Study on Twining Technologies Adopted by Bamboo Sutra Wrappers of the Tang Dynasty

Bamboo sutra wrapper is made of bamboo splints fastened together with silk threads using the warp twining method. Bamboo sutra wrappers dating to the Tang and Song dynasties are mainly found in Cave 17. They are usually in a rectangular shape with the geometrical, dotted or floral pattern which are quite favored in the Tang dynasty. Affected by Buddhism, sutra wrappers of this kind have also been found in other Asian countries such as Japan and Korea, with a similarity in structure, dimension and twining method.


Introduction
The Chinese character zhi means book wrappers or small bags. Jingzhi means sutra wrappers, which were used to store and protect Buddhist sutras in ancient times. Sutra wrappers are of various materials and constructions. Some are decorated with ties and some are not. The decorations' forms and functions are various. Bamboo sutra wrappers are made of flat bamboo splints and silk threads by the method of twining. Twining is an ancient hand weaving technique. Two sets of threads are required when apply twining. A set of threads should be laid in parallel while the other set twining around the former set. Twining appears in many ancient Chinese documents. Xiao Tong, a litterateur of the Southern dynasties (from 420 to 589 CE), wrote in Fu shuzhi [A Poem Dedicated to book wrappers]: "It is lucky that there are book wrappers made of colourful silk and the wrappers are woven by beautiful ladies." Lu You, another litterateur, wrote in RuShu ji [Journey to Sichuan]: "The poem collections of Bai Juyi have once been left in Bai's house then moved several times. The emperor Zhenzong demanded the royal library to compile and edit those poems into a book then cover with a bamboo book wrapper and send to the temple." Most of the twined Buddhist sutra wrappers of the Tang and Song dynasties (from 618 to 1279 CE) were excavated from the Dunhuang library cave (Cave 17). However, at the beginning of the 20th century many of them were taken abroad by foreign archaeologists and have been collected in several museums or libraries overseas such as the British Museum, the Musée Guimet and the Bibliothè que nationale de France etc. Only a very few fragments are collected in Lüshun Museum.

Construction of Sutra Wrappers
Bamboo sutra wrappers can be quite frequently seen in the documents from Dunhuang.Citing the records in document numbered Meng 1704 (Дx1058) as a typical example, Dr. Fang Guangchang has proved that bamboo sutra wrappers were once very popular at Dunhuang. The document says: "6 bamboo wrappers for Benxingji jing [Sutra of the Collection of the Original Acts of the Buddha], a bamboo sutra wrapper for Fa ji jing, a bamboo sutra wrapper for □\□ lun jing, a bamboo sutra wrapper for Dafang deng dayu jing, a bamboo sutra wrapper for Guanfo sanmeihai jing". Bamboo sutra wrappers are also mentioned in the document numbered S.5594, which are divided into two types -crudely made and carefully-crafted.
Chinese bamboo sutra wrappers are all rectangular in shape. The ratio of length to width ranges from 1.2 to 1.6. Mostly, yellowish brown bamboo splints are used as wefts while Z-twisted silk threads in various colours are adopted as warps. Stripes of pattern of different widths, generally 9 in total including 4 wide ones and 5 narrow ones, are created by two silk threads in different colours twining with bamboo splints. The distance between each ass.ccsenet.org Asian Social Science Vol. 16, No. 8 2020 two stripes varies from 1cm to 1.5cm (see Figure 1). None of the survived Bamboo sutra wrappers is complete. Some are even with large areas missing. Some have been stained and the pattern can not be identified.

Materials for Sutra Wrappers
The comparatively well preserved sutra wrappers suggest that necessary materials for making sutra wrappers are bamboo splints and silk threads. Some sutra wrappers are finished by using textiles (such as EO.1209/I shown in Figure 2). Some are pasted with paper documents on the back (such as EO.1208 shown in Figure 3).  Vol. 16, No. 8 2020 Compared with wrappers made of other materials such as textiles, bamboo sutra wrapper are more sturdy and durable. They act as a sound protector of Buddhist documents. The finely made and smooth bamboo splints are mostly in the colour of reddish brown, which are created by careful dyeing. There are 9-13 bamboo splints in very width of 1cm. The lengths of bamboo splints vary from 26.8-32cm, which is determined by the size of Buddhist document. The colours of silk threads are multiple. The most common colours are cream, yellowish brown, brown, dark blue, light blue, dark green and pale green. And in between two colours, there is a colour transition. Therefore, the wrappers should be bright and gorgeous at that time. In a width of 1cm there are 20 to 22 silk threads, all Z-twisted. The sutra wrappers are sometimes finished by textiles such as weft-faced compound twill or warp-faced compound twill, which mostly shows floral pattern.
Why paper documents were attached to bamboo sutra wrappers? Except for protection, there should be social reasons. In ancient times, papers and writing brushes are extremely precious. A legend says that when Cang Jie (a legendary person) created Chinese characters, the heaven was moved into grain tears since information can be spread quickly; ghosts were scared into deep night tears since they have nowhere to hide. Therefore, paper was considered as a symbol of wisdom. Another reason is that temples in Dunhuang are usually lack of paper because of desolate location and poor traffic condition, so it is wasteful to throw away paper with writing. The third reason is that although lack of paper, the temple in Dunhuang has a plenty of papers with writing, mostly of which are sutras donated by Buddhist believers. And finally, sutras were frequently used then reference was necessary, so it is reasonable to paste papers with writing to the back of sutra wrappers.

