdenoted by “Brd” in the table). This regular pattern wasobserved with 15 letters with the exception of 3 letters as
denoted by “One upside down”. Concerning the drying
process, the screen side is regularly in concordance withthe “Brd” side, but the discordance was observed with 6
letters (247-1, 469-1, 535-1, 788-1, 1509-1 and 1543-1).
The result of the fiber orientation analysis revealed thatsuch a reverse drying — the top side attached on awooden drying board — was not too exceptional.
CONCLUSIONS
A nondestructive method of image analysis using fast
Fourier transform has been developed to analyze
anisotropy and angle of fiber orientation of paper
surfaces. When this method was applied to contemporary
Korean and Japanese papers, they showed their own
characteristics in accordance with the motion of apapermaking screen. Anisotropy of fiber orientation was
found to be higher for flow-sheet forming and lower forstill-sheet forming as a result of anisotropy of model
papers hand-made in laboratory. Moreover, theanisotropy was always higher for the screen side than for
the top side, thus meaning that surface fiber orientation ispossible to distinguish the two sides of paper of which
papermaking history is unknown. An application of thistechnique to actual historical documents evidenced thatwrapping papers were used as envelops with a lateralside up, namely, after rotating 90 degrees. A variety ofcultural habits in writing letters was revealed bydiscrimination of the two sides.

Indian ink particles collected. This is the reason why
writers avoided the screen side of Shukushi forbrushstrokes. The fiber orientation analysis helped toknow that it was the screen side with the higher
anisotropy that had a screen mark and that ancient
writers used the top side for writing in spite of the rough2006 Pan Pacific Conference151Analysis of Historical Documents from a Viewpoint of Paper Science

Written side
Non-written side

Anisotropy

1.3
1.2
1.1
1.0

Mino paper

KamakurasuibaraMuromachisuibaraDanshi

Hikiawase

ShukushiHi-gradehikiawaseFig. 6 Anisotropies of Daitokuji document papers classified under papermaking processes and additives etc..

Table 1 Fiber orientation anisotropy of first and second sheets
comprising double sheet letters in Daitokuji document
Doc #First sheet
Second sheet
Written Non-written Non-written Written
side
side
side
side

Pick-up pattern of
double sheets

174

1.20Brd

1.04Brsh

1.18Brd

1.08Brsh

Together196

1.28Brd

1.12Brsh

1.26Brd

1.17Brsh

Together

205

1.19Brd

1.15Brsh

1.10Brd

1.05Brsh

Together206

1.22Brd

1.08Brsh

1.19Brd

1.10Brsh

Together224

1.19Brd

1.12Brsh

1.22Brd

1.13Brsh

Together

247-1

1.14Brd

1.20Brsh

1.16Brd

1.19Brsh

Together253

1.15Brd

1.06Brsh

1.16Brd

1.06Brsh

Together293

1.19Brd

1.08Brsh

1.21Brd

1.10Brsh

Together

419-1

1.21Brd

1.12Brsh

1.21Brd

1.13Brsh

Together

469-1

1.07Brd

1.26Brsh

1.28Brd

1.09?

Together*

535-1

1.09?

1.19?

1.25?

1.10Line

Together*

549-1

1.31Brd

1.15Brsh

1.33Brd

1.11Brsh

Together

566-1

1.24Brd

1.13Brsh

1.23Brd

1.15Brsh

Together

593-1

1.21?

1.09?

1.27?

1.10?

(Together)

788-1

1.22?

1.17Brsh

1.17Brsh

1.09?

One upside down*

1509-1

1.19Brd

1.14Brsh

1.22Brsh

1.12Brd

One upside down*

1543-1

1.06Brd

1.22?

1.19?

1.07?

Together*1557-1

1.19Line

1.10Brsh

1.16?

1.06Line

One upside down*

3) Glossary chapter, Text book of "International courseon conservation of Japanese paper 2002", 123, National
research institute for cultural properties Tokyo (2003).
4) Enomae, T., Han, Y.-H. and Isogai, A., Nondestructive
determination of fiber orientation distribution of paper
surface by image analysis, Nordic Pulp Paper Res J,
21(2), in press (2006).
5) Han, Y.-H., Enomae, T., Isogai, A., Yamamoto, H.,
Hasegawa, S., Song J.-J., and Jang, S.-W, Comparison of
traditional hand papermaking techniques in the Japanese(Washi) and Korean (Hanji) traditions, with particularreference to the study of fiber orientation in historicaland contemporary papers", Studies in conservation, in
press6) Murata, K. and Fujita, M., FFT-estimation of fiber
orientation on wood fiberboard, J. Soc Mat Sci 48(1):77(1999)“Brd”, the side contacting drying board; “Brsh”, the side brushed for spread;“Line”, thick lines found but side unknown; “?”, side unknown; Bold font, theside with higher anisotropy (screen side); “Together”, two sheets taken together
from stack; “ One upside down”, one of two sheets turned upside down; ”*”,
discordance between screen side and Brd side.

REFERENCE
1) Jung, S.-Y., A study on the introduction of paper andpapermaking technique in ancient Korea, PhD
dissertation, Yonsei University (1997) [in Korean].
2) Sukey, H., Washi the world of Japanese paper, 99,
Kodansha International (1978).

1522006 Pan Pacific Conference