Weaving Technique
The bamboo sutra wrappers are made by the technique of twining with silk threads and bamboo splints. Two sets of silk threads in different colours are required in the twining process, when the silk threads are twined once, the colours are alternately arranged while they are twined twice, the colour is the same. By twining, patterns were formed (see fig4.). After observed the sutra wrapper fragments and the repeat of the patterns, most of which are flowers and geometries, some are Chinese characters (see Figure 5.). folded in opposite directions to from a loop and the first splint is arranged in the loop. Such a start is the firmest and the back side of the wrapper looks simple. (see Figure 6). Till the end of threads, new threads should be added. The connection of the threads is made in this way: a length of 5-7cm from the end of the previous threads overlap a length of 5-7cm from the beginning of the newly added threads (generally four threads in total at the meeting point) then continue twining. Such a way of thread connection ensures a secure and firm linkage and a neat view of the back and avoids bulges caused by knots.

The Finish of Twining
When silk threads are approaching the final bamboo splint, some finishing work should be done. For some sutra wrappers, threads wound around the final 10-15 bamboo splints at the back side are of deeper colours than those at the rest part, and some sutra wrappers remain fragments of paper attached to the back the finishing area (see Figure 7.). It is concluded that when finishing, silk threads are tied first then for the purpose of stabilization, paper is pasted to the final bamboo splints with glue or other sticky liquid.

Technique for Creating Chinese Characters
Sutra wrappers unearthed from Dunhuang are almost produced in the same twining technique but some Chinese characters on those wrappers are created in a slight different way. In order to imitate the brush stroke in Chinese calligraphy, the starting and finishing parts of the characters require a special technique. And those characters are usually arranged in rectangles, therefore, they can't be woven along a direction, namely only from left to right or ass.ccsenet.org Asian Social Science Vol. 16, No. 8 2020 just from right to left.
The brush strokes at the beginning or the end of a character are created by the silk threads remaining in between two bamboo splints instead of tightly completely being wound around the bamboo splints after twining (fig8.). Generally, both the silk threads on the front and reverse should be pulled taut, especially those at the reverse side, but when special effect is required, for example, for the purpose of creating an effect like brush strokes, the threads at the back can be loosened but keep the threads on the front tight. This technique is proven by reconstruction (see Figure 9.). Characters are woven in rectangle frames. Therefore, the threads creating characters are not woven along a direction; instead, after a set of threads reaches the frame, it returns and weaves backwards (see fig10.). In addition, the threads in the rectangle are thinner than those outside. At the points where the frame and the main panel outside the frame meet, the threads for the main panel joined the threads for the rectangular frame for the sake of firmness.

The Use of Bamboo Sutra Wrapper
Twined bamboo sutra wrappers are easy to use. Since sutra wrappers are specifically woven for Buddhist documents, therefore they just match the size of documents. To use a wrapper, firstly keep the wrapper upside down then put a Buddhist documents on the wrapper and finally rolls the document with the wrapper from the end to the start (See Figure 11).
The patterns on the wrappers vary from each other. A wrapper may have a title either woven or written on a paper tag then pasted to the wrapper, such as Da zhi lun di yi zhi (indicating this is the first wrapper for the Prajñ āpāramitā Sutra ) or "□□第十帙" □□ di shi zhi (indicating this is the tenth wrapper for □□ Sutra). Temple libraries store documents according to their titles so monks can access any one of the documents easily. Such wrappers are beautiful and useful. They can protect the document and make the documents convenient to read. Figure 11. The use of bamboo sutra wrapper

Japanese and Korean Sutra Wrappers of the Same Period
With the spread of Buddhism, Buddhist sutra wrappers also became popular in Japan and Korea. Their weaving techniques are similar to those in China. According to their pattern, Japanese sutra wrappers fall into two types.
One is similar to those found in Dunhuang. They are formed by alternately arranged wide and narrow strips of patterns. The other shows large patterns instead of patterns in strips. The latter is more difficult and takes more time to make but they are very impressive and gracious.There are two big differences in technique between Chinese sutra wrappers and Japanese ones. One is twining direction and the other is whether all bamboo splints are covered or not.
Firstly, on Chinese sutra wrappers both the threads, including threads at the back or in the front, are twined in the same direction, namely from lower left to upper right while on Japanese ones threads are twined from upper left to lower right but the final twine is from lower left to upper right.
Secondly, the bamboo splints on all Chinese sutra wrappers are covered with threads on both sides while Japanese covers have some parts on the front side not covered with threads if the colour of the bamboo splints is the same as that of the threads. The threads are hidden behind the bamboo splints without twining. When many bamboo splints are skipped, the threads are loose and the wrapper is easy to get twisted and distorted. (See Figure 12 and Figure 13).

Conclusion
Bamboo sutra wrappers of the Tang and Song dynasties (CE 618-1279) are representatives of twined works. They are rectangular in shape and their sizes are determined by the Buddhist documents. The motifs on wrappers are popular in the Tang dynasty such as flowers, scrolled grasses and geometrical patterns. The colours are plentiful and in between two colours there is a colour transition, thus the patterns are very natural. Reddish brown bamboo splints are used as wefts and silk threads of different colours are used as warps and the wrappers are created by warp twinning method. Some wrappers are edged with textile or pasted with paper. Because the wrappers are different in colour, pattern and with different characters or numbers, it is easy for monks to find the Buddhist documents.